@article{kokcu_labib_freericks_kemper_2024, title={A linear response framework for quantum simulation of bosonic and fermionic correlation functions}, volume={15}, ISSN={["2041-1723"]}, url={https://doi.org/10.1038/s41467-024-47729-z}, DOI={10.1038/s41467-024-47729-z}, abstractNote={Abstract Response functions are a fundamental aspect of physics; they represent the link between experimental observations and the underlying quantum many-body state. However, this link is often under-appreciated, as the Lehmann formalism for obtaining response functions in linear response has no direct link to experiment. Within the context of quantum computing, and via a linear response framework, we restore this link by making the experiment an inextricable part of the quantum simulation. This method can be frequency- and momentum-selective, avoids limitations on operators that can be directly measured, and can be more efficient than competing methods. As prototypical examples of response functions, we demonstrate that both bosonic and fermionic Green’s functions can be obtained, and apply these ideas to the study of a charge-density-wave material on the ibm_auckland superconducting quantum computer. The linear response method provides a robust framework for using quantum computers to study systems in physics and chemistry.}, number={1}, journal={NATURE COMMUNICATIONS}, author={Kokcu, Efekan and Labib, Heba A. and Freericks, J. K. and Kemper, A. F.}, year={2024}, month={May} }
@misc{data for: a linear response framework for simulating bosonic and fermionic correlation functions on quantum computers_2024, DOI={10.5061/dryad.51c59zwcn}, journal={Dryad}, year={2024}, month={Apr} }
@article{kemper_yang_gull_2024, title={Denoising and Extension of Response Functions in the Time Domain}, url={https://doi.org/10.1103/PhysRevLett.132.160403}, DOI={10.1103/PhysRevLett.132.160403}, abstractNote={Response functions of quantum systems, such as electron Green's functions, magnetic, or charge susceptibilities, describe the response of a system to an external perturbation. They are the central objects of interest in field theories and quantum computing and measured directly in experiment. Response functions are intrinsically causal. In equilibrium and steady-state systems, they correspond to a positive spectral function in the frequency domain. Since response functions define an inner product on a Hilbert space and thereby induce a positive definite function, the properties of this function can be used to reduce noise in measured data and, in equilibrium and steady state, to construct positive definite extensions for data known on finite time intervals, which are then guaranteed to correspond to positive spectra.}, journal={Physical Review Letters}, author={Kemper, Alexander F. and Yang, Chao and Gull, Emanuel}, year={2024}, month={Apr} }
@article{scott_kisiel_yakhou_agrestini_garcia-fernandez_kummer_choi_zhong_schneeloch_gu_et al._2024, title={Detection of a two-phonon mode in a cuprate superconductor via polarimetric resonant inelastic x-ray scattering}, volume={109}, ISSN={["2469-9969"]}, DOI={10.1103/PhysRevB.109.125126}, abstractNote={Recent improvements in the energy resolution of resonant inelastic x-ray scattering experiments (RIXS) at the Cu-${L}_{3}$ edge have enabled the study of lattice, spin, and charge excitations. Here, we report on the detection of a low-intensity signal at 140 meV, twice the energy of the bond-stretching (BS) phonon mode, in the cuprate superconductor ${\text{Bi}}_{2}{\text{Sr}}_{2}\text{Ca}{\text{Cu}}_{2}{\text{O}}_{8+x}$ (Bi-2212). Ultrahigh-resolution polarimetric RIXS measurements allow us to resolve the outgoing polarization of the signal and identify this feature as a two-phonon excitation. Further, we study the connection between the two-phonon mode and the BS one-phonon mode by constructing a joint density of states toy model that reproduces the key features of the data.}, number={12}, journal={PHYSICAL REVIEW B}, author={Scott, K. and Kisiel, E. and Yakhou, F. and Agrestini, S. and Garcia-Fernandez, M. and Kummer, K. and Choi, J. and Zhong, R. D. and Schneeloch, J. A. and Gu, G. D. and et al.}, year={2024}, month={Mar} }
@article{del re_rost_foss-feig_kemper_freericks_2024, title={Robust Measurements of n-Point Correlation Functions of Driven-Dissipative Quantum Systems on a Digital Quantum Computer}, volume={132}, ISSN={["1079-7114"]}, url={https://doi.org/10.1103/PhysRevLett.132.100601}, DOI={10.1103/PhysRevLett.132.100601}, abstractNote={We propose and demonstrate a unified hierarchical method to measure n-point correlation functions that can be applied to driven, dissipative, or otherwise open or nonequilibrium quantum systems. In this method, the time evolution of the system is repeatedly interrupted by interacting an ancilla qubit with the system through a controlled operation, and measuring the ancilla immediately afterward. We discuss the robustness of this method as compared to other ancilla-based interferometric techniques (such as the Hadamard test), and highlight its advantages for near-term quantum simulations of open quantum systems. We implement the method on a quantum computer in order to measure single-particle Green's functions of a driven-dissipative fermionic system. This Letter shows that dynamical correlation functions for driven-dissipative systems can be robustly measured with near-term quantum computers.}, number={10}, journal={PHYSICAL REVIEW LETTERS}, author={Del Re, Lorenzo and Rost, Brian and Foss-Feig, Michael and Kemper, A. F. and Freericks, J. K.}, year={2024}, month={Mar} }
@article{jabusch_dayal_kemper_2023, title={Catalogue of phonon modes in several cuprate high-temperature superconductors from density functional theory}, volume={6}, ISSN={["2666-9366"]}, DOI={10.21468/SciPostPhysCore.6.1.018}, abstractNote={Cuprates are promising candidates for study in developing higher
temperature superconductors. A thorough understanding of a material’s
phonon modes enables further investigation of its emergent properties,
however, no complete reference of the phonon modes exists. Here, using
density functional theory, we evaluate the phonon frequencies and atomic
displacements for La2CuO4,
Bi2Sr2CuO6,
and Bi2Sr2CaCu2O8,
in their tetragonal structures. The phonon modes for all materials agree
with those expected from space group symmetry and display instabilities
corresponding to known low-temperature structural phase transitions.}, number={1}, journal={SCIPOST PHYSICS CORE}, author={Jabusch, Noah J. and Dayal, Pavan and Kemper, Alexander F.}, year={2023} }
@article{scott_kisiel_boyle_basak_jargot_das_agrestini_garcia-fernandez_choi_pelliciari_et al._2023, title={Low-energy quasi-circular electron correlations with charge order wavelength in Bi2Sr2CaCu2O8+& delta;}, volume={9}, ISSN={["2375-2548"]}, url={http://dx.doi.org/10.1126/sciadv.adg3710}, DOI={10.1126/sciadv.adg3710}, abstractNote={
Most resonant inelastic x-ray scattering (RIXS) studies of dynamic charge order correlations in the cuprates have focused on the high-symmetry directions of the copper oxide plane. However, scattering along other in-plane directions should not be ignored as it may help understand, for example, the origin of charge order correlations or the isotropic scattering resulting in strange metal behavior. Our RIXS experiments reveal dynamic charge correlations over the
q
x
-q
y
scattering plane in underdoped Bi
2
Sr
2
CaCu
2
O
8+δ
. Tracking the softening of the RIXS-measured bond-stretching phonon, we show that these dynamic correlations exist at energies below approximately 70 meV and are centered around a quasi-circular manifold in the
q
x
-q
y
scattering plane with radius equal to the magnitude of the charge order wave vector,
q
CO
. This phonon-tracking procedure also allows us to rule out fluctuations of short-range directional charge order (i.e., centered around [
q
x
= ±
q
CO
,
q
y
= 0] and [
q
x
= 0,
q
y
= ±
q
CO
]) as the origin of the observed correlations.
}, number={29}, journal={SCIENCE ADVANCES}, publisher={American Association for the Advancement of Science (AAAS)}, author={Scott, Kirsty and Kisiel, Elliot and Boyle, Timothy J. and Basak, Rourav and Jargot, Gaetan and Das, Sarmistha and Agrestini, Stefano and Garcia-Fernandez, Mirian and Choi, Jaewon and Pelliciari, Jonathan and et al.}, year={2023}, month={Jul} }
@article{steckmann_keen_kokcu_kemper_dumitrescu_wang_2023, title={Mapping the metal-insulator phase diagram by algebraically fast-forwarding dynamics on a cloud quantum computer}, volume={5}, ISSN={["2643-1564"]}, url={https://doi.org/10.1103/PhysRevResearch.5.023198}, DOI={10.1103/PhysRevResearch.5.023198}, abstractNote={Dynamical mean-field theory (DMFT) maps the local Green's function of the Hubbard model to that of the Anderson impurity model and thus gives an approximate solution of the Hubbard model from the solution of simpler quantum impurity model. Quantum and hybrid quantum-classical algorithms have been proposed to efficiently solve impurity models by preparing and evolving the ground state under the impurity Hamiltonian on a quantum computer that is assumed to have the scalability and accuracy far beyond the current state-of-the-art quantum hardware. As a proof of principle demonstration targeting the Anderson impurity model we close the DMFT loop with current noisy hardware. With a highly optimized fast-forwarding quantum circuit and a noise resilient spectral analysis we observe a Mott phase transition. Based on a Cartan decomposition, our algorithm gives a fixed depth, fast-forwarding, quantum circuit that can evolve the initial state over arbitrarily long times without time-discretization errors typical of other product decomposition formulas such as Trotter decomposition. By exploiting the structure of the fast-forwarding circuits we reduce the gate count (to 77 CNOTs after optimization), simulate the dynamics, and extract frequencies from the Anderson impurity model on noisy quantum hardware. We then demonstrate the Mott transition by mapping the full metal-insulator phase-diagram. Near the Mott phase transition, our method maintains accuracy where Trotter error would otherwise dominate due to the long-time evolution required to resolve quasiparticle resonance frequency extremely close to zero. This work presents the first computation of the Mott phase transition using noisy digital quantum hardware, made viable by a highly optimized computation in terms of gate depth, simulation error, and runtime on quantum hardware.}, number={2}, journal={PHYSICAL REVIEW RESEARCH}, author={Steckmann, Thomas and Keen, Trevor and Kokcu, Efekan and Kemper, Alexander F. and Dumitrescu, Eugene F. and Wang, Yan}, year={2023}, month={Jun} }
@article{yeter-aydeniz_parks_thekkiniyedath_gustafson_kemper_pooser_meurice_dreher_2023, title={Measuring qubit stability in a gate-based NISQ hardware processor}, volume={22}, ISSN={["1573-1332"]}, DOI={10.1007/s11128-023-03826-4}, abstractNote={Some of the most problematic issues that limit the implementation of applications on Noisy Intermediate Scale Quantum (NISQ) machines are the adverse impacts of both incoherent and coherent errors. We conducted an in-depth study of coherent errors on a quantum hardware platform using a transverse field Ising model Hamiltonian as a sample user application. We report here on the results from these computations using several error mitigation protocols that profile these errors and provide an indication of the hardware qubit stability. Through a detailed set of measurements we identify inter-day and intra-day qubit calibration drift and the impacts of quantum circuit placement on groups of qubits in different physical locations on the processor. This paper also discusses how these measurements can provide a better understanding of these types of errors and how they may improve efforts to validate the accuracy of quantum computations.}, number={2}, journal={QUANTUM INFORMATION PROCESSING}, author={Yeter-Aydeniz, Kubra and Parks, Zachary and Thekkiniyedath, Aadithya Nair and Gustafson, Erik and Kemper, Alexander F. and Pooser, Raphael C. and Meurice, Yannick and Dreher, Patrick}, year={2023}, month={Jan} }
@article{mejuto-zaera_kemper_2023, title={Quantum eigenvector continuation for chemistry applications}, volume={5}, ISSN={["2516-1075"]}, url={https://doi.org/10.1088/2516-1075/ad018f}, DOI={10.1088/2516-1075/ad018f}, abstractNote={Abstract}, number={4}, journal={ELECTRONIC STRUCTURE}, author={Mejuto-Zaera, Carlos and Kemper, Alexander F.}, year={2023}, month={Dec} }
@article{camps_kokcou_bassman_de jong_kemper_van beeumen_2022, title={AN ALGEBRAIC QUANTUM CIRCUIT COMPRESSION ALGORITHM FOR HAMILTONIAN SIMULATION}, volume={43}, ISSN={["1095-7162"]}, DOI={10.1137/21m1439298}, abstractNote={Quantum computing is a promising technology that harnesses the peculiarities of quantum mechanics to deliver computational speedups for some problems that are intractable to solve on a classical computer. Current generation noisy intermediate-scale quantum (NISQ) computers are severely limited in terms of chip size and error rates. Shallow quantum circuits with uncomplicated topologies are essential for successful applications in the NISQ era. Based on matrix analysis, we derive localized circuit transformations to efficiently compress quantum circuits for simulation of certain spin Hamiltonians known as free fermions. The depth of the compressed circuits is independent of simulation time and grows linearly with the number of spins. The proposed numerical circuit compression algorithm behaves backward stable and scales cubically in the number of spins enabling circuit synthesis beyond $\mathcal{O}(10^3)$ spins. The resulting quantum circuits have a simple nearest-neighbor topology, which makes them ideally suited for NISQ devices.}, number={3}, journal={SIAM JOURNAL ON MATRIX ANALYSIS AND APPLICATIONS}, author={Camps, Daan and Kokcou, Efekan and Bassman, Lindsay and De Jong, Wibe A. and Kemper, Alexander E. and Van Beeumen, Roel}, year={2022}, pages={1084–1108} }
@article{kokcu_camps_bassman_freericks_jong_van beeumen_kemper_2022, title={Algebraic compression of quantum circuits for Hamiltonian evolution}, volume={105}, ISSN={["2469-9934"]}, url={https://doi.org/10.1103/PhysRevA.105.032420}, DOI={10.1103/PhysRevA.105.032420}, abstractNote={Unitary evolution under a time dependent Hamiltonian is a key component of simulation on quantum hardware. Synthesizing the corresponding quantum circuit is typically done by breaking the evolution into small time steps, also known as Trotterization, which leads to circuits whose depth scales with the number of steps. When the circuit elements are limited to a subset of SU(4) -- or equivalently, when the Hamiltonian may be mapped onto free fermionic models -- several identities exist that combine and simplify the circuit. Based on this, we present an algorithm that compresses the Trotter steps into a single block of quantum gates. This results in a fixed depth time evolution for certain classes of Hamiltonians. We explicitly show how this algorithm works for several spin models, and demonstrate its use for adiabatic state preparation of the transverse field Ising model.}, number={3}, journal={PHYSICAL REVIEW A}, author={Kokcu, Efekan and Camps, Daan and Bassman, Lindsay and Freericks, J. K. and Jong, Wibe A. and Van Beeumen, Roel and Kemper, Alexander F.}, year={2022}, month={Mar} }
@article{data for "quantum computation of magnon spectra"_2022, DOI={10.17605/osf.io/9gx7u}, journal={Open Science Framework}, year={2022}, month={Aug} }
@article{francis_zelleke_zhang_kemper_freericks_2022, title={Determining Ground-State Phase Diagrams on Quantum Computers via a Generalized Application of Adiabatic State Preparation}, volume={14}, ISSN={["2073-8994"]}, url={https://www.mdpi.com/2073-8994/14/4/809}, DOI={10.3390/sym14040809}, abstractNote={Quantum phase transitions materialize as level crossings in the ground-state energy when the parameters of the Hamiltonian are varied. The resulting ground-state phase diagrams are straightforward to determine by exact diagonalization on classical computers, but are challenging on quantum computers because of the accuracy needed and the near degeneracy of the competing states close to the level crossings. On the other hand, classical computers are limited to small system sizes, which quantum computers may help overcome. In this work, we use a local adiabatic ramp for state preparation to allow us to directly compute ground-state phase diagrams on a quantum computer via time evolution. This methodology is illustrated by examining the ground states of the XY model with a magnetic field in the z-direction in one dimension. We are able to calculate an accurate phase diagram on both two- and three-site systems using IBM quantum machines.}, number={4}, journal={SYMMETRY-BASEL}, publisher={MDPI AG}, author={Francis, Akhil and Zelleke, Ephrata and Zhang, Ziyue and Kemper, Alexander F. and Freericks, James K.}, year={2022}, month={Apr} }
@article{wandel_boschini_neto_shen_na_zohar_wang_welch_seaberg_koralek_et al._2022, title={Enhanced charge density wave coherence in a light-quenched, high-temperature superconductor}, volume={376}, ISSN={["1095-9203"]}, DOI={10.1126/science.abd7213}, abstractNote={
Superconductivity and charge density waves (CDWs) are competitive, yet coexisting, orders in cuprate superconductors. To understand their microscopic interdependence, a probe capable of discerning their interaction on its natural length and time scale is necessary. We use ultrafast resonant soft x-ray scattering to track the transient evolution of CDW correlations in YBa
2
Cu
3
O
6+
x
after the quench of superconductivity by an infrared laser pulse. We observe a nonthermal response of the CDW order characterized by a near doubling of the correlation length within ≈1 picosecond of the superconducting quench. Our results are consistent with a model in which the interaction between superconductivity and CDWs manifests inhomogeneously through disruption of spatial coherence, with superconductivity playing the dominant role in stabilizing CDW topological defects, such as discommensurations.
}, number={6595}, journal={SCIENCE}, author={Wandel, S. and Boschini, F. and Neto, E. H. da Silva and Shen, L. and Na, M. X. and Zohar, S. and Wang, Y. and Welch, S. B. and Seaberg, M. H. and Koralek, J. D. and et al.}, year={2022}, month={May}, pages={860-+} }
@article{koekcue_steckmann_wang_freericks_dumitrescu_kemper_2022, title={Fixed Depth Hamiltonian Simulation via Cartan Decomposition}, volume={129}, ISSN={["1079-7114"]}, url={https://doi.org/10.1103/PhysRevLett.129.070501}, DOI={10.1103/PhysRevLett.129.070501}, abstractNote={Simulating quantum dynamics on classical computers is challenging for large systems due to the significant memory requirements. Simulation on quantum computers is a promising alternative, but fully optimizing quantum circuits to minimize limited quantum resources remains an open problem. We tackle this problem by presenting a constructive algorithm, based on Cartan decomposition of the Lie algebra generated by the Hamiltonian, which generates quantum circuits with time-independent depth. We highlight our algorithm for special classes of models, including Anderson localization in one-dimensional transverse field XY model, where O(n^{2})-gate circuits naturally emerge. Compared to product formulas with significantly larger gate counts, our algorithm drastically improves simulation precision. In addition to providing exact circuits for a broad set of spin and fermionic models, our algorithm provides broad analytic and numerical insight into optimal Hamiltonian simulations.}, number={7}, journal={PHYSICAL REVIEW LETTERS}, author={Koekcue, Efekan and Steckmann, Thomas and Wang, Yan and Freericks, J. K. and Dumitrescu, Eugene F. and Kemper, Alexander F.}, year={2022}, month={Aug} }
@article{na_boschini_mills_michiardi_day_zwartsenberg_levy_zhdanovich_kemper_jones_et al._2022, title={Mapping non-thermal regimes in pump-probe electron relaxation dynamics}, DOI={10.1364/up.2022.tu2a.5}, abstractNote={Relaxation of photoexcited carriers in graphite is studied by TR-ARPES. Boltzmann rate-equations are used to model the observed non-thermal electron distributions, and several non-thermal phases in the fluence-delay phase space are identified with broad implications.}, journal={The International Conference on Ultrafast Phenomena (UP) 2022}, publisher={Optica Publishing Group}, author={Na, Meng Xing and Boschini, Fabio and Mills, Arthur K. and Michiardi, Matteo and Day, Ryan P. and Zwartsenberg, Berend and Levy, Giorgio and Zhdanovich, Sergey and Kemper, Alexander F. and Jones, David J. and et al.}, year={2022} }
@article{metcalf_stone_klymko_kemper_sarovar_jong_2022, title={Quantum Markov chain Monte Carlo with digital dissipative dynamics on quantum computers}, volume={7}, ISSN={["2058-9565"]}, url={https://doi.org/10.1088/2058-9565/ac546a}, DOI={10.1088/2058-9565/ac546a}, abstractNote={Abstract}, number={2}, journal={QUANTUM SCIENCE AND TECHNOLOGY}, publisher={IOP Publishing}, author={Metcalf, Mekena and Stone, Emma and Klymko, Katherine and Kemper, Alexander F. and Sarovar, Mohan and Jong, Wibe A.}, year={2022}, month={Apr} }
@article{uzsoy_zareiesfandabadi_jennings_kemper_elting_2021, title={Automated tracking of S. pombe spindle elongation dynamics}, volume={6}, ISSN={["1365-2818"]}, url={https://doi.org/10.1111/jmi.13044}, DOI={10.1111/jmi.13044}, abstractNote={Abstract}, journal={JOURNAL OF MICROSCOPY}, publisher={Wiley}, author={Uzsoy, Ana Sofia M. and Zareiesfandabadi, Parsa and Jennings, Jamie and Kemper, Alexander F. and Elting, Mary Williard}, year={2021}, month={Jul} }
@article{bridging the gap between the transient and the steady state of a nonequilibrium quantum system_2021, year={2021}, month={Jan} }
@article{cong_vetter_yan_li_zhang_xiong_qu_schaller_hoffmann_kemper_et al._2021, title={Coherent control of asymmetric spintronic terahertz emission from two-dimensional hybrid metal halides}, volume={12}, ISSN={["2041-1723"]}, url={https://doi.org/10.1038/s41467-021-26011-6}, DOI={10.1038/s41467-021-26011-6}, abstractNote={Abstract}, number={1}, journal={NATURE COMMUNICATIONS}, author={Cong, Kankan and Vetter, Eric and Yan, Liang and Li, Yi and Zhang, Qi and Xiong, Yuzan and Qu, Hongwei and Schaller, Richard D. and Hoffmann, Axel and Kemper, Alexander F. and et al.}, year={2021}, month={Sep} }
@article{na_boschini_mills_michiardi_day_zwartsenberg_levy_zhdanovich_kemper_jones_et al._2021, title={Evolution of nonthermal electrons in pump-probe electron relaxation dynamics}, DOI={10.1364/cleo_qels.2021.fw4k.1}, abstractNote={We study the relaxation of photoexcited electrons in graphite using TR-ARPES. Observed nonthermal electron distributions are modelled using Boltzmann rate-equations, leading to the identification of nonthermal phases in the fluence-delay phase space with broad implications.}, journal={Conference on Lasers and Electro-Optics}, publisher={Optica Publishing Group}, author={Na, Meng Xing and Boschini, Fabio and Mills, Arthur K. and Michiardi, Matteo and Day, Ryan P. and Zwartsenberg, Berend and Levy, Giorgio and Zhdanovich, Sergey and Kemper, Alexander F. and Jones, David J. and et al.}, year={2021} }
@article{wilmington_ardekani_rustagi_bataller_kemper_younts_gundogdu_2021, title={Fermi liquid theory sheds light on hot electron-hole liquid in 1L-MoS2}, volume={103}, ISSN={["2469-9969"]}, url={https://doi.org/10.1103/PhysRevB.103.075416}, DOI={10.1103/PhysRevB.103.075416}, abstractNote={Room-temperature electron-hole liquid has recently been experimentally identified in low-dimensional transition metal dichalcogenides. Here, the authors demonstrate that a first-principles Fermi liquid model effectively predicts the photoluminescence response of this phenomenon. Using density functional theory, in conjunction with previous Raman and photoluminescence spectroscopy results, they present a consistent quantitative picture of the electron-hole liquid phase transition in suspended, heat-strained 1$L$-MoS${}_{2}$ monolayers. They show a 23-fold increase in photoluminescence per unit of direct gap carrier density and 9:1 indirect-direct hole population ratio at high strain.}, number={7}, journal={PHYSICAL REVIEW B}, author={Wilmington, R. L. and Ardekani, H. and Rustagi, A. and Bataller, A. and Kemper, A. F. and Younts, R. A. and Gundogdu, K.}, year={2021}, month={Feb} }
@article{fixed depth hamiltonian simulation via cartan decomposition_2021, year={2021}, month={Apr} }
@article{huang_wang_pang_wu_cao_mo_rustagi_kemper_wang_yi_et al._2021, title={Flat-band-induced itinerant ferromagnetism in RbCo2Se2}, volume={103}, ISSN={["2469-9969"]}, url={https://doi.org/10.1103/PhysRevB.103.165105}, DOI={10.1103/PhysRevB.103.165105}, abstractNote={$A{\mathrm{Co}}_{2}{\mathrm{Se}}_{2}$ ($A$=K, Rb, Cs) is a homologue of the iron-based superconductor $A{\mathrm{Fe}}_{2}{\mathrm{Se}}_{2}$. From a comprehensive study of ${\mathrm{RbCo}}_{2}{\mathrm{Se}}_{2}$ via measurements of magnetization, transport, neutron diffraction, angle-resolved photoemission spectroscopy, and first-principles calculations, we identify a ferromagnetic order accompanied by an orbital-dependent spin splitting of the electronic dispersions. Furthermore, we identify the ordered moment to be dominated by a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ flat band near the Fermi level, which exhibits the largest spin splitting across the ferromagnetic transition, suggesting an itinerant origin of the ferromagnetism. In the broader context of the iron-based superconductors, we find this ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ flat band to be a common feature in the band structures of both iron chalcogenides and iron pnictides, accessible via heavy electron doping.}, number={16}, journal={PHYSICAL REVIEW B}, author={Huang, Jianwei and Wang, Zhicai and Pang, Hongsheng and Wu, Han and Cao, Huibo and Mo, Sung-Kwan and Rustagi, Avinash and Kemper, A. F. and Wang, Meng and Yi, Ming and et al.}, year={2021}, month={Apr} }
@article{flatband-induced itinerant ferromagnetism in rbco$_2$se$_2$_2021, year={2021}, month={Mar} }
@article{francis_zhu_alderete_johri_xiao_freericks_monroe_linke_kemper_2021, title={Many-body thermodynamics on quantum computers via partition function zeros}, volume={7}, ISSN={["2375-2548"]}, url={https://doi.org/10.1126/sciadv.abf2447}, DOI={10.1126/sciadv.abf2447}, abstractNote={Quantum computers can study thermodynamics by finding zeros of functions in the complex plane.}, number={34}, journal={SCIENCE ADVANCES}, publisher={American Association for the Advancement of Science (AAAS)}, author={Francis, Akhil and Zhu, Daiwei and Alderete, Cinthia Huerta and Johri, Sonika and Xiao, Xiao and Freericks, James K. and Monroe, Christopher and Linke, Norbert M. and Kemper, Alexander F.}, year={2021}, month={Aug} }
@article{zhou_williams_sun_malliakas_kanatzidis_kemper_ruan_2021, title={Nonequilibrium dynamics of spontaneous symmetry breaking into a hidden state of charge-density wave}, volume={12}, ISSN={["2041-1723"]}, url={https://doi.org/10.1038/s41467-020-20834-5}, DOI={10.1038/s41467-020-20834-5}, abstractNote={Abstract}, number={1}, journal={NATURE COMMUNICATIONS}, author={Zhou, Faran and Williams, Joseph and Sun, Shuaishuai and Malliakas, Christos D. and Kanatzidis, Mercouri G. and Kemper, Alexander F. and Ruan, Chong-Yu}, year={2021}, month={Jan} }
@article{paris_nicholson_johnston_tseng_rumo_coslovich_zohar_lin_strocov_saint-martin_et al._2021, title={Probing the interplay between lattice dynamics and short-range magnetic correlations in CuGeO3 with femtosecond RIXS}, volume={6}, ISSN={["2397-4648"]}, DOI={10.1038/s41535-021-00350-5}, abstractNote={Abstract}, number={1}, journal={NPJ QUANTUM MATERIALS}, publisher={Springer Science and Business Media LLC}, author={Paris, E. and Nicholson, C. W. and Johnston, S. and Tseng, Y. and Rumo, M. and Coslovich, G. and Zohar, S. and Lin, M. F. and Strocov, V. N. and Saint-Martin, R. and et al.}, year={2021}, month={May} }
@article{huang_singh_mou_johnston_kemper_brink_chen_lee_okamoto_chu_et al._2021, title={Quantum Fluctuations of Charge Order Induce Phonon Softening in a Superconducting Cuprate}, volume={11}, ISSN={["2160-3308"]}, url={https://doi.org/10.1103/PhysRevX.11.041038}, DOI={10.1103/PhysRevX.11.041038}, abstractNote={H. Y. Huang, A. Singh, C. Y. Mou, S. Johnston, A. F. Kemper, J. van den Brink, P. J. Chen, 7 T. K. Lee, 9 J. Okamoto, Y. Y. Chu, J. H. Li, 1 S. Komiya, A. C. Komarek, A. Fujimori, 1 C. T. Chen, and D. J. Huang 10, ∗ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan Center for Quantum Technology and Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Physics and Astronomy, The University of Tennessee, Knoxville, TN 37996, USA Department of Physics, North Carolina State University, Raleigh, NC 27695, USA Institute for Theoretical Solid State Physics, IFW Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany Department of Mechanical Engineering, City University of Hong Kong, Kowloon, Hong Kong Hong Kong Institute for Advanced Study, City University of Hong Kong, Kowloon, Hong Kong Institute of Physics, Academia Sinica, Taipei 11529, Taiwan Department of Physics, National Sun Yat-sen University, Kaohsiung, 80424, Taiwan Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan Central Research Institute of Electric Power Industry, Yokosuka, Kanagawa, 240-0196, Japan Max Planck Institute for Chemical Physics of Solids, Nöthnitzerstrasse 40, 01187 Dresden, Germany Department of Applied Physics, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan. (Dated: September 28, 2021)}, number={4}, journal={PHYSICAL REVIEW X}, author={Huang, H. Y. and Singh, A. and Mou, C. Y. and Johnston, S. and Kemper, A. F. and Brink, J. and Chen, P. J. and Lee, T. K. and Okamoto, J. and Chu, Y. Y. and et al.}, year={2021}, month={Nov} }
@article{pellatz_roy_lee_schad_kandel_arndt_eom_kemper_reznik_2021, title={Relaxation timescales and electron-phonon coupling in optically pumped YBa2Cu3O6+x revealed by time-resolved Raman scattering}, volume={104}, ISSN={["2469-9969"]}, DOI={10.1103/PhysRevB.104.L180505}, abstractNote={Quantum matter with striking properties has recently been generated by driving materials with ultrafast laser pulses. However, observing these phases by following a single property as a function of time after photoexcitation provides a limited picture. We directly tracked hot phonons and transient heating in a high temperature superconductor YBa$_2$Cu$_3$O$_{6.9}$ by time-resolved Raman scattering with time-resolution of 220 fs. In addition, we used the hardening of the Raman-active apical oxygen phonon right after photoexcitation as an indirect probe of hot electrons, which equilibrated with hot phonons around 0.5 ps. Analysis based on the two-temperature model revealed temperature-independent electron-phonon coupling with $\lambda \approx $ 0.8 as well as significant anharmonicity that increased with temperature. It is essential to reduce this anharmonicity to generate longer lived nonequilibrium states in copper oxides. We also discuss new insights into photoinduced superconductivity recently reported at lower doping that follow from these results.}, number={18}, journal={PHYSICAL REVIEW B}, author={Pellatz, N. and Roy, S. and Lee, J-W and Schad, J. L. and Kandel, H. and Arndt, N. and Eom, C. B. and Kemper, A. F. and Reznik, D.}, year={2021}, month={Nov} }
@article{robust measurement of wave function topology on nisq quantum computers_2021, year={2021}, month={Jan} }
@misc{bassman_urbanek_metcalf_carter_kemper_jong_2021, title={Simulating quantum materials with digital quantum computers}, volume={6}, ISSN={["2058-9565"]}, url={https://doi.org/10.1088/2058-9565/ac1ca6}, DOI={10.1088/2058-9565/ac1ca6}, abstractNote={Abstract}, number={4}, journal={QUANTUM SCIENCE AND TECHNOLOGY}, publisher={IOP Publishing}, author={Bassman, Lindsay and Urbanek, Miroslav and Metcalf, Mekena and Carter, Jonathan and Kemper, Alexander F. and Jong, Wibe A.}, year={2021}, month={Oct} }
@article{nevola_bataller_kumar_sridhar_frick_o'donnell_ade_maggard_kemper_gundogdu_et al._2021, title={Timescales of excited state relaxation in alpha-RuCl3 observed by time-resolved two-photon photoemission spectroscopy}, volume={103}, ISSN={["2469-9969"]}, url={https://doi.org/10.1103/PhysRevB.103.245105}, DOI={10.1103/PhysRevB.103.245105}, abstractNote={The nonequilibrium properties of strongly correlated materials present a target in the search for new phases of matter. It is important to observe the types of excitations that exist in these materials and their associated relaxation dynamics. We have studied the photoexcitations in a spin-orbit assisted Mott insulator $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ using time-resolved two-photon photoemission spectroscopy and transient reflection spectroscopy. We find that photoexcited carriers (doublons) in the upper Hubbard band rapidly relax to Mott-Hubbard excitons on a timescale of less than 200 fs. Subsequently, further relaxation of these lower-energy quasiparticles occurs with an energy-dependent time constant of that ranges from 370 to 600 fs due to exciton cooling. The population of Mott-Hubbard excitons persists for timescales up to several microseconds.}, number={24}, journal={PHYSICAL REVIEW B}, author={Nevola, Dan and Bataller, Alexander and Kumar, Ankit and Sridhar, Samanvitha and Frick, Jordan and O'Donnell, Shaun and Ade, Harald and Maggard, Paul A. and Kemper, Alexander F. and Gundogdu, Kenan and et al.}, year={2021}, month={Jun} }
@article{freericks_kemper_2021, title={What do the two times in two-time correlation functions mean for interpreting tr-ARPES?}, volume={251}, ISSN={["1873-2526"]}, url={https://doi.org/10.1016/j.elspec.2021.147104}, DOI={10.1016/j.elspec.2021.147104}, abstractNote={Time-resolved angle-resolved photoemission spectroscopy is one of the most powerful pump-probe measurements of materials driven far from equilibrium. Unlike the linear-response regime, where the frequency-dependent response function is independent of time, in a far-from-equilibrium experiment, the response function depends on two times in the time domain. In this work, we describe how one can use time-dependent frequency response functions and how they involve contributions from times that are near to each other. This implies that they should not be thought of as a frequency-dependent response at a single definite time. Instead, the Fourier uncertainty relations show that they involve contributions from ranges of times and must be interpreted in this light. We use this insight to help understand what time-resolved photoemission measurements actually measure.}, journal={JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA}, publisher={Elsevier BV}, author={Freericks, J. K. and Kemper, Alexander F.}, year={2021}, month={Aug} }
@article{uzsoy ana sofı́a m._zareiesfandabadi_jennings_kemper_elting_2020, title={Automated tracking of S. pombe spindle elongation dynamics}, volume={10}, url={http://dx.doi.org/10.1101/2020.10.09.333765}, DOI={10.1101/2020.10.09.333765}, abstractNote={The mitotic spindle is a microtubule-based machine that pulls the two identical sets of chromosomes to opposite ends of the cell during cell division. The fission yeastSchizosaccharomyces pombeis an important model organism for studying mitosis due to its simple, stereotyped spindle structure and well-established genetic toolset.S. pombespindle length is a useful metric for mitotic progression, but manually tracking spindle ends in each frame to measure spindle length over time is laborious and can limit experimental throughput. We have developed an ImageJ plugin that can automatically trackS. pombespindle length over time and replace manual or semi-automated tracking of spindle elongation dynamics. Using an algorithm that detects the principal axis of the spindle and then finds its ends, we reliably track the length and angle of the spindle as the cell divides. The plugin integrates with existing ImageJ features, exports its data for further analysis outside of ImageJ, and does not require any programming by the user. Thus, the plugin provides an accessible tool for quantification ofS. pombespindle length that will allow automatic analysis of large microscopy data sets and facilitate screening for effects of cell biological perturbations on mitotic progression.}, journal={[]}, publisher={Cold Spring Harbor Laboratory}, author={Uzsoy Ana Sofı́a M. and Zareiesfandabadi, Parsa and Jennings, Jamie and Kemper, Alexander F. and Elting, Mary Williard}, year={2020}, month={Oct} }
@article{catalogue of phonon modes in several cuprate high-temperature superconductors from density functional theory_2020, year={2020}, month={Oct} }
@article{na_mills_boschini_michiardi_nosarzewski_day_razzoli_sheyerman_schneider_levy_et al._2020, title={Determination of mode-projected electron-phonon coupling from time-domain observations of microscopic scattering processes}, DOI={10.1364/up.2020.m3b.2}, abstractNote={Using a femtosecond extreme ultraviolet source with a narrow linewidth, we observe electrons in graphite scattering with the
A
1
'
phonon from optically-populated initial states to well-defined final states, and extract the electron-phonon matrix element.}, journal={The 22nd International Conference on Ultrafast Phenomena 2020}, publisher={Optica Publishing Group}, author={Na, M. and Mills, A. K. and Boschini, F. and Michiardi, M. and Nosarzewski, B. and Day, R. P. and Razzoli, E. and Sheyerman, A. and Schneider, M. and Levy, G. and et al.}, year={2020} }
@article{re_rost_kemper_freericks_2020, title={Driven-dissipative quantum mechanics on a lattice: Simulating a fermionic reservoir on a quantum computer}, volume={102}, url={https://doi.org/10.1103/PhysRevB.102.125112}, DOI={10.1103/PhysRevB.102.125112}, abstractNote={The driven-dissipative many-body problem remains one of the most challenging unsolved problems in quantum mechanics. The advent of quantum computers may provide a unique platform for efficiently simulating such driven-dissipative systems. But, there are many choices for how one can engineer the reservoir. One can simply employ ancilla qubits to act as a reservoir and then digitally simulate them via algorithmic cooling. A more attractive approach, which allows one to simulate an infinite reservoir, is to integrate out the bath degrees of freedom and describe the driven-dissipative system via a master equation, that can also be simulated on a quantum computer. In this work, we consider the particular case of noninteracting electrons on a lattice driven by an electric field and coupled to a fermionic thermostat. Then, we provide two different quantum circuits: the first one reconstructs the full dynamics of the system using Trotter steps, while the second one dissipatively prepares the final nonequilibrium steady state in a single step. We run both circuits on the IBM quantum experience. For circuit (i), we achieved up to five Trotter steps. When partial resets become available on quantum computers, we expect that the maximum simulation time can be significantly increased. The methods developed here suggest generalizations that can be applied to simulating interacting driven-dissipative systems.}, number={12}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Re, Lorenzo Del and Rost, Brian and Kemper, A. F. and Freericks, J. K.}, year={2020}, month={Sep} }
@article{establishing non-thermal regimes in pump-probe electron-relaxation dynamics_2020, year={2020}, month={Sep} }
@article{na_boschini_mills_michiardi_day_zwartsenberg_levy_zhdanovich_kemper_jones_et al._2020, title={Establishing nonthermal regimes in pump-probe electron relaxation dynamics}, volume={102}, url={https://doi.org/10.1103/PhysRevB.102.184307}, DOI={10.1103/PhysRevB.102.184307}, abstractNote={Time- and angle-resolved photoemission spectroscopy (TR-ARPES) accesses the electronic structure of solids under optical excitation, and is a powerful technique for studying the coupling between electrons and collective modes. One approach to infer electron-boson coupling is through the relaxation dynamics of optically-excited electrons, and the characteristic timescales of energy redistribution. A common description of electron relaxation dynamics is through the effective electronic temperature. Such a description requires that thermodynamic quantities are well-defined, an assumption that is generally violated at early delays. Additionally, precise estimation of the non-thermal window -- within which effective temperature models may not be applied -- is challenging. We perform TR-ARPES on graphite and show that Boltzmann rate equations can be used to calculate the time-dependent electronic occupation function, and reproduce experimental features given by non-thermal electron occupation. Using this model, we define a quantitative measure of non-thermal electron occupation and use it to define distinct phases of electron relaxation in the fluence-delay phase space. More generally, this approach can be used to inform the non-thermal-to-thermal crossover in pump-probe experiments.}, number={18}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Na, M. X. and Boschini, F. and Mills, A. K. and Michiardi, M. and Day, R. P. and Zwartsenberg, B. and Levy, G. and Zhdanovich, S. and Kemper, A. F. and Jones, D. J. and et al.}, year={2020}, month={Nov} }
@article{fermi liquid theory sheds light on "hot" ehl in 1l-mos$_2$_2020, year={2020}, month={Oct} }
@article{light-enhanced charge density wave coherence in a high-temperature superconductor_2020, year={2020}, month={Mar} }
@article{many body thermodynamics on quantum computers via partition function zeros_2020, year={2020}, month={Sep} }
@article{observing coherences with time-resolved photoemission_2020, year={2020}, month={May} }
@article{francis_freericks_kemper_2020, title={Quantum computation of magnon spectra}, volume={101}, ISSN={["2469-9969"]}, url={https://doi.org/10.1103/PhysRevB.101.014411}, DOI={10.1103/PhysRevB.101.014411}, abstractNote={We demonstrate quantum computation of two-point correlation functions for a Heisenberg spin chain. Using the IBM Q 20-qubit quantum machines, we find that, for two sites, the correlation functions produce the exact results reliably. For four sites, results from the IBM Q 20-qubit Tokyo quantum computer are noisy due to read out errors and decoherence. Nevertheless, the correlation functions retain the correct spectral information. This is illustrated in the frequency domain by accurately extracting the magnon energies from peaks in the spectral function.}, number={1}, journal={PHYSICAL REVIEW B}, publisher={American Physical Society (APS)}, author={Francis, Akhil and Freericks, J. K. and Kemper, A. F.}, year={2020}, month={Jan} }
@article{topological quantum computing on a conventional quantum computer_2020, year={2020}, month={Jun} }
@article{band-resolved imaging of photocurrent in a topological insulator_2019, url={http://dx.doi.org/10.1103/physrevlett.122.167401}, DOI={10.1103/physrevlett.122.167401}, abstractNote={We study the microscopic origins of photocurrent generation in the topological insulator Bi_{2}Se_{3} via time- and angle-resolved photoemission spectroscopy. We image the unoccupied band structure as it evolves following a circularly polarized optical excitation and observe an asymmetric electron population in momentum space, which is the spectroscopic signature of a photocurrent. By analyzing the rise times of the population we identify which occupied and unoccupied electronic states are coupled by the optical excitation. We conclude that photocurrents can only be excited via resonant optical transitions coupling to spin-orbital textured states. Our work provides a microscopic understanding of how to control photocurrents in systems with spin-orbit coupling and broken inversion symmetry.}, year={2019}, month={Apr} }
@article{rustagi_kemper_2019, title={Coherent Excitonic Quantum Beats in Time-Resolved Photoemission Measurements}, volume={99}, ISSN={["2469-9969"]}, url={arXiv:1901.07580}, DOI={10.1103/PhysRevB.99.125303}, abstractNote={Coherent excitation of materials via ultrafast laser pulses can have interesting, observable dynamics in time-resolved photoemission measurements. The broad spectral width of ultrafast pump pulses can coherently excite multiple exciton energy levels. When such coherently excited states are probed by means of photoemission spectroscopy, interference between the polarization of different exciton levels can lead to observable coherent exciton beats. Here, we present the theoretical formalism for evaluating the Time- and Angle- Resolved Photoemission Spectra (tr-ARPES) arising from the coherently excited exciton states. We subsequently apply our formalism to a simple model example of hydrogenic exciton energy levels to identify the dependencies that control the quantum beats. Our findings indicate that the most pronounced effect of coherent quantum excitonic beats is seen midway between the excited exciton energy levels and the central energy of the pump pulse provides tunability of this effect.}, number={12}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Rustagi, A. and Kemper, A.F.}, year={2019} }
@article{bataller_younts_rustagi_yu_ardekani_kemper_cao_gundogdu_2019, title={Correction to Dense Electron–Hole Plasma Formation and Ultralong Charge Lifetime in Monolayer MoS2 via Material Tuning}, volume={19}, url={https://doi.org/10.1021/acs.nanolett.9b02578}, DOI={10.1021/acs.nanolett.9b02578}, abstractNote={ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to Dense Electron–Hole Plasma Formation and Ultralong Charge Lifetime in Monolayer MoS2 via Material TuningAlexander W. BatallerAlexander W. BatallerMore by Alexander W. Bataller, Robert A. YountsRobert A. YountsMore by Robert A. Younts, Avinash RustagiAvinash RustagiMore by Avinash Rustagihttp://orcid.org/0000-0001-6776-9496, Yiling YuYiling YuMore by Yiling Yu, Hossein ArdekaniHossein ArdekaniMore by Hossein Ardekani, Alexander KemperAlexander KemperMore by Alexander Kemperhttp://orcid.org/0000-0002-5426-5181, Linyou CaoLinyou CaoMore by Linyou Caohttp://orcid.org/0000-0002-7834-8336, and Kenan Gundogdu*Kenan GundogduMore by Kenan Gundogduhttp://orcid.org/0000-0001-7149-5766Cite this: Nano Lett. 2019, 19, 7, 4816Publication Date (Web):June 27, 2019Publication History Published online27 June 2019Published inissue 10 July 2019https://pubs.acs.org/doi/10.1021/acs.nanolett.9b02578https://doi.org/10.1021/acs.nanolett.9b02578correctionACS PublicationsCopyright © 2019 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views1338Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (233 KB) Get e-Alertsclose Get e-Alerts}, number={7}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Bataller, Alexander W. and Younts, Robert A. and Rustagi, Avinash and Yu, Yiling and Ardekani, Hossein and Kemper, Alexander and Cao, Linyou and Gundogdu, Kenan}, year={2019}, month={Jul}, pages={4816–4816} }
@article{bataller_younts_rustagi_yu_ardekani_kemper_cao_gundogdu_2019, title={Dense Electron–Hole Plasma Formation and Ultralong Charge Lifetime in Monolayer MoS2 via Material Tuning}, volume={19}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/acs.nanolett.8b04408}, DOI={10.1021/acs.nanolett.8b04408}, abstractNote={Many-body interactions in photoexcited semiconductors can bring about strongly interacting electronic states, culminating in the fully ionized matter of electron-hole plasma (EHP) and electron-hole liquid (EHL). These exotic phases exhibit unique electronic properties, such as metallic conductivity and metastable high photoexcitation density, which can be the basis for future transformative applications. However, the cryogenic condition required for its formation has limited the study of dense plasma phases to a purely academic pursuit in a restricted parameter space. This paradigm can potentially change with the recent experimental observation of these phases in atomically thin MoS2 and MoTe2 at room temperature. A fundamental understanding of EHP and EHL dynamics is critical for developing novel applications on this versatile layered platform. In this work, we studied the formation and dissipation of EHP in monolayer MoS2. Unlike previous results in bulk semiconductors, our results reveal that electromechanical material changes in monolayer MoS2 during photoexcitation play a significant role in dense EHP formation. Within the free-standing geometry, photoexcitation is accompanied by an unconstrained thermal expansion, resulting in a direct-to-indirect gap electronic transition at a critical lattice spacing and fluence. This dramatic altering of the material's energetic landscape extends carrier lifetimes by 2 orders of magnitude and allows the density required for EHP formation. The result is a stable dense plasma state that is sustained with modest optical photoexcitation. Our findings pave the way for novel applications based on dense plasma states in two-dimensional semiconductors.}, number={2}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Bataller, Alexander W. and Younts, Robert A. and Rustagi, Avinash and Yu, Yiling and Ardekani, Hossein and Kemper, Alexander and Cao, Linyou and Gundogdu, Kenan}, year={2019}, month={Jan}, pages={1104–1111} }
@article{pfau_rotundu_palmstrom_chen_hashimoto_lu_kemper_fisher_shen_2019, title={Detailed band structure of twinned and detwinned
BaFe2As2
studied with angle-resolved photoemission spectroscopy}, volume={99}, ISSN={2469-9950 2469-9969}, url={http://dx.doi.org/10.1103/physrevb.99.035118}, DOI={10.1103/physrevb.99.035118}, abstractNote={The superconducting dome of most iron-based superconductors emerges from a phase with nematic and magnetic ordering. However, the multiband and multiorbital nature of iron-based superconductors as well as twinning effects pose challenges to fully characterize the implications of these two ordering phenomena for the band structure. Here, the authors report high-quality ARPES measurements on twinned and detwinned BaFe${}_{2}$As${}_{2}$ and present a comprehensive and systematic overview of the band structure in the normal and the magnetically ordered states of this prototype parent compound.}, number={3}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Pfau, H. and Rotundu, C. R. and Palmstrom, J. C. and Chen, S. D. and Hashimoto, M. and Lu, D. and Kemper, A. F. and Fisher, I. R. and Shen, Z.-X.}, year={2019}, month={Jan} }
@article{na_mills_boschini_michiardi_nosarzewski_day_razzoli_sheyerman_schneider_levy_et al._2019, title={Direct determination of mode-projected electron-phonon coupling in the time domain}, volume={366}, ISSN={["1095-9203"]}, DOI={10.1126/science.aaw1662}, abstractNote={A timely look into electron-phonon coupling}, number={6470}, journal={SCIENCE}, publisher={American Association for the Advancement of Science (AAAS)}, author={Na, M. X. and Mills, A. K. and Boschini, F. and Michiardi, M. and Nosarzewski, B. and Day, R. P. and Razzoli, E. and Sheyerman, A. and Schneider, M. and Levy, G. and et al.}, year={2019}, month={Dec}, pages={1231-+} }
@article{kumar_kemper_2019, title={Higgs oscillations in time-resolved optical conductivity}, volume={100}, ISSN={["2469-9969"]}, url={https://doi.org/10.1103/PhysRevB.100.174515}, DOI={10.1103/PhysRevB.100.174515}, abstractNote={Driving superconductors out of equilibrium is a promising avenue to study their equilibrium properties as well as to control the superconducting state. Non-equilibrium superconductors are often studied using time resolved optical conductivity measurements. Thus, the characterization of a superconducting state in a pump driven non-equilibrium state requires careful attention in the time domain. We calculate time-resolved optical conductivity of a pumped superconducting state using a non-equilibrium Keldysh approach. Through functional derivation, the optical conductivity is obtained with full vertex corrections and used to characterize the transient superconducting state. The transient optical conductivity shows the suppression of the superconducting order parameter in the time domain. The subsequent recovery of the order parameter exhibits oscillatory behavior that corresponds to the Higgs amplitude mode, and may be seen in several parts of the spectrum.}, number={17}, journal={PHYSICAL REVIEW B}, publisher={American Physical Society (APS)}, author={Kumar, A. and Kemper, A. F.}, year={2019}, month={Nov} }
@article{kumar_johnston_kemper_2019, title={Identifying a forward-scattering superconductor through pump-probe spectroscopy}, volume={124}, DOI={10.1209/0295-5075/124/67002}, abstractNote={Electron-boson scattering that is peaked in the forward direction has been suggested as an essential ingredient for enhanced superconductivity observed in FeSe monolayers. Here, we study the superconducting state of a system dominated by forward scattering in the time domain and contrast its behavior against the standard isotropic BCS case for both s- and d-wave symmetries. An analysis of the electron's dynamics in the pump-driven non-equilibrium state reveals that the superconducting order in the forward-focused case is robust and persistent against the pump-induced perturbations. The superconducting order parameter also exhibits a non-uniform melting in momentum space. We show that this behavior is in sharp contrast to the isotropic interaction case and propose that time-resolved approaches are a potentially powerful tool to differentiate the nature of the dominant coupling in correlated materials.}, number={6}, journal={EPL (Europhysics Letters)}, publisher={IOP Publishing}, author={Kumar, A. and Johnston, S. and Kemper, A. F.}, year={2019}, month={Jan}, pages={67002} }
@article{kung_goyal_maslov_wang_lee_kemper_cheong_blumberg_2019, title={Observation of chiral surface excitons in a topological insulator Bi2Se3}, volume={116}, ISSN={["0027-8424"]}, url={https://doi.org/10.1073/pnas.1813514116}, DOI={10.1073/pnas.1813514116}, abstractNote={Significance}, number={10}, journal={PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA}, publisher={Proceedings of the National Academy of Sciences}, author={Kung, H-H. and Goyal, A. P. and Maslov, D. L. and Wang, X. and Lee, A. and Kemper, A. F. and Cheong, S-W. and Blumberg, G.}, year={2019}, month={Mar}, pages={4006–4011} }
@article{revelle_kumar_kemper_2019, title={Theory of Time-Resolved Optical Conductivity of Superconductors: Comparing Two Methods for Its Evaluation}, volume={4}, ISSN={["2410-3896"]}, url={https://doi.org/10.3390/condmat4030079}, DOI={10.3390/condmat4030079}, abstractNote={Time-resolved optical conductivity is an often used tool to interrogate quantum materials driven out of equilibrium. Theoretically calculating this observable is a complex topic with several approaches discussed in the literature. Using a nonequilibrium Keldysh formalism and a functional derivative approach to the conductivity, we present a comparison of two particular approaches to the calculation of the optical conductivity and their distinguishing features, as applied to a pumped superconductor. The two methods are distinguished by the relative motion of the probe and gate times; either the probe or gate time is kept fixed while the other is swept. We find that both the methods result in same qualitative features of the time-resolved conductivity after pump is over. However, calculating the conductivity by keeping the gate fixed removes artifacts inherent to the other method. We provide software that, based on data for the first method, is able to construct the second approach.}, number={3}, journal={CONDENSED MATTER}, author={Revelle, John P. and Kumar, Ankit and Kemper, Alexander F.}, year={2019}, month={Sep} }
@article{coslovich_huber_behl_kemper_sasagawa_bechtel_martin_kaindl_2019, title={Ultrafast multi-terahertz probes of symmetry breaking in a stripe-phase correlated oxide}, volume={10756}, ISSN={["1996-756X"]}, DOI={10.1117/12.2322682}, abstractNote={The application of transient terahertz (THz) pulses to excite and probe low-energy quantum and collective excitations in materials represents a powerful tool to study both intrinsic interactions and non-equilibrium phases. In the following, we discuss ultrafast multi-THz studies that resolve the dynamics of electronic itineracy and vibrational symmetries in a strongly-correlated nickelate. Many transition-metal oxides exhibit the emergence of “stripes,” corresponding to quasione- dimensional charge, spin and lattice modulations as a manifestation of strong correlations. In our experiments, optical excitation of a stripe-phase nickel oxide triggers the rapid melting of its atomic-scale charge order and results in dynamics that yields insight into the couplings underlying the stripes. The transient optical conductivity is sensitive to both charges and in-plane vibrations and reveals a succession of ultrafast processes, ranging from rapid delocalization and localization of charges, via a time-delayed reaction of vibrational distortions to the electronic quench, up to the multi-picosecond re-establishment of the symmetry-broken phase.}, journal={TERAHERTZ EMITTERS, RECEIVERS, AND APPLICATIONS IX}, author={Coslovich, G. and Huber, B. and Behl, S. and Kemper, A. F. and Sasagawa, T. and Bechtel, H. A. and Martin, M. C. and Kaindl, R. A.}, year={2019} }
@article{topp_tancogne-dejean_kemper_rubio_sentef_2018, title={All-optical nonequilibrium pathway to stabilising magnetic Weyl semimetals in pyrochlore iridates}, volume={9}, ISSN={["2041-1723"]}, url={https://doi.org/10.1038/s41467-018-06991-8}, DOI={10.1038/s41467-018-06991-8}, abstractNote={Abstract}, number={1}, journal={NATURE COMMUNICATIONS}, publisher={Springer Nature}, author={Topp, Gabriel E. and Tancogne-Dejean, Nicolas and Kemper, Alexander F. and Rubio, Angel and Sentef, Michael A.}, year={2018}, month={Oct} }
@article{cilento_manzoni_sterzi_peli_ronchi_crepaldi_boschini_cacho_chapman_springate_et al._2018, title={Dynamics of correlation-frozen antinodal quasiparticles in superconducting cuprates}, volume={4}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000426845500062&KeyUID=WOS:000426845500062}, DOI={10.1126/sciadv.aar1998}, abstractNote={A novel ultrafast photoemission technique unveils the Mottness of antinodal quasiparticles in superconducting copper oxides.}, number={2}, journal={Science Advances}, author={Cilento, F. and Manzoni, G. and Sterzi, A. and Peli, S. and Ronchi, A. and Crepaldi, A. and Boschini, F. and Cacho, C. and Chapman, R. and Springate, E. and et al.}, year={2018}, pages={7} }
@article{kemper_abdurazakov_freericks_2018, title={General Principles for the Nonequilibrium Relaxation of Populations in Quantum Materials}, volume={8}, ISSN={["2160-3308"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85055139503&partnerID=MN8TOARS}, DOI={10.1103/physrevx.8.041009}, abstractNote={We examine the problem of how excited populations of electrons relax after they have been excited by a pump. We include three of the most important relaxation processes: (i) impurity scattering; (ii) Coulomb scattering; and (iii) electron-phonon scattering. The relaxation of an excited population of electrons is one of the most fundamental processes measured in pump/probe experiments, but it is often not interpreted correctly. We show how to resolve four common and incorrect misconceptions about non-equilibrium relaxation that are pervasive in the field. The resolution of these misconceptions shows that non-equilibrium relaxation is more complex than previously thought, but it yields to recently developed theoretical methods in non-equilibrium theory. We focus much of the discussion on implications of these results for experiment.}, number={4}, journal={PHYSICAL REVIEW X}, publisher={American Physical Society (APS)}, author={Kemper, A. F. and Abdurazakov, O. and Freericks, J. K.}, year={2018}, month={Oct} }
@article{chen_ahmadi-majlan_lim_zhang_ngai_kemper_kumah_2018, title={Interfacial structure of SrZrxTi1−xO3 films on Ge}, volume={113}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.5046394}, DOI={10.1063/1.5046394}, abstractNote={The interfacial structure of SrZrxTi1−xO3 films grown on semiconducting Ge substrates is investigated by synchrotron X-ray diffraction and first-principles density functional theory. By systematically tuning the Zr content x, the effects of bonding at the interface and epitaxial strain on the physical structure of the film can be distinguished. The interfacial perovskite layers are found to be polarized as a result of cation-anion ionic displacements perpendicular to the perovskite/semiconductor interface. We find a correlation between the observed buckling and valence band offsets at the SrZrxTi1−xO3/Ge interface. The trends in the theoretical valence band offsets as a function of Zr content for the polar structures are in agreement with reported X-ray photoelectron spectroscopy measurements. These results have important implications for the integration of functional oxide materials with established semiconductor based technologies.}, number={20}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Chen, Tongjie and Ahmadi-Majlan, Kamyar and Lim, Zheng Hui and Zhang, Zhan and Ngai, Joseph H. and Kemper, Alexander F. and Kumah, Divine P.}, year={2018}, month={Nov}, pages={201601} }
@article{konstantinova_rameau_reid_abdurazakov_wu_li_shen_gu_huang_rettig_et al._2018, title={Nonequilibrium electron and lattice dynamics of strongly correlated Bi2Sr2CaCu2O8+delta single crystals}, volume={4}, ISSN={["2375-2548"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000431374900021&KeyUID=WOS:000431374900021}, DOI={10.1126/sciadv.aap7427}, abstractNote={Both electron and lattice dynamics are directly observed in the nonequilibrium state of strongly correlated Bi-2212.}, number={4}, journal={SCIENCE ADVANCES}, publisher={American Association for the Advancement of Science (AAAS)}, author={Konstantinova, Tatiana and Rameau, Jonathan D. and Reid, Alexander H. and Abdurazakov, Omadillo and Wu, Lijun and Li, Renkai and Shen, Xiaozhe and Gu, Genda and Huang, Yuan and Rettig, Laurenz and et al.}, year={2018}, month={Apr} }
@article{abdurazakov_nevola_rustagi_freericks_dougherty_kemper_2018, title={Nonequilibrium electron dynamics in pump-probe spectroscopy: Role of excited phonon populations}, volume={98}, ISSN={["2469-9969"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85058282950&partnerID=MN8TOARS}, DOI={10.1103/physrevb.98.245110}, abstractNote={We study the role of excited phonon populations in the relaxation rates of nonequilibrium electrons using a nonequilibrium Green's function formalism. The transient modifications in the phononic properties are accounted for by self-consistently solving the Dyson equation for the electron and phonon Green's functions. The pump-induced changes manifest in both the electronic and phononic spectral functions. We find that the excited phonon populations suppress the decay rates of nonequilibrium electrons due to enhanced phonon absorption. The increased phonon occupation also sets the nonequilibrium decay rates and the equilibrium scattering rates apart. The decay rates are found to be time dependent, and this is illustrated in the experimentally observed population decay of photoexcited ${\mathrm{Bi}}_{1.5}{\mathrm{Sb}}_{0.5}{\mathrm{Te}}_{1.7}{\mathrm{Se}}_{1.3}$.}, number={24}, journal={PHYSICAL REVIEW B}, publisher={American Physical Society (APS)}, author={Abdurazakov, O. and Nevola, D. and Rustagi, A. and Freericks, J. K. and Dougherty, D. B. and Kemper, A. F.}, year={2018}, month={Dec} }
@article{rustagi_kemper_2018, title={Photoemission signature of excitons}, volume={97}, ISSN={["2469-9969"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000435441500003&KeyUID=WOS:000435441500003}, DOI={10.1103/physrevb.97.235310}, abstractNote={Excitons - the particle-hole bound states - composed of localized electron-hole states in semiconducting systems are crucial to explaining the optical spectrum. Spectroscopic measurements can contain signatures of these two particle bound states and can be particularly useful in determining the characteristics of these excitons. We formulate an expression for evaluating the angle-resolved photoemission spectrum arising from the ionization of excitons given their steady-state distribution in a semiconductor. We show that the spectrum contains information about the direct/indirect band gap nature of the semiconductor and is located below the conduction band minimum displaced by the binding energy. The dispersive features of the spectrum contains remnants of the valence band. Our results indicate that for most exciton probability distributions, the energy integrated photoemission spectrum provides an estimate of the exciton Bohr radius.}, number={23}, journal={PHYSICAL REVIEW B}, publisher={American Physical Society (APS)}, author={Rustagi, Avinash and Kemper, Alexander F.}, year={2018}, month={Jun} }
@article{yi_frano_lu_he_wang_frandsen_kemper_yu_si_wang_et al._2018, title={Spectral Evidence for Emergent Order in
Ba1−xNaxFe2As2}, volume={121}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/physrevlett.121.127001}, DOI={10.1103/physrevlett.121.127001}, abstractNote={We report an angle-resolved photoemission spectroscopy study of the iron-based superconductor family, Ba_{1-x}Na_{x}Fe_{2}As_{2}. This system harbors the recently discovered double-Q magnetic order appearing in a reentrant C_{4} phase deep within the underdoped regime of the phase diagram that is otherwise dominated by the coupled nematic phase and collinear antiferromagnetic order. From a detailed temperature-dependence study, we identify the electronic response to the nematic phase in an orbital-dependent band shift that strictly follows the rotational symmetry of the lattice and disappears when the system restores C_{4} symmetry in the low temperature phase. In addition, we report the observation of a distinct electronic reconstruction that cannot be explained by the known electronic orders in the system.}, number={12}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Yi, M. and Frano, A. and Lu, D. H. and He, Y. and Wang, Meng and Frandsen, B. A. and Kemper, A. F. and Yu, R. and Si, Q. and Wang, L. and et al.}, year={2018}, month={Sep} }
@article{rustagi_kemper_2018, title={Theoretical Phase Diagram for the Room-Temperature Electron-Hole Liquid in Photoexcited Quasi-Two-Dimensional Monolayer MoS2}, volume={18}, ISSN={["1530-6992"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000420000000061&KeyUID=WOS:000420000000061}, DOI={10.1021/acs.nanolett.7b04377}, abstractNote={Strong correlations between electrons and holes can drive the existence of an electron-hole liquid (EHL) state, typically at high carrier densities and low temperatures. The recent emergence of quasi-two-dimensional (2D) monolayer transition metal dichalcogenides (TMDCs) provides ideal systems to explore the EHL state since ineffective screening of the out-of-plane field lines in these quasi-2D systems allows for stronger charge carrier correlations in contrast to conventional 3D bulk semiconductors and enables the existence of the EHL at high temperatures. Here we construct the phase diagram for the photoinduced first-order phase transition from a plasma of electron-hole pairs to a correlated EHL state in suspended monolayer MoS2. We show that the quasi-2D nature of monolayer TMDCs and the ineffective screening of the out-of-plane field lines allow for this phase transition to occur at and above room temperature, thereby opening avenues for studying many-body phenomena without the constraint of cryogenics.}, number={1}, journal={NANO LETTERS}, publisher={American Chemical Society (ACS)}, author={Rustagi, Avinash and Kemper, Alexander F.}, year={2018}, month={Jan}, pages={455–459} }
@inproceedings{freericks_abdurazakov_kemper_2017, place={Bellingham}, title={
Relaxation of nonequilibrium populations after a pump: the breaking of Mathiessen's rule
}, volume={10193}, url={http://dx.doi.org/10.1117/12.2261872}, DOI={10.1117/12.2261872}, abstractNote={From the early days of many-body physics, it was realized that the self-energy governs the relaxation or lifetime of the retarded Green’s function. So it seems reasonable to directly extend those results into the nonequilibrium domain. But experiments and calculations of the response of quantum materials to a pump show that the relationship between the relaxation and the self-energy only holds in special cases. Experimentally, the decay time for a population to relax back to equilibrium and the linewidth measured in a linear-response angle-resolved photoemission spectroscopy differ by large amounts. Theoretically, aside from the weak-coupling regime where the relationship holds, one also finds deviations and additionally one sees violations of Mathiessen’s rule. In this work, we examine whether looking at an effective transport relaxation time helps to analyze the decay times of excited populations as they relax back to equilibrium. We conclude that it may do a little better, but it has a fitting parameter for the overall scale which must be determined.}, booktitle={Ultrafast Bandgap Photonics II}, publisher={SPIE}, author={Freericks, J. K. and Abdurazakov, O. and Kemper, A. F.}, editor={Rafailov, Michael K.Editor}, year={2017}, month={May} }
@article{nosarzewski_moritz_freericks_kemper_devereaux_2017, title={Amplitude mode oscillations in pump-probe photoemission spectra from a d-wave superconductor}, volume={96}, ISSN={["2469-9969"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000415675600006&KeyUID=WOS:000415675600006}, DOI={10.1103/physrevb.96.184518}, abstractNote={Recent developments in the techniques of ultrafast pump-probe photoemission have made possible the search for collective modes in strongly correlated systems out of equilibrium. Including inelastic scattering processes and a retarded interaction, we simulate time- and angle- resolved photoemission spectroscopy (trARPES) to study the amplitude mode of a d-wave superconductor, a collective mode excited through the nonlinear light-matter coupling to the pump pulse. We find that the amplitude mode oscillations of the d-wave order parameter occur in phase at a single frequency that is twice the quasi-steady-state maximum gap size after pumping. We comment on the necessary conditions for detecting the amplitude mode in trARPES experiments.}, number={18}, journal={PHYSICAL REVIEW B}, author={Nosarzewski, B. and Moritz, B. and Freericks, J. K. and Kemper, A. F. and Devereaux, T. P.}, year={2017}, month={Nov} }
@article{hübener_sentef_giovannini_kemper_rubio_2017, title={Creating stable Floquet–Weyl semimetals by laser-driving of 3D Dirac materials}, volume={8}, url={https://doi.org/10.1038/ncomms13940}, DOI={10.1038/ncomms13940}, abstractNote={Abstract}, journal={Nature Communications}, publisher={Springer Nature}, author={Hübener, Hannes and Sentef, Michael A. and Giovannini, Umberto De and Kemper, Alexander F. and Rubio, Angel}, year={2017}, month={Jan}, pages={13940} }
@article{creating stable floquet–weyl semimetals by laser-driving of 3d dirac materials_2017, DOI={10.3204/pubdb-2017-10018}, journal={Deutsches Elektronen-Synchrotron, DESY, Hamburg}, year={2017} }
@article{hubener_sentef_de giovannini_kemper_rubio_2017, title={Creating stable floquet-weyl semimetals by laser-driving of 3d dirac materials}, volume={8}, journal={Nature Communications}, author={Hubener, H. and Sentef, M. A. and De Giovannini, U. and Kemper, A. F. and Rubio, A.}, year={2017} }
@article{kemper_sentef_moritz_devereaux_freericks_2017, title={Review of the Theoretical Description of Time‐Resolved Angle‐Resolved Photoemission Spectroscopy in Electron‐Phonon Mediated Superconductors}, volume={7}, url={https://doi.org/10.1002/andp.201600235}, DOI={10.1002/andp.201600235}, abstractNote={Abstract}, number={9}, journal={Annalen der Physik}, publisher={Wiley-Blackwell}, author={Kemper, A. F. and Sentef, M. A. and Moritz, B. and Devereaux, T. P. and Freericks, J. K.}, year={2017}, month={Sep}, pages={1600235} }
@article{kung_salehi_boulares_kemper_koirala_brahlek_lostak_uher_merlin_wang_et al._2017, title={Surface vibrational modes of the topological insulator Bi2Se3 observed by Raman spectroscopy}, volume={95}, ISSN={["2469-9969"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000402973800007&KeyUID=WOS:000402973800007}, DOI={10.1103/physrevb.95.245406}, abstractNote={We present a polarization resolved Raman scattering study of surface vibration modes in the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ single crystal and thick films. Besides the four Raman active bulk phonons, we observed four additional modes with much weaker intensity and slightly lower energy than the bulk counterparts. Using symmetry analysis, we assigned these additional modes to out-of-plane surface phonons. Comparing with first-principle calculations, we conclude that the appearance of these modes is due to $c$-axis lattice distortion and van der Waals gap expansion near the crystal surface. Two of the surface modes at 60 and 173 ${\mathrm{cm}}^{\ensuremath{-}1}$ are associated with Raman active ${A}_{1g}$ bulk phonon modes, the other two at 136 and 158 ${\mathrm{cm}}^{\ensuremath{-}1}$ are associated with infrared active bulk phonons with ${A}_{2u}$ symmetry. The latter become Raman allowed due to reduction of crystalline symmetry from ${D}_{3d}$ in the bulk to ${C}_{3v}$ on the crystal surface. In particular, the 158 ${\mathrm{cm}}^{\ensuremath{-}1}$ surface phonon mode shows a Fano line shape under resonant excitation, suggesting interference in the presence of electron-phonon coupling of the surface excitations.}, number={24}, journal={PHYSICAL REVIEW B}, publisher={American Physical Society (APS)}, author={Kung, H. -H. and Salehi, M. and Boulares, I. and Kemper, A. F. and Koirala, N. and Brahlek, M. and Lostak, P. and Uher, C. and Merlin, R. and Wang, X. and et al.}, year={2017}, month={Jun} }
@article{kemper_krishnamurthy_freericks_2017, title={The role of average time dependence on the relaxation of excited electron populations in nonequilibrium many-body physics}, volume={65}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000403351700012&KeyUID=WOS:000403351700012}, DOI={10.1002/prop.201600042}, abstractNote={We examine the exact equation of motion for the relaxation of populations of strongly correlated electrons after a nonequilibrium excitation by a pulsed field, and prove that the populations do not change when the Green's functions have no average time dependence. We show how the average time dependence enters into the equation of motion to lowest order and describe what governs the relaxation process of the electron populations in the long‐time limit. While this result may appear, on the surface, to be required by any steady‐state solution, the proof is nontrivial, and provides new critical insight into how nonequilibrium populations relax, which goes beyond the assumption that they thermalize via a simple relaxation rate determined by the imaginary part of the self‐energy, or that they can be described by a quasi‐equilibrium condition with a Fermi‐Dirac distribution and a time‐dependent temperature. We also discuss the implications of this result to approximate theories, which may not satisfy the exact relation in the equation of motion.}, number={6-8}, journal={Fortschritte Der Physik-Progress of Physics}, publisher={Wiley-Blackwell}, author={Kemper, A. F. and Krishnamurthy, H. R. and Freericks, J. K.}, year={2017}, pages={8} }
@article{kemper_krishnamurthy_freericks_2017, title={The role of average time dependence on the relaxation of excited electron populations in nonequilibrium many-body physics}, volume={65}, number={6-8}, journal={Fortschritte der Physik-Progress of Physics}, author={Kemper, A. F. and Krishnamurthy, H. R. and Freericks, J. K.}, year={2017} }
@article{coslovich_kemper_behl_huber_bechtel_sasagawa_martin_lanzara_kaindl_2017, title={Ultrafast dynamics of vibrational symmetry breaking in a charge-ordered nickelate}, volume={3}, url={https://doi.org/10.1126/sciadv.1600735}, DOI={10.1126/sciadv.1600735}, abstractNote={Terahertz pulses reveal the multiscale dynamics of the crystal symmetry upon melting and formation of atomic-scale stripes.}, number={11}, journal={Science Advances}, publisher={American Association for the Advancement of Science (AAAS)}, author={Coslovich, Giacomo and Kemper, Alexander F. and Behl, Sascha and Huber, Bernhard and Bechtel, Hans A. and Sasagawa, Takao and Martin, Michael C. and Lanzara, Alessandra and Kaindl, Robert A.}, year={2017}, month={Nov} }
@article{rameau_freutel_kemper_sentef_freericks_avigo_ligges_rettig_yoshida_eisaki_et al._2016, title={Energy dissipation from a correlated system driven out of equilibrium}, volume={7}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000390052600001&KeyUID=WOS:000390052600001}, DOI={10.1038/ncomms13761}, abstractNote={Abstract}, journal={Nature Communications}, publisher={Springer Nature}, author={Rameau, J. D. and Freutel, S. and Kemper, A. F. and Sentef, M. A. and Freericks, J. K. and Avigo, I. and Ligges, M. and Rettig, L. and Yoshida, Y. and Eisaki, H. and et al.}, year={2016}, pages={13761} }
@article{kemper_freericks_2016, title={Relationship between Population Dynamics and the Self-Energy in Driven Non-Equilibrium Systems}, volume={18}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000377262900023&KeyUID=WOS:000377262900023}, DOI={10.3390/e18050180}, abstractNote={We compare the decay rates of excited populations directly calculated within a Keldysh formalism to the equation of motion of the population itself for a Hubbard-Holstein model in two dimensions. While it is true that these two approaches must give the same answer, it is common to make a number of simplifying assumptions within the differential equation for the populations that allows one to interpret the decay in terms of hot electrons interacting with a phonon bath. Here we show how care must be taken to ensure an accurate treatment of the equation of motion for the populations due to the fact that there are identities that require cancellations of terms that naively look like they contribute to the decay rates. In particular, the average time dependence of the Green's functions and self-energies plays a pivotal role in determining these decay rates.}, number={5}, journal={Entropy}, publisher={MDPI AG}, author={Kemper, Alexander and Freericks, James}, year={2016}, month={May}, pages={180} }
@article{jozwiak_sobota_gotlieb_kemper_rotundu_birgeneau_hussain_lee_shen_lanzara_et al._2016, title={Spin-polarized surface resonances accompanying topological surface state formation}, volume={7}, url={https://doi.org/10.1038/ncomms13143}, DOI={10.1038/ncomms13143}, abstractNote={Abstract}, number={1}, journal={Nature Communications}, publisher={Springer Nature}, author={Jozwiak, Chris and Sobota, Jonathan A. and Gotlieb, Kenneth and Kemper, Alexander F. and Rotundu, Costel R. and Birgeneau, Robert J. and Hussain, Zahid and Lee, Dung-Hai and Shen, Zhi-Xun and Lanzara, Alessandra and et al.}, year={2016}, month={Oct}, pages={13143} }
@article{hinton_thewalt_alpichshev_mahmood_koralek_chan_veit_dorow_barišić_kemper_et al._2016, title={The rate of quasiparticle recombination probes the onset of coherence in cuprate superconductors}, volume={6}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000373964900001&KeyUID=WOS:000373964900001}, DOI={10.1038/srep23610}, abstractNote={Abstract}, journal={Sci. Rep.}, publisher={Nature Publishing Group}, author={Hinton, J. P. and Thewalt, E. and Alpichshev, Z. and Mahmood, F. and Koralek, J. D. and Chan, M. K. and Veit, M. J. and Dorow, C. J. and Barišić, N. and Kemper, A. F. and et al.}, year={2016}, month={Apr}, pages={23610} }
@article{sentef_kemper_georges_kollath_2016, title={Theory of light-enhanced phonon-mediated superconductivity}, volume={93}, ISSN={["2469-9969"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000373568900004&KeyUID=WOS:000373568900004}, DOI={10.1103/physrevb.93.144506}, abstractNote={We investigate the dynamics of a phonon-mediated superconductor driven out of equilibrium. The electronic hopping amplitude is ramped down in time, resulting in an increased electronic density of states. The dynamics of the coupled electron-phonon model is investigated by solving Migdal-Eliashberg equations for the double-time Keldysh Green's functions. The increase of the density of states near the Fermi level leads to an enhancement of superconductivity when the system thermalizes to the new state at the same temperature. We provide a time- and momentum-resolved view on this thermalization process and show that it involves fast processes associated with single-particle scattering and much slower dynamics associated with the superconducting order parameter. The importance of electron-phonon coupling for the rapid enhancement and the efficient thermalization of superconductivity is demonstrated, and the results are compared to a BCS time-dependent mean-field approximation.}, number={14}, journal={PHYSICAL REVIEW B}, publisher={American Physical Society (APS)}, author={Sentef, M. A. and Kemper, A. F. and Georges, A. and Kollath, C.}, year={2016}, month={Apr} }
@article{moore_lee_kirchman_chuang_kemper_trigo_patthey_lu_krupin_yi_et al._2016, title={Ultrafast resonant soft x-ray diffraction dynamics of the charge density wave in TbTe 3}, volume={93}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000369218500004&KeyUID=WOS:000369218500004}, DOI={10.1103/physrevb.93.024304}, abstractNote={Understanding the emergence of collective behavior in correlated electron systems remains at the forefront of modern condensed matter physics. Disentangling the degrees of freedom responsible for collective behavior can lead to insights into the microscopic origins of emergent properties and phase transitions. Utilizing an optical pump, resonant soft x-ray diffraction probe we are able to track, in real time, the dynamics of the charge density wave (CDW) in ${\mathrm{TbTe}}_{3}$, a model system that violates traditional views of a Fermi surface nested CDW. We observe coherent oscillations corresponding to the CDW amplitude mode at $2.4$ THz and a coherent optical phonon mode at $\ensuremath{\sim}1.7\phantom{\rule{0.16em}{0ex}}\mathrm{THz}$. We show how such observations reveal the anisotropic energy optimization between in-plane Te charge density modulations and the three-dimensional lattice coupling.}, number={2}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Moore, R. G. and Lee, W. S. and Kirchman, P. S. and Chuang, Y. D. and Kemper, A. F. and Trigo, M. and Patthey, L. and Lu, D. H. and Krupin, O. and Yi, M. and et al.}, year={2016}, month={Jan} }
@article{yi_wang_kemper_mo_hussain_bourret-courchesne_lanzara_hashimoto_lu_shen_et al._2015, title={Bandwidth and Electron Correlation-Tuned Superconductivity in Rb 0.8 Fe 2 ( Se 1 − z S z ) 2}, volume={115}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000366484900007&KeyUID=WOS:000366484900007}, DOI={10.1103/physrevlett.115.256403}, abstractNote={We present a systematic angle-resolved photoemission spectroscopy study of the substitution dependence of the electronic structure of Rb_{0.8}Fe_{2}(Se_{1-z}S_{z})_{2} (z=0, 0.5, 1), where superconductivity is continuously suppressed into a metallic phase. Going from the nonsuperconducting Rb_{0.8}Fe_{2}S_{2} to superconducting Rb_{0.8}Fe_{2}Se_{2}, we observe little change of the Fermi surface topology, but a reduction of the overall bandwidth by a factor of 2. Hence, for these heavily electron-doped iron chalcogenides, we have identified electron correlation as explicitly manifested in the quasiparticle bandwidth to be the important tuning parameter for superconductivity, and that moderate correlation is essential to achieving high T_{C}.}, number={25}, journal={Phys. Rev. Lett.}, publisher={American Physical Society (APS)}, author={Yi, M. and Wang, Meng and Kemper, A. F. and Mo, S.-K. and Hussain, Z. and Bourret-Courchesne, E. and Lanzara, A. and Hashimoto, M. and Lu, D. H. and Shen, Z.-X. and et al.}, year={2015} }
@article{leuenberger_sobota_yang_kemper_giraldo-gallo_moore_fisher_kirchmann_devereaux_shen_2015, title={Classification of collective modes in a charge density wave by momentum-dependent modulation of the electronic band structure}, volume={91}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000354985300001&KeyUID=WOS:000354985300001}, DOI={10.1103/physrevb.91.201106}, abstractNote={We present time- and angle-resolved photoemission spectroscopy (trARPES) measurements on the charge density wave system CeTe3. Optical excitation transiently populates the unoccupied band structure and reveals a gap size of 2� = 0.59 eV. The occupied Te-5p band dispersion is coherently modified by three modes at}, number={20}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Leuenberger, D. and Sobota, J. A. and Yang, S.-L. and Kemper, A. F. and Giraldo-Gallo, P. and Moore, R. G. and Fisher, I. R. and Kirchmann, P. S. and Devereaux, T. P. and Shen, Z.-X.}, year={2015}, month={May} }
@article{gerber_kim_zhang_zhu_plonka_yi_dakovski_leuenberger_kirchmann_moore_et al._2015, title={Direct characterization of photoinduced lattice dynamics in BaFe2As2}, volume={6}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000357175300010&KeyUID=WOS:000357175300010}, DOI={10.1038/ncomms8377}, abstractNote={Abstract}, journal={Nat Comms}, publisher={Nature Publishing Group}, author={Gerber, S. and Kim, K. W. and Zhang, Y. and Zhu, D. and Plonka, N. and Yi, M. and Dakovski, G. L. and Leuenberger, D. and Kirchmann, P.S. and Moore, R. G. and et al.}, year={2015}, month={Jun}, pages={7377} }
@article{kemper_sentef_moritz_freericks_devereaux_2015, title={Direct observation of Higgs mode oscillations in the pump-probe photoemission spectra of electron-phonon mediated superconductors}, volume={92}, ISSN={["2469-9969"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000367376200003&KeyUID=WOS:000367376200003}, DOI={10.1103/physrevb.92.224517}, abstractNote={Using the non-equilibrium Keldysh formalism, we solve the equations of motion for electron-phonon superconductivity, including an ultrafast pump field. We present results for time-dependent photoemission spectra out of equilibrium which probes the dynamics of the superconducting gap edge. The partial melting of the order by the pump field leads to oscillations at twice the melted gap frequency, a hallmark of the Higgs or amplitude mode. Thus the Higgs mode can be directly excited through the nonlinear effects of an electromagnetic field and detected without any additional symmetry breaking.}, number={22}, journal={PHYSICAL REVIEW B}, publisher={American Physical Society (APS)}, author={Kemper, A. F. and Sentef, M. A. and Moritz, B. and Freericks, J. K. and Devereaux, T. P.}, year={2015}, month={Dec} }
@article{arenas_middleton_kemper_2015, title={First-principles study of the phonon modes in bismuth sillenites}, volume={91}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000352469300003&KeyUID=WOS:000352469300003}, DOI={10.1103/physrevb.91.144103}, abstractNote={Density functional theory calculations of the vibrational modes of ${\mathrm{Bi}}_{12}{\mathrm{SiO}}_{20}$ are presented, with a detailed assignment and geometry investigation of the vibrational modes in the sillenite structure. We show that the symmetry and geometry of the strong Raman peaks allow direct probing of the Bi-O(1) and Bi-O(2) bonds. The physical significance of the vibrational modes is considered to discuss experimental data on the trivalent sillenites and show evidence of O(1) and/or O(2) vacancies in the Bi-O framework. The infrared modes are also discussed to motivate future systematic studies of sillenites. The Born effective charges of sillenites are studied and the results show the existence of large and anisotropic charges. The magnitude of the LO-TO splits is also calculated and compared with experiment.}, number={14}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Arenas, D. J. and Middleton, Carl and Kemper, A. F.}, year={2015}, month={Apr} }
@article{liu_lin_vigil-fowler_lischner_kemper_sharifzadeh_jornada_deslippe_yang_neaton_et al._2015, title={Numerical integration for ab initio many-electron self energy calculations within the GW approximation}, volume={286}, url={https://doi.org/10.1016/j.jcp.2015.01.023}, DOI={10.1016/j.jcp.2015.01.023}, abstractNote={We present a numerical integration scheme for evaluating the convolution of a Green's function with a screened Coulomb potential on the real axis in the GW approximation of the self energy. Our scheme takes the zero broadening limit in Green's function first, replaces the numerator of the integrand with a piecewise polynomial approximation, and performs principal value integration on subintervals analytically. We give the error bound of our numerical integration scheme and show by numerical examples that it is more reliable and accurate than the standard quadrature rules such as the composite trapezoidal rule. We also discuss the benefit of using different self energy expressions to perform the numerical convolution at different frequencies.}, journal={Journal of Computational Physics}, publisher={Elsevier BV}, author={Liu, Fang and Lin, Lin and Vigil-Fowler, Derek and Lischner, Johannes and Kemper, Alexander F. and Sharifzadeh, Sahar and Jornada, Felipe H. and Deslippe, Jack and Yang, Chao and Neaton, Jeffrey B. and et al.}, year={2015}, month={Apr}, pages={1–13} }
@article{langner_roy_kemper_chuang_mishra_versteeg_zhu_hertlein_glover_dumesnil_et al._2015, title={Scattering bottleneck for spin dynamics in metallic helical antiferromagnetic dysprosium}, volume={92}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000365506600006&KeyUID=WOS:000365506600006}, DOI={10.1103/physrevb.92.184423}, abstractNote={We measure dynamics of the helical ordering in the Lanthanide metal Dy resulting from transient changes in the conduction electron Fermi surface and subsequent scattering events that transfer the excitation to the core spin.}, number={18}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Langner, M. C. and Roy, S. and Kemper, A. F. and Chuang, Y.-D. and Mishra, S. K. and Versteeg, R. B. and Zhu, Y. and Hertlein, M. P. and Glover, T. E. and Dumesnil, K. and et al.}, year={2015}, month={Nov} }
@article{sentef_claassen_kemper_moritz_oka_freericks_devereaux_2015, title={Theory of Floquet band formation and local pseudospin textures in pump-probe photoemission of graphene}, volume={6}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000355530700003&KeyUID=WOS:000355530700003}, DOI={10.1038/ncomms8047}, abstractNote={Ultrafast materials science promises optical control of physical properties of solids. Continuous-wave circularly polarized laser driving was predicted to induce a light-matter coupled state with an energy gap and a quantum Hall effect, coined Floquet topological insulator. Whereas the envisioned Floquet topological insulator requires high-frequency pumping to obtain well-separated Floquet bands, a follow-up question regards the creation of Floquet-like states in graphene with realistic low-frequency laser pulses. Here we predict that short optical pulses attainable in experiments can lead to local spectral gaps and novel pseudospin textures in graphene. Pump-probe photoemission spectroscopy can track these states by measuring sizeable energy gaps and Floquet band formation on femtosecond time scales. Analysing band crossings and pseudospin textures near the Dirac points, we identify new states with optically induced nontrivial changes of sublattice mixing that leads to Berry curvature corrections of electrical transport and magnetization.}, journal={Nat Comms}, publisher={Nature Publishing Group}, author={Sentef, M.A. and Claassen, M. and Kemper, A.F. and Moritz, B. and Oka, T. and Freericks, J.K. and Devereaux, T.P.}, year={2015}, month={May}, pages={7047} }
@article{yang_sobota_leuenberger_kemper_lee_schmitt_li_moore_kirchmann_shen_2015, title={Thickness-Dependent Coherent Phonon Frequency in Ultrathin FeSe/SrTiO 3 Films}, volume={15}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000356316900074&KeyUID=WOS:000356316900074}, DOI={10.1021/acs.nanolett.5b01274}, abstractNote={Ultrathin FeSe films grown on SrTiO3 substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. By using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO3 films grown by molecular beam epitaxy. After optical excitation, we observe periodic modulations of the photoelectron spectrum as a function of pump-probe delay for 1-unit-cell, 3-unit-cell, and 60-unit-cell thick FeSe films. The frequencies of the coherent intensity oscillations increase from 5.00 ± 0.02 to 5.25 ± 0.02 THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A1g phonon. The dominant mechanism for the phonon softening in 1-unit-cell thick FeSe films is a substrate-induced lattice strain. Our results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.}, number={6}, journal={Nano Lett.}, publisher={American Chemical Society (ACS)}, author={Yang, Shuolong and Sobota, Jonathan A. and Leuenberger, Dominik and Kemper, Alexander F. and Lee, James J. and Schmitt, Felix T. and Li, Wei and Moore, Rob G. and Kirchmann, Patrick S. and Shen, Zhi-Xun}, year={2015}, month={Jun}, pages={4150–4154} }
@inproceedings{langner_roy_kemper_chuang_mishra_versteeg_zhu_hertlein_glover_dumesnil_et al._2015, title={Transient Exchange Interaction in a Helical Antiferromagnet}, ISBN={9781557529688}, url={http://dx.doi.org/10.1364/cleo_qels.2015.ftu1b.6}, DOI={10.1364/cleo_qels.2015.ftu1b.6}, abstractNote={We measure dynamics of the helical ordering in the Lanthanide metal Dy resulting from transient changes in the conduction electron Fermi surface and subsequent scattering events that transfer the excitation to the core spin.}, booktitle={CLEO: 2015}, publisher={OSA}, author={Langner, Matt and Roy, Sujoy and Kemper, A. F. and Chuang, Y.-D. and Mishra, S. and Versteeg, R. B. and Zhu, Yi and Hertlein, M. P. and Glover, T. E. and Dumesnil, K. and et al.}, year={2015} }
@inbook{langner_roy_kemper_chuang_misra_versteeg_zhu_hertlein_glover_dumesnil_et al._2015, place={New York}, title={Transient Exchange Interaction in a Helical Antiferromagnet}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000370627100375&KeyUID=WOS:000370627100375}, booktitle={2015 Conference on Lasers and Electro-Optics}, author={Langner, M. C. and Roy, S. and Kemper, A. F. and Chuang, Y. D. and Misra, S. K. and Versteeg, R. B. and Zhu, Y. and Hertlein, M. P. and Glover, T. E. and Dumesnil, K. and et al.}, year={2015} }
@article{eiter_lavagnini_hackl_nowadnick_kemper_devereaux_chu_analytis_fisher_degiorgi_et al._2014, title={Balancing Act: Evidence for a Strong Subdominant d -Wave Pairing Channel in Ba 0.6 K 0.4 Fe 2 As 2}, volume={4}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000349072800001&KeyUID=WOS:000349072800001}, DOI={10.1103/physrevx.4.041046}, abstractNote={We present an analysis of the Raman spectra of optimally doped ${\rm Ba_{0.6}K_{0.4}Fe_2As_2}$ based on LDA band structure calculations and the subsequent estimation of effective Raman vertices. Experimentally a narrow, emergent mode appears in the $B_{1g}$ ($d_{x^2-y^2}$) Raman spectra only below $T_c$, well into the superconducting state and at an energy below twice the energy gap on the electron Fermi surface sheets. The Raman spectra can be reproduced quantitatively with estimates for the magnitude and momentum space structure of the s$_{+-}$ pairing gap on different Fermi surface sheets, as well as the identification of the emergent sharp feature as a Bardasis-Schrieffer exciton, formed as a Cooper pair bound state in a subdominant $d_{x^2-y^2}$ channel. The binding energy of the exciton relative to the gap edge shows that the coupling strength in this subdominant $d_{x^2-y^2}$ channel is as strong as 60% of that in the dominant $s_{+-}$ channel. This result suggests that $d_{x^2-y^2}$ may be the dominant pairing symmetry in Fe-based sperconductors which lack central hole bands.}, number={4}, journal={Phys. Rev. X}, publisher={American Physical Society (APS)}, author={Eiter, H.-M. and Lavagnini, M. and Hackl, R. and Nowadnick, E.A. and Kemper, A.F. and Devereaux, T.P. and Chu, J.-H. and Analytis, J.G. and Fisher, I.R. and Degiorgi, L. and et al.}, year={2014}, pages={64–69} }
@article{wang_kemper_baldini_shapiro_riggs_zhao_liu_devereaux_geballe_fisher_et al._2014, title={Bandgap closure and reopening in at high pressure}, volume={89}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000336976400003&KeyUID=WOS:000336976400003}, DOI={10.1103/physrevb.89.245109}, abstractNote={Shibing Wang,1,2,* Alexander F. Kemper,2,3 Maria Baldini,4 M. C. Shapiro,5 Scott C. Riggs,5 Zhao Zhao,6 Zhenxian Liu,7 Thomas P. Devereaux,2,8 Ted H. Geballe,5 Ian R. Fisher,5 and Wendy L. Mao1,8 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA 2SIMES, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 3Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 4HPSynC, Carnegie Institution of Washington, Washington, DC 20015, USA 5Department of Applied Physics and Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, USA 6Department of Physics, Stanford University, Stanford, California 94305, USA 7Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA 8Photon Science, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA (Received 27 February 2014; revised manuscript received 23 May 2014; published 9 June 2014)}, number={24}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Wang, Shibing and Kemper, Alexander F. and Baldini, Maria and Shapiro, M. C. and Riggs, Scott C. and Zhao, Zhao and Liu, Zhenxian and Devereaux, Thomas P. and Geballe, Ted H. and Fisher, Ian R. and et al.}, year={2014}, month={Jun} }
@article{sobota_yang_leuenberger_kemper_analytis_fisher_kirchmann_devereaux_shen_2014, title={Distinguishing Bulk and Surface Electron-Phonon Coupling in the Topological Insulator Using Time-Resolved Photoemission Spectroscopy}, volume={113}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000344052200011&KeyUID=WOS:000344052200011}, DOI={10.1103/physrevlett.113.157401}, abstractNote={We report time- and angle-resolved photoemission spectroscopy measurements on the topological insulator Bi(2)Se(3). We observe oscillatory modulations of the electronic structure of both the bulk and surface states at a frequency of 2.23 THz due to coherent excitation of an A(1g) phonon mode. A distinct, additional frequency of 2.05 THz is observed in the surface state only. The lower phonon frequency at the surface is attributed to the termination of the crystal and thus reduction of interlayer van der Waals forces, which serve as restorative forces for out-of-plane lattice distortions. Density functional theory calculations quantitatively reproduce the magnitude of the surface phonon softening. These results represent the first band-resolved evidence of the A(1g) phonon mode coupling to the surface state in a topological insulator.}, number={15}, journal={Phys. Rev. Lett.}, publisher={American Physical Society (APS)}, author={Sobota, J. A. and Yang, S.-L. and Leuenberger, D. and Kemper, A. F. and Analytis, J. G. and Fisher, I. R. and Kirchmann, P. S. and Devereaux, T. P. and Shen, Z.-X.}, year={2014}, month={Oct} }
@article{yi_zhang_liu_ding_chu_kemper_plonka_moritz_hashimoto_mo_et al._2014, title={Dynamic competition between spin-density wave order and superconductivity in underdoped Ba1−xKxFe2As2}, volume={5}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000335223100002&KeyUID=WOS:000335223100002}, DOI={10.1038/ncomms4711}, abstractNote={An intriguing aspect of unconventional superconductivity is that it always appears in the vicinity of other competing phases, whose suppression brings the full emergence of superconductivity. In the iron pnictides, these competing phases are marked by a tetragonal-to-orthorhombic structural transition and a collinear spin-density wave (SDW) transition. There has been macroscopic evidence for competition between these phases and superconductivity as the magnitude of both the orthorhombicity and magnetic moment are suppressed in the superconducting state. Here, using angle-resolved photoemission spectroscopy on detwinned underdoped Ba(1-x)K(x)Fe2As2, we observe a coexistence of both the SDW gap and superconducting gap in the same electronic structure. Furthermore, our data reveal that following the onset of superconductivity, the SDW gap decreases in magnitude and shifts in a direction consistent with a reduction of the orbital anisotropy. This observation provides direct spectroscopic evidence for the dynamic competition between superconductivity and both SDW and electronic nematic orders in these materials.}, journal={Nat Comms}, publisher={Nature Publishing Group}, author={Yi, M. and Zhang, Y. and Liu, Z.-K. and Ding, X. and Chu, J.-H. and Kemper, A.F. and Plonka, N. and Moritz, B. and Hashimoto, M. and Mo, S.-K. and et al.}, year={2014}, month={Apr} }
@article{kemper_sentef_moritz_freericks_devereaux_2014, title={Effect of dynamical spectral weight redistribution on effective interactions in time-resolved spectroscopy}, volume={90}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000341256600003&KeyUID=WOS:000341256600003}, DOI={10.1103/physrevb.90.075126}, abstractNote={The redistribution of electrons in an ultrafast pump-probe experiment causes significant changes to the spectral distribution of the retarded interaction between electrons and bosonic modes. We study the influence of these changes on pump-probe photoemission spectroscopy for a model electron-phonon coupled system using the nonequilibrium Keldysh formalism. We show that spectral rearrangement due to the driving field preserves an overall sum rule for the electronic self-energy, but modifies the effective electron-phonon scattering as a function of energy. Experimentally, this pump-modified scattering can be tracked by analyzing the fluence or excitation energy dependence of population decay rates and transient changes in dispersion kinks.}, number={7}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Kemper, A. F. and Sentef, M. A. and Moritz, B. and Freericks, J. K. and Devereaux, T. P.}, year={2014}, month={Aug} }
@article{shen_kemper_devereaux_freericks_2014, title={Exact solution for high harmonic generation and the response to an ac driving field for a charge density wave insulator}, volume={90}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000341259600002&KeyUID=WOS:000341259600002}, DOI={10.1103/physrevb.90.115113}, abstractNote={We develop and exactly solve a model for electrons driven by pulsed or continuous ac fields. The theory includes both the photoexcitation process as well as the subsequent acceleration of the electrons. In the case of an ac response, we examine both the nonequilibrium density of states and the current. In the case of pulsed light for high harmonic generation, we find the radiated light assumes a nearly universal behavior, with only limited dependence on the parameters of the system, except for the amplitude of the driving field, which determines the range of high harmonics generated and a tendency toward a narrowing of the peaks in a charge density wave versus a metal. This type of high harmonic generation can potentially be used for the creation of solid-state-based ultrafast light sources.}, number={11}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Shen, Wen and Kemper, A. F. and Devereaux, T. P. and Freericks, J. K.}, year={2014}, month={Sep} }
@article{kang_kemper_fernandes_2014, title={Manipulation of Gap Nodes by Uniaxial Strain in Iron-Based Superconductors}, volume={113}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000345356900006&KeyUID=WOS:000345356900006}, DOI={10.1103/physrevlett.113.217001}, abstractNote={In the iron pnictides and chalcogenides, multiple orbitals participate in the superconducting state, enabling different gap structures to be realized in distinct materials. Here we argue that the spectral weights of these orbitals can, in principle, be controlled by a tetragonal symmetry-breaking uniaxial strain, due to the enhanced nematic susceptibility of many iron-based superconductors. By investigating multiorbital microscopic models in the presence of orbital order, we show that not only Tc can be enhanced, but pairs of accidental gap nodes can be annihilated and created in the Fermi surface by an increasing external strain. We explain our results as a mixture of nearly degenerate superconducting states promoted by strain, and show that the annihilation and creation of nodes can be detected experimentally via anisotropic penetration depth measurements. Our results provide a promising framework to externally control the superconducting properties of iron-based materials.}, number={21}, journal={Phys. Rev. Lett.}, publisher={American Physical Society (APS)}, author={Kang, Jian and Kemper, Alexander F. and Fernandes, Rafael M.}, year={2014}, month={Nov} }
@inproceedings{freericks_najafi_kemper_devereaux_2014, title={Nonequilibrium sum rules for the Holstein model}, url={arXiv:1403.5604}, booktitle={Conference Proceedings for FEIS 2013}, author={Freericks, J.K. and Najafi, K. and Kemper, A.F. and Devereaux, T.P.}, year={2014} }
@article{kemper_sentef_moritz_freericks_devereaux_2014, title={Publisher's Note: Effect of dynamical spectral weight redistribution on effective interactions in time-resolved spectroscopy [Phys. Rev. B 90, 075126 (2014)]}, volume={90}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000341299800010&KeyUID=WOS:000341299800010}, DOI={10.1103/physrevb.90.079905}, number={7}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Kemper, A. F. and Sentef, M. A. and Moritz, B. and Freericks, J. K. and Devereaux, T. P.}, year={2014}, month={Aug} }
@article{sobota_yang_leuenberger_kemper_analytis_fisher_kirchmann_devereaux_shen_2014, title={Ultrafast electron dynamics in the topological insulator Bi2Se3 studied by time-resolved photoemission spectroscopy}, volume={195}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000342872800036&KeyUID=WOS:000342872800036}, DOI={10.1016/j.elspec.2014.01.005}, abstractNote={We characterize the topological insulator Bi2Se3 using time- and angle-resolved photoemission spectroscopy. By employing two-photon photoemission, a complete picture of the unoccupied electronic structure from the Fermi level up to the vacuum level is obtained. We demonstrate that the unoccupied states host a second Dirac surface state which can be resonantly excited by 1.5 eV photons. We then study the ultrafast relaxation processes following optical excitation. We find that they culminate in a persistent non-equilibrium population of the first Dirac surface state, which is maintained by a meta-stable population of the bulk conduction band. Finally, we perform a temperature-dependent study of the electron–phonon scattering processes in the conduction band, and find the unexpected result that their rates decrease with increasing sample temperature. We develop a model of phonon emission and absorption from a population of electrons, and show that this counter-intuitive trend is the natural consequence of fundamental electron–phonon scattering processes. This analysis serves as an important reminder that the decay rates extracted by time-resolved photoemission are not in general equal to single electron scattering rates, but include contributions from filling and emptying processes from a continuum of states.}, journal={Journal of Electron Spectroscopy and Related Phenomena}, publisher={Elsevier BV}, author={Sobota, J.A. and Yang, S.-L. and Leuenberger, D. and Kemper, A.F. and Analytis, J.G. and Fisher, I.R. and Kirchmann, P.S. and Devereaux, T.P. and Shen, Z.-X.}, year={2014}, month={Aug}, pages={249–257} }
@article{eiter_lavagnini_hackl_nowadnick_kemper_devereaux_chu_analytis_fisher_degiorgi_2013, title={Alternative route to charge density wave formation in multiband systems}, volume={110}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000313630300027&KeyUID=WOS:000313630300027}, DOI={10.1073/pnas.1214745110}, abstractNote={
Charge and spin density waves, periodic modulations of the electron, and magnetization densities, respectively, are among the most abundant and nontrivial low-temperature ordered phases in condensed matter. The ordering direction is widely believed to result from the Fermi surface topology. However, several recent studies indicate that this common view needs to be supplemented. Here, we show how an enhanced electron–lattice interaction can contribute to or even determine the selection of the ordering vector in the model charge density wave system ErTe
3
. Our joint experimental and theoretical study allows us to establish a relation between the selection rules of the electronic light scattering spectra and the enhanced electron–phonon coupling in the vicinity of band degeneracy points. This alternative proposal for charge density wave formation may be of general relevance for driving phase transitions into other broken-symmetry ground states, particularly in multiband systems, such as the iron-based superconductors.
}, number={1}, journal={PNAS}, publisher={Proceedings of the National Academy of Sciences}, author={Eiter, H.-M. and Lavagnini, M. and Hackl, R. and Nowadnick, E.A. and Kemper, A.F. and Devereaux, T.P. and Chu, J.-H. and Analytis, J.G. and Fisher, I.R. and Degiorgi, L.}, year={2013}, pages={64–69} }
@article{sobota_yang_kemper_lee_schmitt_li_moore_analytis_fisher_kirchmann_et al._2013, title={Direct Optical Coupling to an Unoccupied Dirac Surface State in the Topological Insulator Bi_{2}Se_{3}}, volume={111}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000324762300016&KeyUID=WOS:000324762300016}, DOI={10.1103/physrevlett.111.136802}, abstractNote={We characterize the occupied and unoccupied electronic structure of the topological insulator Bi2Se3 by one-photon and two-photon angle-resolved photoemission spectroscopy and slab band structure calculations. We reveal a second, unoccupied Dirac surface state with similar electronic structure and physical origin to the well-known topological surface state. This state is energetically located 1.5 eV above the conduction band, which permits it to be directly excited by the output of a Ti:sapphire laser. This discovery demonstrates the feasibility of direct ultrafast optical coupling to a topologically protected, spin-textured surface state.}, number={13}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Sobota, J. A. and Yang, S. L. and Kemper, A. F. and Lee, J. J. and Schmitt, F. T. and Li, W. and Moore, R. G. and Analytis, J. G. and Fisher, I. R. and Kirchmann, P. S. and et al.}, year={2013}, pages={5} }
@article{moritz_kemper_sentef_devereaux_freericks_2013, title={Electron-Mediated Relaxation Following Ultrafast Pumping of Strongly Correlated Materials: Model Evidence of a Correlation-Tuned Crossover between Thermal and Nonthermal States}, volume={111}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000323036500004&KeyUID=WOS:000323036500004}, DOI={10.1103/physrevlett.111.077401}, abstractNote={We examine electron-electron mediated relaxation following ultrafast electric field pump excitation of the fermionic degrees of freedom in the Falicov-Kimball model for correlated electrons. The results reveal a dichotomy in the temporal evolution of the system as one tunes through the Mott metal-to-insulator transition: in the metallic regime relaxation can be characterized by evolution toward a steady state well described by Fermi-Dirac statistics with an increased effective temperature; however, in the insulating regime this quasithermal paradigm breaks down with relaxation toward a nonthermal state with a complicated electronic distribution as a function of momentum. We characterize the behavior by studying changes in the energy, photoemission response, and electronic distribution as functions of time. This relaxation may be observable qualitatively on short enough time scales that the electrons behave like an isolated system not in contact with additional degrees of freedom which would act as a thermal bath, especially when using strong driving fields and studying materials whose physics may manifest the effects of correlations.}, number={7}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Moritz, B. and Kemper, A. F. and Sentef, M. and Devereaux, T. P. and Freericks, J. K.}, year={2013} }
@article{sentef_kemper_moritz_freericks_shen_devereaux_2013, title={Examining Electron-Boson Coupling Using Time-Resolved Spectroscopy}, volume={3}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000330128900001&KeyUID=WOS:000330128900001}, DOI={10.1103/physrevx.3.041033}, abstractNote={Nonequilibrium pump-probe time domain spectroscopies can become an important tool to disentangle degrees of freedom whose coupling leads to broad structures in the frequency domain. Here, using the time-resolved solution of a model photoexcited electron-phonon system we show that the relaxational dynamics are directly governed by the equilibrium self-energy so that the phonon frequency sets a window for "slow" versus "fast" recovery. The overall temporal structure of this relaxation spectroscopy allows for a reliable and quantitative extraction of the electron-phonon coupling strength without requiring an effective temperature model or making strong assumptions about the underlying bare electronic band dispersion.}, number={4}, journal={Physical Review X}, publisher={American Physical Society (APS)}, author={Sentef, Michael and Kemper, Alexander and Moritz, Brian and Freericks, James and Shen, Zhi-Xun and Devereaux, Thomas}, year={2013} }
@article{rana_yajima_parui_kemper_devereaux_hikita_hwang_banerjee_2013, title={Hot electron transport in a strongly correlated transition-metal oxide}, volume={3}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000314864800002&KeyUID=WOS:000314864800002}, DOI={10.1038/srep01274}, abstractNote={Oxide heterointerfaces are ideal for investigating strong correlation effects to electron transport, relevant for oxide-electronics. Using hot-electrons, we probe electron transport perpendicular to the La₀.₇Sr₀.₃MnO₃ (LSMO)- Nb-doped SrTiO₃ (Nb:STO) interface and find the characteristic hot-electron attenuation length in LSMO to be 1.48 ± 0.10 unit cells (u.c.) at -1.9 V, increasing to 2.02 ± 0.16 u.c. at -1.3 V at room temperature. Theoretical analysis of this energy dispersion reveals the dominance of electron-electron and polaron scattering. Direct visualization of the local electron transport shows different transmission at the terraces and at the step-edges.}, journal={Sci. Rep.}, publisher={Nature Publishing Group}, author={Rana, Kumari Gaurav and Yajima, Takeaki and Parui, Subir and Kemper, Alexander F. and Devereaux, Thomas P. and Hikita, Yasuyuki and Hwang, Harold Y. and Banerjee, Tamalika}, year={2013} }
@article{lee_moon_riggs_shapiro_fisher_fulfer_chan_kemper_basov_2013, title={Infrared study of the electronic structure of the metallic pyrochlore iridate Bi_{2}Ir_{2}O_{7}}, volume={87}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000319729900003&KeyUID=WOS:000319729900003}, DOI={10.1103/physrevb.87.195143}, abstractNote={We investigated the electronic properties of a single crystal of metallic pyrochlore iridate Bi${}_{2}$Ir${}_{2}$O${}_{7}$ by means of infrared spectroscopy. Our optical conductivity data show the splitting of ${t}_{2g}$ bands into ${J}_{\mathrm{eff}}$ ones due to strong spin-orbit coupling. We observed a sizable midinfrared absorption near 0.2 eV which can be attributed to the optical transition within the ${J}_{\mathrm{eff},1/2}$ bands. More interestingly, we found an abrupt suppression of optical conductivity in the very far-infrared region. Our results suggest that the electronic structure of Bi${}_{2}$Ir${}_{2}$O${}_{7}$ is governed by the strong spin-orbit coupling and correlation effects, which are a prerequisite for theoretically proposed nontrivial topological phases in pyrochlore iridates.}, number={19}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Lee, Y. S. and Moon, S. J. and Riggs, Scott C. and Shapiro, M. C. and Fisher, I. R. and Fulfer, Bradford W. and Chan, Julia Y. and Kemper, A. F. and Basov, D. N.}, year={2013}, month={May}, pages={7} }
@article{kemper_sentef_moritz_kao_shen_freericks_devereaux_2013, title={Mapping of unoccupied states and relevant bosonic modes via the time-dependent momentum distribution}, volume={87}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000321061000001&KeyUID=WOS:000321061000001}, DOI={10.1103/physrevb.87.235139}, abstractNote={The unoccupied states of complex materials are difficult to measure, yet play a key role in determining their properties. We propose a technique that can measure the unoccupied states, called time-resolved Compton scattering, which measures the time-dependent momentum distribution (TDMD). Using a non-equilibrium Keldysh formalism, we study the TDMD for electrons coupled to a lattice in a pump-probe setup. We find a direct relation between temporal oscillations in the TDMD and the dispersion of the underlying unoccupied states, suggesting that both can be measured by time-resolved Compton scattering. We demonstrate the experimental feasibility by applying the method to a model of MgB$_2$ with realistic material parameters.}, number={23}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Kemper, A. F. and Sentef, M. and Moritz, B. and Kao, C. C. and Shen, Z. X. and Freericks, J. K. and Devereaux, T. P.}, year={2013} }
@article{hamdan_kemper_cao_cheng_2013, title={Structure and functionality of bromine doped graphite}, volume={138}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000318550800060&KeyUID=WOS:000318550800060}, DOI={10.1063/1.4801786}, abstractNote={First-principles calculations are used to study the enhanced in-plane conductivity observed experimentally in Br-doped graphite, and to study the effect of external stress on the structure and functionality of such systems. The model used in the numerical calculations is that of stage two doped graphite. The band structure near the Fermi surface of the doped systems with different bromine concentrations is compared to that of pure graphite, and the charge transfer between carbon and bromine atoms is analyzed to understand the conductivity change along different high symmetry directions. Our calculations show that, for large interlayer separation between doped graphite layers, bromine is stable in the molecular form (Br2). However, with increased compression (decreased layer-layer separation) Br2 molecules tend to dissociate. While in both forms, bromine is an electron acceptor. The charge exchange between the graphite layers and Br atoms is higher than that with Br2 molecules. Electron transfer to the Br atoms increases the number of hole carriers in the graphite sheets, resulting in an increase of conductivity.}, number={16}, journal={J. Chem. Phys.}, publisher={AIP Publishing}, author={Hamdan, Rashid and Kemper, A. F. and Cao, Chao and Cheng, H. P.}, year={2013}, pages={164702} }
@article{kemper_moritz_freericks_devereaux_2013, title={Theoretical description of high-order harmonic generation in solids}, volume={15}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000314516500003&KeyUID=WOS:000314516500003}, DOI={10.1088/1367-2630/15/2/023003}, abstractNote={We consider several aspects of high-order harmonic generation in solids: the effects of elastic and inelastic scattering, varying pulse characteristics and inclusion of material-specific parameters through a realistic band structure. We reproduce many observed characteristics of high harmonic generation experiments in solids including the formation of only odd harmonics in inversion-symmetric materials, and the nonlinear formation of high harmonics with increasing field. We find that the harmonic spectra are fairly robust against elastic and inelastic scattering. Furthermore, we find that the pulse characteristics can play an important role in determining the harmonic spectra.}, number={2}, journal={New Journal of Physics}, publisher={IOP Publishing}, author={Kemper, A F and Moritz, B and Freericks, J K and Devereaux, T P}, year={2013}, pages={023003} }
@article{kung_lee_chen_kemper_sorini_moritz_devereaux_2013, title={Time-dependent charge-order and spin-order recovery in striped systems}, volume={88}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000324231000003&KeyUID=WOS:000324231000003}, DOI={10.1103/physrevb.88.125114}, abstractNote={Using time-dependent Ginzburg-Landau theory, we study the role of amplitude and phase fluctuations in the recovery of charge and spin stripe phases in response to a pump pulse that melts the orders. For parameters relevant to the case where charge order precedes spin order thermodynamically, amplitude recovery governs the initial time scales, while phase recovery controls behavior at longer times. In addition to these intrinsic effects, there is a longer spin re-orientation time scale related to the scattering geometry that dominates the recovery of the spin phase. Coupling between the charge and spin orders locks the amplitude and similarly the phase recovery, reducing the number of distinct time scales. Our results well reproduce the major experimental features of pump-probe x-ray diffraction measurements on the striped nickelate La$_{1.75}$Sr$_{0.25}$NiO$_4$. They highlight the main idea of this work, which is the use of time-dependent Ginzburg-Landau theory to study systems with multiple coexisting order parameters.}, number={12}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Kung, Y. F. and Lee, W.-S. and Chen, C.-C. and Kemper, A. F. and Sorini, A. P. and Moritz, B. and Devereaux, T. P.}, year={2013} }
@article{plonka_kemper_graser_kampf_devereaux_2013, title={Tunneling spectroscopy for probing orbital anisotropy in iron pnictides}, volume={88}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000327933500007&KeyUID=WOS:000327933500007}, DOI={10.1103/physrevb.88.174518}, abstractNote={Using realistic multi-orbital tight-binding Hamiltonians and the T-matrix formalism, we explore the effects of a non-magnetic impurity on the local density of states in Fe-based compounds. We show that scanning tunneling spectroscopy (STS) has very specific anisotropic signatures that track the evolution of orbital splitting (OS) and antiferromagnetic gaps. Both anisotropies exhibit two patterns that split in energy with decreasing temperature, but for OS these two patterns map onto each other under 90 degree rotation. STS experiments that observe these signatures should expose the underlying magnetic and orbital order as a function of temperature across various phase transitions.}, number={17}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Plonka, N. and Kemper, A. F. and Graser, S. and Kampf, A. P. and Devereaux, T. P.}, year={2013} }
@article{srivastava_wang_kemper_cheng_2012, title={Density functional study of gold and iron clusters on perfect and defected graphene}, volume={85}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000303236200006&KeyUID=WOS:000303236200006}, DOI={10.1103/physrevb.85.165444}, abstractNote={Metal clusters adsorbed on graphene can give rise to interesting physical properties. Using density-functional theory calculations, we investigate Au${}_{n}$ and Fe${}_{n}$ ($n=1--5$) clusters adsorbed on perfect and defected graphene with a single vacancy. With the exception of Fe clusters on defected graphene, clusters are bonded to graphene through an anchor atom. Geometries of clusters on graphene are similar to their free-standing structures except for the Fe${}_{5}$ cluster on perfect graphene. Compared to Au, Fe clusters are more strongly bonded to graphene. We find that it is important to include long-range van der Waals interactions for Au clusters adsorbed on perfect graphene. An Au${}_{5}$ cluster becomes parallel to the graphene only when the van der Waals interactions are taken into account. Charge transfer between clusters and graphene shows strong size dependency, and the amount is larger in the presence of the single vacancy on the graphene than a pristine sheet. Perfect graphene is found to be doped for Au clusters with an odd number of atoms and undoped with an even number of atoms. Magnetic moments are also calculated as a function of cluster size and an odd-even oscillation is observed in Au${}_{n}$-perfect as well as defected graphene system. While Fe${}_{n}$ clusters remain to be magnetic for all $n$, the spin of a single Fe atom on a defect site is very small due to a covalent bonding to C atoms.}, number={16}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Srivastava, Manoj K. and Wang, Yan and Kemper, A. F. and Cheng, Hai-Ping}, year={2012} }
@article{lee_chuang_moore_zhu_patthey_trigo_lu_kirchmann_krupin_yi_et al._2012, title={Phase fluctuations and the absence of topological defects in a photo-excited charge-ordered nickelate}, volume={3}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000304611400033&KeyUID=WOS:000304611400033}, DOI={10.1038/ncomms1837}, abstractNote={The dynamics of an order parameter's amplitude and phase determines the collective behaviour of novel states emerged in complex materials. Time- and momentum-resolved pump-probe spectroscopy, by virtue of its ability to measure material properties at atomic and electronic time scales and create excited states not accessible by the conventional means can decouple entangled degrees of freedom by visualizing their corresponding dynamics in the time domain. Here, combining time-resolved femotosecond optical and resonant x-ray diffraction measurements on striped La1.75Sr0.25NiO4, we reveal unforeseen photo-induced phase fluctuations of the charge order parameter. Such fluctuations preserve long-range order without creating topological defects, unlike thermal phase fluctuations near the critical temperature in equilibrium10. Importantly, relaxation of the phase fluctuations are found to be an order of magnitude slower than that of the order parameter's amplitude fluctuations, and thus limit charge order recovery. This discovery of new aspect to phase fluctuation provides more holistic view for the importance of phase in ordering phenomena of quantum matter.}, journal={Nature Communications}, publisher={Nature Publishing Group}, author={Lee, W.S. and Chuang, Y.D. and Moore, R.G. and Zhu, Y. and Patthey, L. and Trigo, M. and Lu, D.H. and Kirchmann, P.S. and Krupin, O. and Yi, M. and et al.}, year={2012}, pages={838} }
@article{freericks_liu_kemper_devereaux_2012, title={Pulsed high harmonic generation of light due to pumped Bloch oscillations in noninteracting metals}, volume={T151}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000311961700063&KeyUID=WOS:000311961700063}, DOI={10.1088/0031-8949/2012/t151/014062}, abstractNote={We derive a simple theory for high-order harmonic generation due to pumping a noninteracting metal with a large amplitude oscillating electric field. The model assumes that the radiated light field arises from the acceleration of electrons due to the time-varying current generated by the pump, and also assumes that the system has a constant density of photoexcited carriers, hence it ignores the dipole excitation between bands (which would create carriers in semiconductors). We examine the circumstances under which odd harmonic frequencies would be expected to dominate the spectrum of radiated light, and we also apply the model to real materials like ZnO, for which high-order harmonic generation has already been demonstrated in experiments.}, journal={Phys. Scr.}, publisher={IOP Publishing}, author={Freericks, J K and Liu, A Y and Kemper, A F and Devereaux, T P}, year={2012}, pages={014062} }
@article{kemper_korshunov_devereaux_fry_cheng_hirschfeld_2011, title={Anisotropic quasiparticle lifetimes in Fe-based superconductors}, volume={83}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000290864400005&KeyUID=WOS:000290864400005}, DOI={10.1103/physrevb.83.184516}, abstractNote={We study the dynamical quasiparticle scattering by spin and charge fluctuations in Fe-based pnictides within a five-orbital model with on-site interactions. The leading contribution to the scattering rate is calculated from the second-order diagrams with the polarization operator calculated in the random-phase approximation. We find one-particle scattering rates which are highly anisotropic on each Fermi surface sheet due to the momentum dependence of the spin susceptibility and the multi-orbital composition of each Fermi pocket. This fact, combined with the anisotropy of the effective mass, produces disparity between electrons and holes in conductivity, the Hall coefficient, and the Raman initial slope, in qualitative agreement with experimental data.}, number={18}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Kemper, A. F. and Korshunov, M. M. and Devereaux, T. P. and Fry, J. N. and Cheng, H-P. and Hirschfeld, P. J.}, year={2011} }
@article{yi_lu_chu_analytis_sorini_kemper_moritz_mo_moore_hashimoto_et al._2011, title={Symmetry-breaking orbital anisotropy observed for detwinned Ba(Fe1-xCox)2As2 above the spin density wave transition}, volume={108}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000289888500040&KeyUID=WOS:000289888500040}, DOI={10.1073/pnas.1015572108}, abstractNote={
Nematicity, defined as broken rotational symmetry, has recently been observed in competing phases proximate to the superconducting phase in the cuprate high-temperature superconductors. Similarly, the new iron-based high-temperature superconductors exhibit a tetragonal-to-orthorhombic structural transition (i.e., a broken C
4
symmetry) that either precedes or is coincident with a collinear spin density wave (SDW) transition in undoped parent compounds, and superconductivity arises when both transitions are suppressed via doping. Evidence for strong in-plane anisotropy in the SDW state in this family of compounds has been reported by neutron scattering, scanning tunneling microscopy, and transport measurements. Here, we present an angle-resolved photoemission spectroscopy study of detwinned single crystals of a representative family of electron-doped iron-arsenide superconductors, Ba(Fe
1-
x
Co
x
)
2
As
2
in the underdoped region. The crystals were detwinned via application of in-plane uniaxial stress, enabling measurements of single domain electronic structure in the orthorhombic state. At low temperatures, our results clearly demonstrate an in-plane electronic anisotropy characterized by a large energy splitting of two orthogonal bands with dominant
d
xz
and
d
yz
character, which is consistent with anisotropy observed by other probes. For compositions
x
> 0, for which the structural transition (
T
S
) precedes the magnetic transition (
T
SDW
), an anisotropic splitting is observed to develop above
T
SDW
, indicating that it is specifically associated with
T
S
. For unstressed crystals, the band splitting is observed close to
T
S
, whereas for stressed crystals, the splitting is observed to considerably higher temperatures, revealing the presence of a surprisingly large in-plane nematic susceptibility in the electronic structure.
}, number={17}, journal={Proceedings of the National Academy of Sciences}, publisher={Proceedings of the National Academy of Sciences}, author={Yi, M. and Lu, D. and Chu, J.-H. and Analytis, J. G. and Sorini, A. P. and Kemper, A. F. and Moritz, B. and Mo, S.-K. and Moore, R. G. and Hashimoto, M. and et al.}, year={2011}, pages={6878â6883} }
@article{chen_jia_kemper_singh_devereaux_2011, title={Theory of Two-Magnon Raman Scattering in Iron Pnictides and Chalcogenides}, volume={106}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000287196900011&KeyUID=WOS:000287196900011}, DOI={10.1103/physrevlett.106.067002}, abstractNote={Although the parent iron-based pnictides and chalcogenides are itinerant antiferromagnets, the use of local moment picture to understand their magnetic properties is still widespread. We study magnetic Raman scattering from a local moment perspective for various quantum spin models proposed for this new class of superconductors. These models vary greatly in the level of magnetic frustration and show a vastly different two-magnon Raman response. Light scattering by two-magnon excitations thus provides a robust and independent measure of the underlying spin interactions. In accord with other recent experiments, our results indicate that the amount of magnetic frustration in these systems may be small.}, number={6}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Chen, C.-C. and Jia, C. J. and Kemper, A. F. and Singh, R. R. P. and Devereaux, T. P.}, year={2011} }
@article{kemper_cao_hirschfeld_cheng_2010, title={Erratum: Effects of cobalt doping and three-dimensionality in BaFe_{2}As_{2} [Phys. Rev. B 80, 104511 (2009)]}, volume={81}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000278386900007&KeyUID=WOS:000278386900007}, DOI={10.1103/physrevb.81.229902}, number={22}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Kemper, A. F. and Cao, C. and Hirschfeld, P. J. and Cheng, H.-P.}, year={2010} }
@article{kemper_maier_graser_cheng_hirschfeld_scalapino_2010, title={Sensitivity of the superconducting state and magnetic susceptibility to key aspects of electronic structure in ferropnictides}, volume={12}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000281232800003&KeyUID=WOS:000281232800003}, DOI={10.1088/1367-2630/12/7/073030}, abstractNote={Experiments on the iron–pnictide superconductors appear to show some materials where the ground state is fully gapped, and others where low-energy excitations dominate, possibly indicative of gap nodes. Within the framework of a five-orbital spin fluctuation theory for these systems, we discuss how changes in the doping, the electronic structure or interaction parameters can tune the system from a fully gapped to a nodal sign-changing gap with s-wave (A1g) symmetry (s±). In particular, we focus on the role of the hole pocket at the (π, π) point of the unfolded Brillouin zone, identified as crucial to the pairing by Kuroki et al (2009 Phys. Rev. B 79 224511), and show that its presence leads to additional nesting of hole and electron pockets, which stabilizes the isotropic s± state. The pocket's contribution to the pairing can be tuned by doping, surface effects and by changes in interaction parameters, which we examine. Analytic expressions for orbital pairing vertices calculated within the random phase approximation (RPA) fluctuation exchange approximation allow us to draw connections between aspects of the electronic structure, interaction parameters and the form of the superconducting gap.}, number={7}, journal={New Journal of Physics}, publisher={IOP Publishing}, author={Kemper, A F and Maier, T A and Graser, S and Cheng, H-P and Hirschfeld, P J and Scalapino, D J}, year={2010}, pages={073030} }
@article{graser_kemper_maier_cheng_hirschfeld_scalapino_2010, title={Spin fluctuations and superconductivity in a three-dimensional tight-binding model for BaFe_{2}As_{2}}, volume={81}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000278386700006&KeyUID=WOS:000278386700006}, DOI={10.1103/physrevb.81.214503}, abstractNote={Despite the wealth of experimental data on the Fe-pnictide compounds of the KFe2As2 type, K=Ba, Ca, or Sr, the main theoretical work based on multiorbital tight-binding models has been restricted so far to the study of the related 1111 compounds. This can be ascribed to the more three-dimensional electronic structure found by ab initio calculations for the 122 materials, making this system less amenable to model development. In addition, the more complicated Brillouin zone BZ of the body-centered tetragonal symmetry does not allow a straightforward unfolding of the electronic band structure into an effective 1Fe/unit cell BZ. Here we present an effective five-orbital tight-binding fit of the full density functional theory band structure for BaFe2As2 including the kz dispersions. We compare the five-orbital spin fluctuation model to one previously studied for LaOFeAs and calculate the random-phase approximation enhanced susceptibility. Using the fluctuation ex- change approximation to determine the leading pairing instability, we then examine the differences between a strictly two-dimensional model calculation over a single kz cut of the BZ and a completely three-dimensional approach. We find pairing states quite similar to the 1111 materials, with generic quasi-isotropic pairing on the hole sheets and nodal states on the electron sheetsmore » at kz=0, which however are gapped as the system is hole doped. On the other hand, a substantial kz dependence of the order parameter remains, with most of the pairing strength deriving from processes near kz=?. These states exhibit a tendency for an enhanced anisotropy on the hole sheets and a reduced anisotropy on the electron sheets near the top of the BZ.« less}, number={21}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Graser, S. and Kemper, A. F. and Maier, T. A. and Cheng, H.-P. and Hirschfeld, P. J. and Scalapino, D. J.}, year={2010} }
@article{kemper_cheng_kébaïli_benrezzak_schmidt_masson_bréchignac_2009, title={Curvature effect on the interaction between folded graphitic surface and silver clusters}, volume={79}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000266501300025&KeyUID=WOS:000266501300025}, DOI={10.1103/physrevb.79.193403}, abstractNote={Evidence of curvature effects on the interaction and binding of silver clusters on folded graphitic surfaces has been shown from both experiment and theory. Density Functional Theory (DFT) calculations within the local density and generalized gradient approximations have been performed for the structural relaxation of both Ag and Ag$_2$ on curved surfaces, showing a cross-over from quantum to classical behaviour. Using Lennard-Jones potential to model the interaction between a single cluster and the graphene surface, evidence is found for the curvature effect on the binding of silver nano-particles to folding graphitic surfaces. The theoretical results are compared to SEM and AFM images of samples obtained from pre-formed silver cluster deposition on carboneous substrates exhibiting anisotropic pleat structures.}, number={19}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Kemper, A. F. and Cheng, H-P. and Kébaïli, N. and Benrezzak, S. and Schmidt, M. and Masson, A. and Bréchignac, C.}, year={2009}, month={May} }
@article{kemper_cao_hirschfeld_cheng_2009, title={Effects of cobalt doping and three-dimensionality in BaFe2As2}, volume={80}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000270383100094&KeyUID=WOS:000270383100094}, DOI={10.1103/physrevb.80.104511}, abstractNote={We investigate the dual roles of a cobalt impurity in the Ba-122 ferropnictide superconductor in the state with coexisting collinear spin-density wave (SDW) order as a dopant and as a scattering center, using first-principles electronic-structure methods. The Co atom is found to dope the FeAs plane where it is located with a single delocalized electron as expected, but also induces a strong perturbation of the SDW ground state of the system. This in turn induces a stripelike modulation of the density of states in nearby planes which may be observable in scanning tunnel microscope (STM) experiments. The defect is found to have an intermediate strength nonmagnetic scattering potential with a range of roughly $1\text{ }\text{\AA{}}$, and the Co gives rise to a smaller but longer range magnetic scattering potential. The impurity potential in both channels is highly anisotropic, reflecting the broken symmetry of the SDW ground state. We give values for the effective Co potentials for each $d$ orbital on the impurity and nearby sites. The calculation also shows a clear local resonance comprised of Co states about 200 meV above the Fermi level, in quantitative agreement with a recent report from STM. Finally, we discuss the issue of the effective dimensionality of the 122 materials, and show that the hybridization of the out-of-phase As atoms leads to a higher density of states between the FeAs planes relative to the 1111 counterparts.}, number={10}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Kemper, A. and Cao, C. and Hirschfeld, P. and Cheng, H.-P.}, year={2009} }
@article{kemper_doluweera_maier_jarrell_hirschfeld_cheng_2009, title={Insensitivity of d-wave pairing to disorder in the high-temperature cuprate superconductors}, volume={79}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000264768600096&KeyUID=WOS:000264768600096}, DOI={10.1103/physrevb.79.104502}, abstractNote={Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831, USA(Dated: February 2, 2009)Using a dynamical cluster quantum Monte Carlo approximation, we investigate the effect of localdisorder on the stability of d-wave superconductivity including the effect of electronic correlationsin both particle-particle and particle-hole channels. With increasing impurity potential, we findan initial rise of the critical temperature due to an enhancement of antiferromagnetic spin correla-tions, followed by a decrease of T}, number={10}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Kemper, A. and Doluweera, D. and Maier, T. and Jarrell, M. and Hirschfeld, P. and Cheng, H-P.}, year={2009} }
@article{cao_kemper_agapito_zhang_he_rinzler_cheng_zhang_rocha_sanvito_2009, title={Nonequilibrium Green’s function study of Pd4-cluster-functionalized carbon nanotubes as hydrogen sensors}, volume={79}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000263815800049&KeyUID=WOS:000263815800049}, DOI={10.1103/physrevb.79.075127}, abstractNote={Pd-cluster-functionalized carbon nanotubes (CNTs) have been shown experimentally to be effective hydrogen sensors. Semiconducting CNTs exhibit much higher sensitivity than ensemble (mixed) ones. Using the non-equilibrium Green's function method combined with the density functional theory, we simulate and contrast the (8,0) semiconducting and the (5,5) metallic CNT model systems. We find that the electron localization effect plays a crucial role in determining electron transport. Pd clusters and hydrogen adsorption causes opposite effects on electron localization in the CNT backbone for the semiconducting CNT based systems. Consequently Pd-functionalization dramatically increases the conductance but then it is strongly supressed by hydrogen absorption. For the metallic CNT based systems, there is a tiny shift of the transmission peak near the Fermi energy. These results offer a consistent explanation for the experiments.}, number={7}, journal={Phys. Rev. B}, publisher={American Physical Society (APS)}, author={Cao, Chao and Kemper, A. and Agapito, Luis and Zhang, Jian-Wei and He, Yao and Rinzler, Andrew and Cheng, Hai-Ping and Zhang, X.-G. and Rocha, Alexandre and Sanvito, Stefano}, year={2009}, month={Feb} }