@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{sun_yang_yang_kumar_vetter_xue_li_li_li_zhang_et al._2020, title={Visualizing Tailored Spin Phenomena in a Reduced-Dimensional Topological Superlattice}, volume={32}, ISSN={["1521-4095"]}, DOI={10.1002/adma.202005315}, abstractNote={Emergent topological insulators (TIs) and their design are in high demand for manipulating and transmitting spin information toward ultralow‐power‐consumption spintronic applications. Here, distinct topological states with tailored spin properties can be achieved in a single reduced‐dimensional TI‐superlattice, (Bi2/Bi2Se3)‐(Bi2/Bi2Se3)N or (□/Bi2Se3)‐(Bi2/Bi2Se3)N (N is the repeating unit, □ represents an empty layer) by controlling the termination via molecular beam epitaxy. The Bi2‐terminated superlattice exhibits a single Dirac cone with a spin momentum splitting ≈0.5 Å−1, producing a pronounced inverse Edelstein effect with a coherence length up to 1.26 nm. In contrast, the Bi2Se3‐terminated superlattice is identified as a dual TI protected by coexisting time reversal and mirror symmetries, showing an unexpectedly long spin lifetime up to 1 ns. The work elucidates the key role of dimensionality and dual topological phases in selecting desired spin properties, suggesting a promise route for engineering topological superlattices for high‐performance TI‐spintronic devices.}, number={49}, journal={ADVANCED MATERIALS}, author={Sun, Rui and Yang, Shijia and Yang, Xu and Kumar, A. and Vetter, Eric and Xue, Wenhua and Li, Yan and Li, Na and Li, Yang and Zhang, Shihao and et al.}, year={2020}, month={Dec} }
 @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{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} }