@article{arnault_al-tawhid_salmani-rezaie_muller_kumah_bahramy_finkelstein_ahadi_2023, title={Anisotropic superconductivity at KTaO3(111) interfaces}, volume={9}, ISSN={["2375-2548"]}, url={https://doi.org/10.1126/sciadv.adf1414}, DOI={10.1126/sciadv.adf1414}, abstractNote={A two-dimensional, anisotropic superconductivity was recently found at the KTaO 3 (111) interfaces. The nature of the anisotropic superconducting transition remains a subject of debate. To investigate the origins of the observed behavior, we grew epitaxial KTaO 3 (111)-based heterostructures. We show that the superconductivity is robust against the in-plane magnetic field and violates the Pauli limit. We also show that the Cooper pairs are more resilient when the bias is along [11 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mn>2</mml:mn> <mml:mo accent="false">¯</mml:mo> </mml:mover> </mml:math> ] (I ∥ [11 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mn>2</mml:mn> <mml:mo accent="false">¯</mml:mo> </mml:mover> </mml:math> ]) and the magnetic field is along [1 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mn>1</mml:mn> <mml:mo accent="false">¯</mml:mo> </mml:mover> </mml:math> 0] ( B ∥ [1 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mn>1</mml:mn> <mml:mo accent="false">¯</mml:mo> </mml:mover> </mml:math> 0]). We discuss the anisotropic nature of superconductivity in the context of electronic structure, orbital character, and spin texture at the KTaO 3 (111) interfaces. The results point to future opportunities to enhance superconducting transition temperatures and critical fields in crystalline, two-dimensional superconductors with strong spin-orbit coupling.}, number={7}, journal={SCIENCE ADVANCES}, author={Arnault, Ethan G. and Al-Tawhid, Athby H. and Salmani-Rezaie, Salva and Muller, David A. and Kumah, Divine P. and Bahramy, Mohammad S. and Finkelstein, Gleb and Ahadi, Kaveh}, year={2023}, month={Feb} }
 @article{al-tawhid_poage_salmani-rezaie_gonzalez_chikara_muller_kumah_gastiasoro_lorenzana_ahadi_2023, title={Enhanced Critical Field of Superconductivity at an Oxide Interface}, volume={7}, ISSN={["1530-6992"]}, DOI={10.1021/acs.nanolett.3c01571}, abstractNote={The nature of superconductivity and its interplay with strong spin-orbit coupling at the KTaO3(111) interfaces remains a subject of debate. To address this problem, we grew epitaxial LaMnO3/KTaO3(111) heterostructures. We show that superconductivity is robust against the in-plane magnetic field, with the critical field of superconductivity reaching 25 T in optimally doped heterostructures. The superconducting order parameter is highly sensitive to carrier density. We argue that spin-orbit coupling drives the formation of anomalous quasiparticles with vanishing magnetic moment, providing the condensate significant immunity against magnetic fields beyond the Pauli paramagnetic limit. These results offer design opportunities for superconductors with extreme resilience against magnetic field.}, journal={NANO LETTERS}, author={Al-Tawhid, Athby H. H. and Poage, Samuel J. J. and Salmani-Rezaie, Salva and Gonzalez, Antonio and Chikara, Shalinee and Muller, David A. A. and Kumah, DivineP. P. and Gastiasoro, Maria N. N. and Lorenzana, Josei and Ahadi, Kaveh}, year={2023}, month={Jul} }
 @article{negi_kim_hua_timofeeva_zhang_zhu_peters_kumah_jiang_liu_2023, title={Ferroelectric Domain Wall Engineering Enables Thermal Modulation in PMN-PT Single Crystals}, volume={4}, ISSN={["1521-4095"]}, url={https://doi.org/10.1002/adma.202211286}, DOI={10.1002/adma.202211286}, abstractNote={Acting like thermal resistances, ferroelectric domain walls can be manipulated to realize dynamic modulation of thermal conductivity (k), which is essential for developing novel phononic circuits. Despite the interest, little attention has been paid to achieving room-temperature thermal modulation in bulk materials due to challenges in obtaining a high thermal conductivity switching ratio (khigh /klow ), particularly in commercially viable materials. Here, room-temperature thermal modulation in 2.5 mm-thick Pb(Mg1/3 Nb2/3 )O3 -xPbTiO3 (PMN-xPT) single crystals is demonstrated. With the use of advanced poling conditions, assisted by the systematic study on composition and orientation dependence of PMN-xPT, a range of thermal conductivity switching ratios with a maximum of ≈1.27 is observed. Simultaneous measurements of piezoelectric coefficient (d33 ) to characterize the poling state, domain wall density using polarized light microscopy (PLM), and birefringence change using quantitative PLM reveal that compared to the unpoled state, the domain wall density at intermediate poling states (0< d33 <d33,max ) is lower due to the enlargement in domain size. At optimized poling conditions (d33,max ), the domain sizes show increased inhomogeneity that leads to enhancement in the domain wall density. This work highlights the potential of commercially available PMN-xPT single crystals among other relaxor-ferroelectrics for achieving temperature control in solid-state devices.}, journal={ADVANCED MATERIALS}, author={Negi, Ankit and Kim, Hwang Pill and Hua, Zilong and Timofeeva, Anastasia and Zhang, Xuanyi and Zhu, Yong and Peters, Kara and Kumah, Divine and Jiang, Xiaoning and Liu, Jun}, year={2023}, month={Apr} }
 @article{yang_zhang_reynolds_kumah_xu_2023, title={The Role of Carbon Content: A Comparison of the Nickel Particle Size and Magnetic Property of Nickel/Polysiloxane-Derived Silicon Oxycarbide}, volume={1}, ISSN={["1527-2648"]}, url={https://doi.org/10.1002/adem.202201453}, DOI={10.1002/adem.202201453}, abstractNote={A facile and novel processable method to synthesize the Ni nanoparticles (Ni NPs) by tailoring their size in the matrix of the silicon oxycarbide (SiOC) ceramic system is reported. This method is based on polymer‐derived ceramics (PDCs), instead of the conventional powder route. The specific structural characteristics and magnetic properties of the various Ni NPs/SiOC composites as a function of carbon content are systematically investigated. The magnetic properties are experimentally investigated as a function of NP size and measurement temperature. It is demonstrated that the change in the size of Ni NPs (average from ≈4 to ≈ 19 nm) determines the magnetic nature of superparamagnetism. Zero‐field‐cooled (ZFC) and field‐cooled (FC) magnetization studies under magnetic fields of 100 Oe are performed. The saturated M versus H ( M – H ) loops (saturation magnetization) increase and the coercivity decreases with the size reduction of Ni NPs. It is an indicator of the presence of superparamagnetic behavior and single‐domain NP for ceramic materials.}, journal={ADVANCED ENGINEERING MATERIALS}, author={Yang, Ni and Zhang, Xuanyi and Reynolds, Lewis and Kumah, Divine and Xu, Chengying}, year={2023}, month={Jan} }
 @article{negi_rodriguez_zhang_comstock_yang_sun_jiang_kumah_hu_liu_2023, title={Thickness-Dependent Thermal Conductivity and Phonon Mean Free Path Distribution in Single-Crystalline Barium Titanate}, volume={4}, ISSN={["2198-3844"]}, DOI={10.1002/advs.202301273}, abstractNote={Abstract Nanosized perovskite ferroelectrics are widely employed in several electromechanical, photonics, and thermoelectric applications. Scaling of ferroelectric materials entails a severe reduction in the lattice (phonon) thermal conductivity, particularly at sub‐100 nm length scales. Such thermal conductivity reduction can be accurately predicted using the information of phonon mean free path (MFP) distribution. The current understanding of phonon MFP distribution in perovskite ferroelectrics is still inconclusive despite the critical thermal management implications. Here, high‐quality single‐crystalline barium titanate (BTO) thin films, a representative perovskite ferroelectric material, are grown at several thicknesses. Using experimental thermal conductivity measurements and first‐principles based modeling (including four‐phonon scattering), the phonon MFP distribution is determined in BTO. The simulation results agree with the measured thickness‐dependent thermal conductivity. The results show that the phonons with sub‐100 nm MFP dominate the thermal transport in BTO, and phonons with MFP exceeding 10 nm contribute ≈35% to the total thermal conductivity, in significant contrast to previously published experimental results. The experimentally validated phonon MFP distribution is consistent with the theoretical predictions of other complex crystals with strong anharmonicity. This work paves the way for thermal management in nanostructured and ferroelectric‐domain‐engineered systems for oxide perovskite‐based functional materials.}, journal={ADVANCED SCIENCE}, author={Negi, Ankit and Rodriguez, Alejandro and Zhang, Xuanyi and Comstock, Andrew H. H. and Yang, Cong and Sun, Dali and Jiang, Xiaoning and Kumah, Divine and Hu, Ming and Liu, Jun}, year={2023}, month={Apr} }
 @article{chen_ahmadi-majlan_lim_zhang_ngai_kumah_2022, title={Effect of buffer termination on intermixing and conductivity in LaTiO3/SrTiO3 heterostructures integrated on Si(100)}, volume={40}, ISSN={["1520-8559"]}, url={https://doi.org/10.1116/6.0001464}, DOI={10.1116/6.0001464}, abstractNote={The control of chemical exchange across heterointerfaces formed between ultrathin functional transition-metal oxide layers provides an effective route to manipulate the electronic properties of these systems. By determining the layer-resolved structural profile across the interface between the Mott insulator, LaTiO3 (LTO) grown epitaxially on SrTiO3 (STO)-buffered silicon by molecular beam epitaxy, we find that interfacial cationic exchange depends on the surface termination of the strained STO buffer. Using a combination of temperature-dependent transport and synchrotron x-ray crystal truncation rods and reciprocal space mapping, an enhanced conductivity in STO/LTO/SrO-terminated STO buffers compared to heterostructures with TiO2-terminated STO buffers is correlated with La/Sr exchange and the formation of metallic La1−xSrxTiO3. La/Sr exchange effectively reduces the strain energy of the system due to the large lattice mismatch between the nominal oxide layers and the Si substrate.}, number={1}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Chen, Tongjie and Ahmadi-Majlan, Kamyar and Lim, Zheng Hui and Zhang, Zhan and Ngai, Joseph H. and Kumah, Divine P.}, year={2022}, month={Jan} }
 @article{al-tawhid_kanter_hatefipour_irving_kumah_shabani_ahadi_2022, title={Oxygen Vacancy-Induced Anomalous Hall Effect in a Nominally Non-magnetic Oxide}, volume={9}, ISSN={["1543-186X"]}, url={https://doi.org/10.1007/s11664-022-09941-9}, DOI={10.1007/s11664-022-09941-9}, abstractNote={The anomalous Hall effect, a hallmark of broken time-reversal symmetry and spin–orbit coupling, is frequently observed in magnetically polarized systems. However, its realization in nominally non-magnetic systems remains elusive. Here, we report on the observation of the anomalous Hall effect in nominally non-magnetic KTaO3. Anomalous Hall effect emerges in reduced KTaO3 and shows an extrinsic to intrinsic crossover. A paramagnetic behavior is observed in reduced samples and confirmed using first principles calculations. The observed anomalous Hall effect follows the oxygen vacancy-induced magnetization response, suggesting that the localized magnetic moments of the oxygen vacancies scatter conduction electrons asymmetrically and give rise to the anomalous Hall effect. The anomalous Hall conductivity is found to be insensitive to the scattering rate in the low temperature limit (T < 5 K), implying that the Berry curvature of the electrons on the Fermi surface controls the anomalous Hall effect. Our observations provide a detailed picture of many-body interactions, which trigger the anomalous Hall effect in a non-magnetic system.}, journal={JOURNAL OF ELECTRONIC MATERIALS}, author={Al-Tawhid, Athby H. and Kanter, Jesse and Hatefipour, Mehdi and Irving, Douglas L. and Kumah, Divine P. and Shabani, Javad and Ahadi, Kaveh}, year={2022}, month={Sep} }
 @article{ozbek_brooks_zhang_al-tawhid_stoica_zhang_kumah_2022, title={Superconducting phase of TixOy thin films grown by molecular beam epitaxy}, volume={6}, ISSN={["2475-9953"]}, url={https://doi.org/10.1103/PhysRevMaterials.6.064805}, DOI={10.1103/PhysRevMaterials.6.064805}, abstractNote={We investigate the complex relationship between the growth conditions and the structural and transport properties of $\mathrm{Ti_xO_y}$ thin films grown by molecular beam epitaxy. Transport properties ranging from metallicity to superconductivity and insulating states are stabilized by effectively tuning the O/Ti ratio via the Ti flux rate and the O partial pressure, $P_{Ox}$, for films grown on (0001)-$\mathrm{Al_2 O_3}$ substrates at 850$^{\circ}$ C. A cubic $c-\mathrm{TiO_{1\pm\delta}}$ buffer layer is formed for low O/Ti ratios while a corundum cr-$\mathrm{Ti_2 O_3}$ layer is formed under higher oxidizing conditions. Metallicity is observed for c-$\mathrm{TiO_{1-\delta}}$ buffer layers. The superconducting $\mathrm{\gamma-Ti_3 O_5}$ Magn\'eli phase is found to nucleate on a c-$\mathrm{TiO_{1-\delta}}$ buffer for intermediate $P_{Ox}$ conditions and an insulator-superconducting transition is observed at 4.5 K (T$_C^{onset}=6 K$) for 85 nm thick films. Strain relaxation of the $\mathrm{\gamma-Ti_3 O_5}$ occurs with increasing film thickness and correlates with a thickness-dependent increase in T$_C$ observed for $\mathrm{Ti_xO_y}$ thin films.}, number={6}, journal={PHYSICAL REVIEW MATERIALS}, author={Ozbek, Yasemin and Brooks, Cooper and Zhang, Xuanyi and Al-Tawhid, Athby and Stoica, Vladmir A. and Zhang, Zhan and Kumah, Divine P.}, year={2022}, month={Jun} }
 @article{al-tawhid_kanter_hatefipour_kumah_shabani_ahadi_2022, title={Superconductivity and Weak Anti-localization at KTaO3 (111) Interfaces}, volume={8}, ISSN={["1543-186X"]}, url={https://doi.org/10.1007/s11664-022-09844-9}, DOI={10.1007/s11664-022-09844-9}, abstractNote={The intersection of two-dimensional superconductivity and topologically nontrivial states hosts a wide range of quantum phenomena, including Majorana fermions. We report on the observation of two-dimensional superconductivity and weak anti-localization at the TiOx/KTaO3 (111) interfaces. A remnant, saturating resistance persists below the transition temperature as superconducting puddles fail to reach phase coherence. Signatures of weak anti-localization are observed near the superconducting transition, suggesting the coexistence of superconducting fluctuations and quantum coherent quasiparticle effects. The superconducting interfaces show roughly one order of magnitude larger weak anti-localization correction, compared to non-superconducting interfaces, alluding to a relatively large coherence length in these interfaces.}, journal={JOURNAL OF ELECTRONIC MATERIALS}, author={Al-Tawhid, Athby H. and Kanter, Jesse and Hatefipour, Mehdi and Kumah, Divine P. and Shabani, Javad and Ahadi, Kaveh}, year={2022}, month={Aug} }
 @article{zhang_penn_wysocki_zhang_loosdrecht_kornblum_lebeau_lindfors-vrejoiu_kumah_2022, title={Thickness and temperature dependence of the atomic-scale structure of SrRuO3 thin films}, volume={10}, ISSN={["2166-532X"]}, url={https://doi.org/10.1063/5.0087791}, DOI={10.1063/5.0087791}, abstractNote={The temperature-dependent layer-resolved structure of 3 to 44 unit cell thick SrRuO3 (SRO) films grown on Nb-doped SrTiO3 substrates is investigated using a combination of high-resolution synchrotron x-ray diffraction and high-resolution electron microscopy to understand the role that structural distortions play in suppressing ferromagnetism in ultra-thin SRO films. The oxygen octahedral tilts and rotations and Sr displacements characteristic of the bulk orthorhombic phase are found to be strongly dependent on temperature, the film thickness, and the distance away from the film–substrate interface. For thicknesses, t, above the critical thickness for ferromagnetism (t > 3 uc), the orthorhombic distortions decrease with increasing temperature above TC. Below TC, the structure of the films remains constant due to the magneto-structural coupling observed in bulk SRO. The orthorhombic distortions are found to be suppressed in the 2–3 interfacial layers due to structural coupling with the SrTiO3 substrate and correlate with the critical thickness for ferromagnetism in uncapped SRO films.}, number={5}, journal={APL MATERIALS}, author={Zhang, Xuanyi and Penn, Aubrey N. and Wysocki, Lena and Zhang, Zhan and Loosdrecht, Paul H. M. and Kornblum, Lior and LeBeau, James M. and Lindfors-Vrejoiu, Ionela and Kumah, Divine P.}, year={2022}, month={May} }
 @article{koohfar_ozbek_bland_zhang_kumah_2021, title={Interface-driven magnetic anisotropy in relaxed La0.7Sr0.3CrO3/La0.7Sr0.3MnO3 heterostructures on MgO}, volume={129}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0033236}, DOI={10.1063/5.0033236}, abstractNote={We investigate the structural and magnetic properties of La$_{0.7}$Sr$_{0.3}$CrO$_3$ (LSCO)/La$_{0.7}$Sr$_{0.3}$MnO$_3$(LSMO) heterostructures grown on (001)-oriented MgO by molecular beam epitaxy. Due to the large film-substrate lattice mismatch, strain relaxation is found to occur within the first 2-3 unit cells (uc) of the film as evidenced by reflection high energy electron diffraction and high-resolution synchrotron X-ray reciprocal space mapping. We find that the presence of the LSCO spacer and capping layers leads to ferromagnetism in ultra-thin LSMO layers with thicknesses on the order of 2 uc with the magnetic easy axis oriented in the film plane. Net magnetic moments of 1.4 and 2.4 $\mu_B$/Mn are measured for [2 uc LSCO/ 2 uc LSMO] and [2 uc LSCO/ 4 uc LSMO] superlattices, respectively by SQUID magnetometry. The effective magnetic anisotropy of the relaxed [2 uc LSCO/ 4 uc LSMO] heterostructure is found to be an order of magnitude higher than bulk LSMO highlighting the critical role of interfacial magnetic exchange interactions in tuning magnetic anisotropy at complex oxide interfaces.}, number={5}, journal={JOURNAL OF APPLIED PHYSICS}, author={Koohfar, Sanaz and Ozbek, Yasemin and Bland, Hayden and Zhang, Zhan and Kumah, Divine P.}, year={2021}, month={Feb} }
 @article{al-tawhid_kumah_ahadi_2021, title={Two-dimensional electron systems and interfacial coupling in LaCrO3/KTaO3 heterostructures}, volume={118}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0049119}, DOI={10.1063/5.0049119}, abstractNote={The strong interfacial coupling at the 3d-5d transition metal-oxide interfaces has generated excitement due to the possibility of engineering a wide range of quantum phenomena and functionalities. Here, we investigate the electronic interfacial coupling and structural properties of LaCrO3/KTaO3 heterostructures. High-quality LaCrO3 films were grown on KTaO3 substrates using molecular beam epitaxy. These heterostructures show a robust two-dimensional electron gas and a metallic behavior down to liquid helium temperature. Using magnetoresistance measurements, we analyze the coupling of electronic orders between Cr 3d and Ta 5d states and observe signatures of weak anti-localization and Kondo scattering at low-temperature transport. The results provide direct evidence that a crossover (weak anti-localization to Kondo) occurs with increasing temperature as the dephasing scattering events reduce the coherence length. Our observations allow for a clear and detailed picture of two distinct quantum corrections to conductivity at low temperature.}, number={19}, journal={APPLIED PHYSICS LETTERS}, author={Al-Tawhid, Athby H. and Kumah, Divine P. and Ahadi, Kaveh}, year={2021}, month={May} }
 @article{al-tawhid_kumah_2020, title={Correlating polar distortions and interfacial charge at the polar/non-polar LaCrO3/SrTiO3 (001) interface}, url={https://doi.org/10.1063/5.0002298}, DOI={10.1063/5.0002298}, abstractNote={The relationship between the sheet carrier concentration, ns, of LaCrO3(LCO)/SrTiO3(STO) heterostructures and their structural properties has been investigated. Under low oxygen partial pressure, the STO substrate is reduced during growth as evidenced by a high ns of 1016 cm−2. By controlling the post-growth annealing conditions, heterostructures with ns of 1013–1016 cm−2 are achieved. The atomic-scale structure of the samples is obtained using high-resolution synchrotron x-ray diffraction measurements. For heterostructures with ns at or below 3 × 1013 cm−2, polar distortions are present within the LCO layers and increase in magnitude with a decrease in sheet carrier concentration. These distortions are absent for samples with ns on the order of 1015–1016 cm−2 where interfacial carriers play a role in alleviating the polar discontinuity at the LCO/STO interface. These results suggest that interfacial charge carriers and polar distortions can act as complementary mechanisms to alleviate the polar discontinuity at polar/non-polar complex oxide interfaces.}, journal={AIP Advances}, author={Al-Tawhid, A. H. and Kumah, D. P.}, year={2020}, month={Apr} }
 @article{koohfar_georgescu_hallsteinsen_sachan_roldan_arenholz_kumah_2020, title={Effect of strain on magnetic and orbital ordering of LaSrCrO3/LaSrMnO3 heterostructures}, volume={101}, url={https://doi.org/10.1103/PhysRevB.101.064420}, DOI={10.1103/PhysRevB.101.064420}, abstractNote={We investigate the effect of strain and film thickness on the orbital and magnetic properties of ${\mathrm{LaSrCrO}}_{3}\phantom{\rule{0.16em}{0ex}}(\mathrm{LSCO})/{\mathrm{LaSrMnO}}_{3}$ (LSMO) heterostructures using bulk magnetometry, soft x-ray magnetic spectroscopy, first-principles density-functional theory, high-resolution electron microscopy, and x-ray diffraction. We observe an antiparallel ordering of the magnetic moments between the ferromagnetic LSMO layers and the LSCO spacers, leading to a strain-independent ferromagnetic ground state of the LSCO/LSMO heterostructures for LSMO layers as thin as two unit cells. As the LSMO thickness is increased, a net ferromagnetic state is maintained, however, the average magnetic moment per Mn is found to be dependent on the magnitude of the substrate-induced strain. The differences in the magnetic responses are related to preferential occupation of the Mn ${x}^{2}\ensuremath{-}{y}^{2}$ (in-plane) $d$ orbitals for tensile strain and $3{z}^{2}\ensuremath{-}{r}^{2}$ (out-of-plane) orbitals under compressive strain, leading to competing ferromagnetic and antiferromagnetic exchange interactions within the LSMO layers. These results underscore the relative contributions of orbital, structural, and spin degrees of freedom and their tunability in atomically thin crystalline complex oxide layers.}, number={6}, journal={Physical Review B}, author={Koohfar, Sanaz and Georgescu, Alexandru B. and Hallsteinsen, Ingrid and Sachan, Ritesh and Roldan, Manuel A. and Arenholz, Elke and Kumah, Divine P.}, year={2020}, month={Feb} }
 @article{olmos_iturriaga_blazer_koohfar_gandha_nlebedim_kumah_singamaneni_2020, title={Exchange bias in La0.7Sr0.3CrO3/La0.7Sr0.3MnO3/La0.7Sr0.3CrO3 heterostructures}, volume={10}, url={https://doi.org/10.1063/1.5130453}, DOI={10.1063/1.5130453}, abstractNote={In the recent past, heterostructures of magnetic oxide thin films have attracted a great deal of research excitement due to very interesting physical properties such as antiferromagnetic interlayer coupling, tunable exchange-bias, interfacial driven magnetic properties and high mobility electron gas across the interfaces. In this work, we report on the comprehensive magnetic properties observed from the heterostructures of (2 unit cells) La0.7Sr0.3CrO3/(8 unit cells) La0.7Sr0.3MnO3/(2 unit cells) La0.7Sr0.3CrO3, which are epitaxially deposited on SrTiO3 substrate by plasma-assisted oxide molecular beam epitaxy. Using SQUID magnetometer, the magnetic properties are studied when the magnetic field was applied both in plane and out of plane. The Curie temperature of this structure is found to be at 290 K. Most significantly, at 2 K, we observed a complete up/down shift (along magnetization axis) of hysteresis loop when the sample was cooled under a magnetic field of ± 5000 Oe in the in-plane configuration. We believe that the strong antiferromagnetic (super) exchange coupling of Mn-Cr across the two interfaces is responsible for the observed exchange bias. We will present and discuss our in-detailed experimental findings collected on this heterostructure as a function of temperature and magnetic field.}, number={1}, journal={AIP Advances}, publisher={AIP Publishing}, author={Olmos, Rubyann and Iturriaga, Hector and Blazer, Dawn S. and Koohfar, Sanaz and Gandha, Kinjal and Nlebedim, Ikenna C. and Kumah, Divine P. and Singamaneni, Srinivasa R.}, year={2020}, month={Jan}, pages={015001} }
 @article{penn_koohfar_kumah_lebeau_2020, title={On the redistribution of charge in La0.7Sr0.3CrO3/La0.7Sr0.3MnO3 multilayer thin films}, url={https://doi.org/10.1063/1.5140352}, DOI={10.1063/1.5140352}, abstractNote={The atomic and electronic structures of La0.7Sr0.3MnO3 (LSMO)/La0.7Sr0.3CrO3 (LSCO) multilayer thin films are investigated using aberration corrected scanning transmission electron microscopy (STEM) imaging and spectroscopy. Atomic resolution high angle annular dark-field reveals that LSMO layers have an expanded out-of-plane lattice parameter compared to compressed LSCO layers, contrasting with x-ray diffraction measurements. The expansion is found to result from preferential oxygen vacancy formation in LSMO during STEM sample preparation as determined by electron energy-loss spectroscopy. The La/Sr atom column intensity is also found to oscillate by about 4% between the LSMO and LSCO layers, indicative of La/Sr concentration variation. Using energy-dispersive x-ray spectroscopy in combination with image simulations, we confirm the La/Sr inhomogeneity and elucidate the origin of charge redistribution within the multilayer. These results illuminate the sensitivity of the technique to subtle structural, chemical, and electronic features that can arise to compensate charge imbalances in complex oxide heterostructures.}, journal={AIP Advances}, author={Penn, Aubrey N. and Koohfar, Sanaz and Kumah, Divine P. and LeBeau, James M.}, year={2020}, month={Apr} }
 @article{lim_chrysler_kumar_mauthe_kumah_richardson_lebeau_ngai_2020, title={Suspended single-crystalline oxide structures on silicon through wet-etch techniques: Effects of oxygen vacancies and dislocations on etch rates}, volume={38}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85077449474&partnerID=MN8TOARS}, DOI={10.1116/1.5135035}, abstractNote={Wet-etch techniques to realize suspended microscale structures of single-crystalline SrTiO3 and BaTiO3 grown on Si are explored. The authors examine the effects of oxygen vacancies and dislocations on etch rates. Both oxygen vacancies and dislocations enhance etching, yielding rates that are sufficiently high to enable conventional photoresist to serve as a mask layer. Suspended bridge structures are realized by etching the underlying Si substrate using a potassium hydroxide solution. The ability to realize suspended microscale structures using wet-etch techniques that are compatible with standard semiconductor device processing opens a pathway to integrate multifunctional oxides in microelectromechanical systems.}, number={1}, journal={Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films}, author={Lim, Z.H. and Chrysler, M. and Kumar, A. and Mauthe, J.P. and Kumah, D.P. and Richardson, C. and Lebeau, J.M. and Ngai, J.H.}, year={2020} }
 @article{vetter_vonwald_yang_yan_koohfar_kumah_yu_you_sun_2020, title={Tuning of spin-orbit coupling in metal-free conjugated polymers by structural conformation}, url={https://doi.org/10.1103/PhysRevMaterials.4.085603}, DOI={10.1103/PhysRevMaterials.4.085603}, abstractNote={Manipulating spin-orbit coupling (SOC) is a key achievement for spin-orbitronic applications since SOC determines spin-diffusion lengths and spin-to-charge conversion efficiencies. While in most organic semiconductors SOC is inherently very weak due to being composed of primarily light elements, the SOC in conjugated polymer systems is also intimately tied to the polymer's structural conformation and thus may be manipulated. Here we report a modification of SOC in conjugated polymers by altering torsion angle between conjugated units. Spin-pumping experiments are performed on three poly(3-alkylthiophene) polymer films with decreasing conjugation lengths and concomitantly increasing torsion angle. The more twisted polymer exhibits a shorter spin-diffusion length and a giant spin-mixing conductance (up to ${10}^{21}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}$ ), which is attributed to an increased SOC by structural conformation. This work offers a route for enhancing SOC and spin-injection efficiency in organic materials for spintronic applications.}, journal={Physical Review Materials}, author={Vetter, Eric and VonWald, Ian and Yang, Shijia and Yan, Liang and Koohfar, Sanaz and Kumah, Divine and Yu, Zhi-Gang and You, Wei and Sun, Dali}, year={2020}, month={Aug} }
 @article{koohfar_georgescu_penn_lebeau_arenholz_kumah_2019, title={Confinement of magnetism in atomically thin La0.7Sr0.3CrO3/La0.7Sr0.3MnO3 heterostructures}, volume={4}, ISSN={2397-4648}, url={http://dx.doi.org/10.1038/s41535-019-0164-1}, DOI={10.1038/s41535-019-0164-1}, abstractNote={At crystalline interfaces where a valence mismatch exists, electronic and structural interactions may occur to relieve the polar mismatch leading to the stabilization of non-bulklike phases. We show that spontaneous reconstructions at polar $La_{0.7}Sr_{0.3}MnO_3$ interfaces are correlated with suppressed ferromagnetism for film thicknesses on the order of a unit cell. We investigate the structural and magnetic properties of valence-matched $La_{0.7}Sr_{0.3}CrO_3$ - $La_{0.7}Sr_{0.3}MnO_3$ interfaces using a combination of high-resolution electron microscopy, first principles theory, synchrotron X-ray scattering and magnetic spectroscopy and temperature-dependent magnetometry. A combination of an antiferromagnetic coupling between the $La_{0.7}Sr_{0.3}CrO_3$ and $La_{0.7}Sr_{0.3}MnO_3$ layers and a suppression of interfacial polar distortions are found to result in robust long range ferromagnetic ordering for ultra-thin $La_{0.7}Sr_{0.3}MnO_3$. These results underscore the critical importance of interfacial structural and magnetic interactions in the design of devices based on two-dimensional oxide magnetic systems.}, number={1}, journal={npj Quantum Materials}, publisher={Springer Science and Business Media LLC}, author={Koohfar, Sanaz and Georgescu, Alexandru B. and Penn, Aubrey N. and LeBeau, James M. and Arenholz, Elke and Kumah, Divine P.}, year={2019}, month={May} }
 @article{kumah_ngai_kornblum_2019, title={Epitaxial Oxides on Semiconductors: From Fundamentals to New Devices}, volume={7}, ISSN={1616-301X 1616-3028}, url={http://dx.doi.org/10.1002/adfm.201901597}, DOI={10.1002/adfm.201901597}, abstractNote={Abstract Functional oxides are an untapped resource for futuristic devices and functionalities. These functionalities can range from high temperature superconductivity to multiferroicity and novel catalytic schemes. The most prominent route for transforming these ideas from a single device in the lab to practical technologies is by integration with semiconductors. Moreover, coupling oxides with semiconductors can herald new and unexpected functionalities that exist in neither of the individual materials. Therefore, oxide epitaxy on semiconductors provides a materials platform for novel device technologies. As oxides and semiconductors exhibit properties that are complementary to one another, epitaxial heterostructures comprised of the two are uniquely poised to deliver rich functionalities. This review discusses recent advancements in the growth of epitaxial oxides on semiconductors, and the electronic and physical structure of their interfaces. Leaning on these fundamentals and practicalities, the material behavior and functionality of semiconductor–oxide heterostructures is discussed, and their potential as device building blocks is highlighted. The culmination of this discussion is a review of recent advances in the development of prototype devices based on semiconductor–oxide heterostructures, in areas ranging from silicon photonics to photocatalysis. This overview is intended to stimulate ideas for new concepts of functional devices and lay the groundwork for their realization.}, journal={Advanced Functional Materials}, publisher={Wiley}, author={Kumah, Divine P. and Ngai, Joseph H. and Kornblum, Lior}, year={2019}, month={Jul}, pages={1901597} }
 @article{al-tawhid_frick_dougherty_kumah_2019, title={Growth-temperature dependence of conductivity at the LaCrO3/SrTiO3 (001) interface}, volume={37}, ISSN={0734-2101 1520-8559}, url={http://dx.doi.org/10.1116/1.5085334}, DOI={10.1116/1.5085334}, abstractNote={The effect of growth conditions on the structural and electronic properties of the polar/non-polar LaCrO$_3$/SrTiO$_3$ (LCO/STO) interface has been investigated. The interface is either insulating or metallic depending on growth conditions. A high sheet carrier concentration of 2x10$^{16}$ cm$^{-2}$ and mobility of 30,000 cm$^2$/V s is reported for the metallic interfaces, which is similar to the quasi-two dimensional gas at the LaAlO$_{3}$/SrTiO$_{3}$ interface with similar growth conditions. High-resolution synchrotron X-ray-based structural determination of the atomic-scale structures of both metallic and insulating LCO/STO interfaces show chemical intermixing and an interfacial lattice expansion. Angle resolved photoemission spectroscopy of 2 and 4 uc metallic LCO/STO shows no intensity near the Fermi level indicating that the conducting region is occurring deep enough in the substrate to be inaccessible to photoemission spectroscopy. Post-growth annealing in flowing oxygen causes a reduction in the sheet carrier concentration and mobility for the metallic interface while annealing the insulating interface at high temperatures and low oxygen partial pressures results in metallicity. These results highlight the critical role of defects related to oxygen vacancies on the creation of mobile charge carriers at the LCO/STO heterointerface.}, number={2}, journal={Journal of Vacuum Science & Technology A}, publisher={American Vacuum Society}, author={Al-Tawhid, Athby and Frick, Jordan R. and Dougherty, Daniel B. and Kumah, Divine P.}, year={2019}, month={Mar}, pages={021102} }
 @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 SrZr$_{x}$Ti$_{1-x}$O$_3$ films grown on semiconducting Ge substrates are investigated by synchrotron X-ray diffraction and first-principles density functional theory. By systematically tuning 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 SrZr$_{x}$Ti$_{1-x}$O$_3$/Ge interface. The theoretical valence band offsets 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{ahmadi-majlan_chen_lim_conlin_hensley_chrysler_su_chen_kumah_ngai_et al._2018, title={Tuning metal-insulator behavior in LaTiO3/SrTiO3 heterostructures integrated directly on Si(100) through control of atomic layer thickness}, volume={112}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.5018069}, DOI={10.1063/1.5018069}, abstractNote={We present electrical and structural characterization of epitaxial LaTiO3/SrTiO3 heterostructures integrated directly on Si(100). By reducing the thicknesses of the heterostructures, an enhancement in carrier-carrier scattering is observed in the Fermi liquid behavior, followed by a metal to insulator transition in the electrical transport. The insulating behavior is described by activated transport, and its onset occurs near an occupation of 1 electron per Ti site within the SrTiO3, providing evidence for a Mott driven transition. We also discuss the role that structure and gradients in strain could play in enhancing the carrier density. The manipulation of Mott metal-insulator behavior in oxides grown directly on Si opens the pathway to harnessing strongly correlated phenomena in device technologies.}, number={19}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Ahmadi-Majlan, Kamyar and Chen, Tongjie and Lim, Zheng Hui and Conlin, Patrick and Hensley, Ricky and Chrysler, Matthew and Su, Dong and Chen, Hanghui and Kumah, D.P. and Ngai, Joseph H. and et al.}, year={2018}, month={May}, pages={193104} }
 @article{disa_georgescu_hart_kumah_shafer_arenholz_arena_ismail-beigi_taheri_walker_et al._2017, title={Control of hidden ground-state order in 
NdNiO3
 superlattices}, volume={1}, ISSN={2475-9953}, url={http://dx.doi.org/10.1103/physrevmaterials.1.024410}, DOI={10.1103/physrevmaterials.1.024410}, abstractNote={The combination of charge and spin degrees of freedom with electronic correlations in condensed matter systems leads to a rich array of phenomena, such as magnetism, superconductivity, and novel conduction mechanisms. While such phenomena are observed in bulk materials, a richer array of behaviors becomes possible when these degrees of freedom are controlled in atomically layered heterostructures, where one can constrain dimensionality and impose interfacial boundary conditions. Here, we unlock a host of unique, hidden electronic and magnetic phase transitions in NdNiO$_3$ while approaching the two-dimensional (2D) limit, resulting from the differing influences of dimensional confinement and interfacial coupling. Most notably, we discover a new phase in fully 2D, single layer NdNiO$_3$, in which all signatures of the bulk magnetic and charge ordering are found to vanish. In addition, for quasi two-dimensional layers down to a thickness of two unit cells, bulk-type ordering persists but separates from the onset of insulating behavior in a manner distinct from that found in the bulk or thin film nickelates. Using resonant x-ray spectroscopies, first-principles theory, and model calculations, we propose that the single layer phase suppression results from a new mechanism of interfacial electronic reconstruction based on ionicity differences across the interface, while the phase separation in multi-layer NdNiO$_3$ emerges due to enhanced 2D fluctuations. These findings provide insights into the intertwined mechanisms of charge and spin ordering in strongly correlated systems in reduced dimensions and illustrate the ability to use atomic layering to access hidden phases.}, number={2}, journal={Physical Review Materials}, publisher={American Physical Society (APS)}, author={Disa, Ankit S. and Georgescu, Alexandru B. and Hart, James L. and Kumah, Divine P. and Shafer, Padraic and Arenholz, Elke and Arena, Dario A. and Ismail-Beigi, Sohrab and Taheri, Mitra L. and Walker, Frederick J. and et al.}, year={2017}, month={Jul} }
 @article{ngai_ahmadi-majlan_moghadam_chrysler_kumah_walker_ahn_droubay_du_chambers_et al._2017, title={Electrically coupling complex oxides to semiconductors: A route to novel material functionalities}, volume={32}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85009385527&partnerID=MN8TOARS}, DOI={10.1557/jmr.2016.496}, abstractNote={Abstract}, number={2}, journal={Journal of Materials Research}, author={Ngai, J.H. and Ahmadi-Majlan, K. and Moghadam, J. and Chrysler, M. and Kumah, D. and Walker, F.J. and Ahn, C.H. and Droubay, T. and Du, Y. and Chambers, S.A. and et al.}, year={2017}, pages={249–259} }
 @article{dogan_fernandez-peña_kornblum_jia_kumah_reiner_krivokapic_kolpak_ismail-beigi_ahn_et al._2017, title={Single Atomic Layer Ferroelectric on Silicon}, volume={18}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/acs.nanolett.7b03988}, DOI={10.1021/acs.nanolett.7b03988}, abstractNote={A single atomic layer of ZrO2 exhibits ferroelectric switching behavior when grown with an atomically abrupt interface on silicon. Hysteresis in capacitance–voltage measurements of a ZrO2 gate stack demonstrate that a reversible polarization of the ZrO2 interface structure couples to the carriers in the silicon. First-principles computations confirm the existence of multiple stable polarization states and the energy shift in the semiconductor electron states that result from switching between these states. This monolayer ferroelectric represents a new class of materials for achieving devices that transcend conventional complementary metal oxide semiconductor (CMOS) technology. Significantly, a single atomic layer ferroelectric allows for more aggressively scaled devices than bulk ferroelectrics, which currently need to be thicker than 5–10 nm to exhibit significant hysteretic behavior (Park, et al. Adv. Mater. 2015, 27, 1811).}, number={1}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Dogan, Mehmet and Fernandez-Peña, Stéphanie and Kornblum, Lior and Jia, Yichen and Kumah, Divine P. and Reiner, James W. and Krivokapic, Zoran and Kolpak, Alexie M. and Ismail-Beigi, Sohrab and Ahn, Charles H. and et al.}, year={2017}, month={Dec}, pages={241–246} }
 @article{koohfar_disa_marshall_walker_ahn_kumah_2017, title={Structural distortions at polar manganite interfaces}, volume={96}, ISSN={["2469-9969"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85027360935&partnerID=MN8TOARS}, DOI={10.1103/physrevb.96.024108}, abstractNote={Electronic, lattice, and spin interactions at the interfaces between crystalline complex transition metal oxides can give rise to a wide range of functional electronic and magnetic phenomena not found in bulk. At hetero-interfaces, these interactions may be enhanced by combining oxides where the polarity changes at the interface. The physical structure between non-polar SrTiO$_3$ and polar La$_{1-x}$Sr$_x$MnO$_3$(x=0.2) is investigated using high resolution synchrotron x-ray diffraction to directly determine the role of structure in compensating the polar discontinuity. At both the oxide-oxide interface and vacuum-oxide interfaces, the lattice is found to expand and rumple along the growth direction. The SrTiO$_3$/La$_{1-x}$Sr$_x$MnO$_3$ interface also exhibits intermixing of La and Sr over a few unit cells. The results, hence, demonstrate that polar distortions and ionic intermixing coexist and both pathways play a significant role at interfaces with polar discontinuities.}, number={2}, journal={PHYSICAL REVIEW B}, author={Koohfar, S. and Disa, A. S. and Marshall, M. S. J. and Walker, F. J. and Ahn, C. H. and Kumah, D. P.}, year={2017}, month={Jul} }
 @article{ngai_ahmadi-majlan_moghadam_chrysler_kumah_ahn_walker_droubay_bowden_chambers_et al._2016, title={Electrically Coupling Multifunctional Oxides to Semiconductors: A
                    Route to Novel Material Functionalities}, volume={1}, ISSN={2059-8521}, url={http://dx.doi.org/10.1557/adv.2016.101}, DOI={10.1557/adv.2016.101}, number={4}, journal={MRS Advances}, publisher={Cambridge University Press (CUP)}, author={Ngai, J. H. and Ahmadi-Majlan, K. and Moghadam, J. and Chrysler, M. and Kumah, D. P. and Ahn, C. H. and Walker, F. J. and Droubay, T. and Bowden, M. and Chambers, S. A. and et al.}, year={2016}, pages={255–263} }
 @article{kumah_dogan_ngai_qiu_zhang_su_specht_ismail-beigi_ahn_walker_et al._2016, title={Engineered Unique Elastic Modes at a BaTiO3/(2×1)−Ge(001) Interface}, volume={116}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/physrevlett.116.106101}, DOI={10.1103/physrevlett.116.106101}, abstractNote={The strong interaction at an interface between a substrate and thin film leads to epitaxy and provides a means of inducing structural changes in the epitaxial film. These induced material phases often exhibit technologically relevant electronic, magnetic, and functional properties. The 2×1 surface of a Ge(001) substrate applies a unique type of epitaxial constraint on thin films of the perovskite oxide BaTiO_{3} where a change in bonding and symmetry at the interface leads to a non-bulk-like crystal structure of the BaTiO_{3}. While the complex crystal structure is predicted using first-principles theory, it is further shown that the details of the structure are a consequence of hidden phases found in the bulk elastic response of the BaTiO_{3} induced by the symmetry of forces exerted by the germanium substrate.}, number={10}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Kumah, D.P. and Dogan, M. and Ngai, J. H. and Qiu, D. and Zhang, Z. and Su, D. and Specht, E. D. and Ismail-Beigi, S. and Ahn, C. H. and Walker, F. J. and et al.}, year={2016}, month={Mar} }
 @article{zou_mandal_albright_peng_pu_kumah_lau_simon_dagdeviren_he_et al._2016, title={Role of double TiO2 layers at the interface of FeSe/SrTiO3 superconductors}, volume={93}, ISSN={2469-9950 2469-9969}, url={http://dx.doi.org/10.1103/physrevb.93.180506}, DOI={10.1103/physrevb.93.180506}, abstractNote={We determine the surface reconstruction of $\mathrm{SrTi}{\mathrm{O}}_{3}$ used to achieve superconducting FeSe films in experiments, which is different from the $1\ifmmode\times\else\texttimes\fi{}1\mathrm{Ti}{\mathrm{O}}_{2}$-terminated $\mathrm{SrTi}{\mathrm{O}}_{3}$ assumed by most previous theoretical studies. In particular, we identify the existence of a double $\mathrm{Ti}{\mathrm{O}}_{2}$ layer at the FeSe/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interface that plays two important roles. First, it facilitates the epitaxial growth of FeSe. Second, ab initio calculations reveal a strong tendency for electrons to transfer from an oxygen deficient $\mathrm{SrTi}{\mathrm{O}}_{3}$ surface to FeSe when the double $\mathrm{Ti}{\mathrm{O}}_{2}$ layer is present. The double layer helps to remove the hole pocket in the FeSe at the \ensuremath{\Gamma} point of the Brillouin zone and leads to a band structure characteristic of superconducting samples. The characterization of the interface structure presented here is a key step towards the resolution of many open questions about this superconductor.}, number={18}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Zou, Ke and Mandal, Subhasish and Albright, Stephen D. and Peng, Rui and Pu, Yujia and Kumah, Divine and Lau, Claudia and Simon, Georg H. and Dagdeviren, Omur E. and He, Xi and et al.}, year={2016}, month={May} }
 @article{disa_kumah_malashevich_chen_arena_specht_ismail-beigi_walker_ahn_2015, title={Orbital Engineering in Symmetry-Breaking Polar Heterostructures}, volume={114}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/physrevlett.114.026801}, DOI={10.1103/physrevlett.114.026801}, abstractNote={We experimentally demonstrate a novel approach to substantially modify orbital occupations and symmetries in electronically correlated oxides. In contrast to methods using strain or confinement, this orbital tuning is achieved by exploiting charge transfer and inversion symmetry breaking using atomically layered heterostructures. We illustrate the technique in the LaTiO3−LaNiO3−LaAlO3 system; a combination of x-ray absorption spectroscopy and ab initio theory reveals electron transfer and concomitant polar fields, resulting in a ∼50% change in the occupation of Ni d orbitals. This change is sufficiently large to remove the orbital degeneracy of bulk LaNiO3 and creates an electronic configuration approaching a single-band Fermi surface. Furthermore, we theoretically show that such three-component heterostructuring is robust and tunable by choice of insulator in the heterostructure, providing a general method for engineering orbital configurations and designing novel electronic systems.Received 14 November 2014DOI:https://doi.org/10.1103/PhysRevLett.114.026801© 2015 American Physical Society}, number={2}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Disa, Ankit S. and Kumah, Divine P. and Malashevich, Andrei and Chen, Hanghui and Arena, Dario A. and Specht, Eliot D. and Ismail-Beigi, Sohrab and Walker, F. J. and Ahn, Charles H.}, year={2015}, month={Jan} }
 @article{kornblum_jin_kumah_ernst_broadbridge_ahn_walker_2015, title={Oxide 2D electron gases as a route for high carrier densities on (001) Si}, volume={106}, url={https://doi.org/10.1063/1.4921437}, DOI={10.1063/1.4921437}, abstractNote={Two dimensional electron gases (2DEGs) formed at the interfaces of oxide heterostructures draw considerable interest owing to their unique physics and potential applications. Growing such heterostructures on conventional semiconductors has the potential to integrate their functionality with semiconductor device technology. We demonstrate 2DEGs on a conventional semiconductor by growing GdTiO3-SrTiO3 on silicon. Structural analysis confirms the epitaxial growth of heterostructures with abrupt interfaces and a high degree of crystallinity. Transport measurements show the conduction to be an interface effect, ∼9 × 1013 cm−2 electrons per interface. Good agreement is demonstrated between the electronic behavior of structures grown on Si and on an oxide substrate, validating the robustness of this approach to bridge between lab-scale samples to a scalable, technologically relevant materials system.}, number={20}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Kornblum, Lior and Jin, Eric N. and Kumah, Divine P. and Ernst, Alexis T. and Broadbridge, Christine C. and Ahn, Charles H. and Walker, Fred J.}, year={2015}, month={May}, pages={201602} }
 @article{jin_kornblum_kumah_zou_broadbridge_ngai_ahn_walker_2014, title={A high density two-dimensional electron gas in an oxide heterostructure on Si (001)}, volume={2}, url={https://doi.org/10.1063/1.4902433}, DOI={10.1063/1.4902433}, abstractNote={We present the growth and characterization of layered heterostructures comprised of LaTiO3 and SrTiO3 epitaxially grown on Si (001). Magnetotransport measurements show that the sheet carrier densities of the heterostructures scale with the number of LaTiO3/SrTiO3 interfaces, consistent with the presence of an interfacial 2-dimensional electron gas (2DEG) at each interface. Sheet carrier densities of 8.9 × 1014 cm−2 per interface are observed. Integration of such high density oxide 2DEGs on silicon provides a bridge between the exceptional properties and functionalities of oxide 2DEGs and microelectronic technologies.}, number={11}, journal={APL Materials}, publisher={AIP Publishing}, author={Jin, E. N. and Kornblum, L. and Kumah, D. P. and Zou, K. and Broadbridge, C. C. and Ngai, J. H. and Ahn, C. H. and Walker, F. J.}, year={2014}, month={Nov}, pages={116109} }
 @inproceedings{xiong_pernice_ngai_reiner_kumah_walker_ahn_tang_2014, title={A silicon integrated BaTiO<inf>3</inf> electro-optic modulator}, volume={2014-January}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84944698664&partnerID=MN8TOARS}, booktitle={Conference on Lasers and Electro-Optics Europe - Technical Digest}, author={Xiong, C. and Pernice, W.H.P. and Ngai, J.H. and Reiner, J.W. and Kumah, D. and Walker, F.J. and Ahn, C.H. and Tang, H.X.}, year={2014} }
 @article{xiong_pernice_ngai_reiner_kumah_walker_ahn_tang_2014, title={Active Silicon Integrated Nanophotonics: Ferroelectric BaTiO3 Devices}, volume={14}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/nl404513p}, DOI={10.1021/nl404513p}, abstractNote={The integration of complex oxides on silicon presents opportunities to extend and enhance silicon technology with novel electronic, magnetic, and photonic properties. Among these materials, barium titanate (BaTiO3) is a particularly strong ferroelectric perovskite oxide with attractive dielectric and electro-optic properties. Here we demonstrate nanophotonic circuits incorporating ferroelectric BaTiO3 thin films on the ubiquitous silicon-on-insulator (SOI) platform. We grow epitaxial, single-crystalline BaTiO3 directly on SOI and engineer integrated waveguide structures that simultaneously confine light and an RF electric field in the BaTiO3 layer. Using on-chip photonic interferometers, we extract a large effective Pockels coefficient of 213 plus minus 49 pm/V, a value more than six times larger than found in commercial optical modulators based on lithium niobate. The monolithically integrated BaTiO3 optical modulators show modulation bandwidth in the gigahertz regime, which is promising for broadband applications.}, number={3}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Xiong, Chi and Pernice, Wolfram H. P. and Ngai, Joseph H. and Reiner, James W. and Kumah, Divine and Walker, Frederick J. and Ahn, Charles H. and Tang, Hong X.}, year={2014}, month={Feb}, pages={1419–1425} }
 @article{kumah_malashevich_disa_arena_walker_ismail-beigi_ahn_2014, title={Effect of Surface Termination on the Electronic Properties of LaNiO3 Films}, volume={2}, ISSN={2331-7019}, url={http://dx.doi.org/10.1103/physrevapplied.2.054004}, DOI={10.1103/physrevapplied.2.054004}, abstractNote={Tailoring the microscopic structures of oxide interfaces and surfaces to realize desired electronic properties is of intense interest. The authors modulate the conductivity of LaNiO${}_{3}$ films as thin as three unit cells by precisely controlling the composition of the topmost atomic layer. Three-dimensional x-ray imaging combined with first-principles theory exposes the correlation between surface termination and electrical conductivity, which could be widely exploited in logic, memory, or sensor applications.}, number={5}, journal={Physical Review Applied}, publisher={American Physical Society (APS)}, author={Kumah, Divine P. and Malashevich, Andrei and Disa, Ankit S. and Arena, Dario A. and Walker, Frederick J. and Ismail-Beigi, Sohrab and Ahn, Charles H.}, year={2014}, month={Nov}, pages={054004} }
 @article{ngai_kumah_ahn_walker_2014, title={Hysteretic electrical transport in BaTiO3/Ba1−xSrxTiO3/Ge heterostructures}, volume={104}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4864648}, DOI={10.1063/1.4864648}, abstractNote={We present electrical transport measurements of heterostructures comprised of BaTiO3 and Ba1−xSrxTiO3 epitaxially grown on Ge. Sr alloying imparts compressive strain to the BaTiO3, which enables the thermal expansion mismatch between BaTiO3 and Ge to be overcome to achieve c-axis oriented growth. The conduction bands of BaTiO3 and Ba1−xSrxTiO3 are nearly aligned with the conduction band of Ge, which facilitates electron transport. Electrical transport measurements through the dielectric stack exhibit rectifying behavior and hysteresis, where the latter is consistent with ferroelectric switching.}, number={6}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Ngai, J. H. and Kumah, D. P. and Ahn, C. H. and Walker, F. J.}, year={2014}, month={Feb}, pages={062905} }
 @article{kumah_disa_ngai_chen_malashevich_reiner_ismail-beigi_walker_ahn_2014, title={Tuning the Structure of Nickelates to Achieve Two-Dimensional Electron Conduction}, volume={26}, ISSN={0935-9648}, url={http://dx.doi.org/10.1002/adma.201304256}, DOI={10.1002/adma.201304256}, abstractNote={Metallic electronic transport in nickelate heterostructures can be induced and confined to two dimensions (2D) by controlling the structural parameters of the nickel–oxygen planes.}, number={12}, journal={Advanced Materials}, publisher={Wiley}, author={Kumah, Divine P. and Disa, Ankit S. and Ngai, Joseph H. and Chen, Hanghui and Malashevich, Andrei and Reiner, James W. and Ismail-Beigi, Sohrab and Walker, Frederick J. and Ahn, Charles H.}, year={2014}, month={Feb}, pages={1935–1940} }
 @article{kumah_yacoby_pauli_willmott_clarke_2013, title={La-doped BaTiO3 heterostructures: Compensating the polarization discontinuity}, volume={1}, ISSN={2166-532X}, url={http://dx.doi.org/10.1063/1.4849735}, DOI={10.1063/1.4849735}, abstractNote={Abstract}, number={6}, journal={APL Materials}, publisher={AIP Publishing}, author={Kumah, D. P. and Yacoby, Y. and Pauli, S. A. and Willmott, P. R. and Clarke, R.}, year={2013}, month={Dec}, pages={062107} }
 @article{chen_kumah_disa_walker_ahn_ismail-beigi_2013, title={Modifying the Electronic Orbitals of Nickelate Heterostructures via Structural Distortions}, volume={110}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876953473&partnerID=MN8TOARS}, DOI={10.1103/PhysRevLett.110.186402}, abstractNote={We describe a general materials design approach that produces large orbital energy splittings (orbital polarization) in nickelate heterostructures, creating a two-dimensional single-band electronic surface at the Fermi energy. The resulting electronic structure mimics that of the high temperature cuprate superconductors. The two key ingredients are: (i) the construction of atomic-scale distortions about the Ni site via charge transfer and internal electric fields, and (ii) the use of three component (tri-component) superlattices to break inversion symmetry. We use {\it ab initio} calculations to implement the approach, with experimental verification of the critical structural motif that enables the design to succeed.}, number={18}, journal={Physical Review Letters}, author={Chen, Hanghui and Kumah, Divine P. and Disa, Ankit S. and Walker, Frederick J. and Ahn, Charles H. and Ismail-Beigi, Sohrab}, year={2013} }
 @article{disa_kumah_ngai_specht_arena_walker_ahn_2013, title={Phase diagram of compressively strained nickelate thin films}, volume={1}, ISSN={2166-532X}, url={http://dx.doi.org/10.1063/1.4820431}, DOI={10.1063/1.4820431}, abstractNote={The complex phase diagrams of strongly correlated oxides arise from the coupling between physical and electronic structure. This can lead to a renormalization of the phase boundaries when considering thin films rather than bulk crystals due to reduced dimensionality and epitaxial strain. The well-established bulk RNiO3 phase diagram shows a systematic dependence between the metal-insulator transition and the perovskite A-site rare-earth ion, R. Here, we explore the equivalent phase diagram for nickelate thin films under compressive epitaxial strain. We determine the metal-insulator phase diagram for the solid solution of Nd1-yLayNiO3 thin films within the range 0 ≤ y ≤ 1. We find qualitative similarity between the films and their bulk analogs, but with an overall renormalization in the metal-insulator transition to lower temperature. A combination of x-ray diffraction measurements and soft x-ray absorption spectroscopy indicates that the renormalization is due to increased Ni–O bond hybridization for coherently strained thin films.}, number={3}, journal={APL Materials}, publisher={AIP Publishing}, author={Disa, A. S. and Kumah, D. P. and Ngai, J. H. and Specht, E. D. and Arena, D. A. and Walker, F. J. and Ahn, C. H.}, year={2013}, month={Sep}, pages={032110} }
 @article{moyer_kumah_vaz_arena_henrich_2013, title={Role of epitaxial strain on the magnetic structure of Fe-doped CoFe2O4}, volume={345}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84885147976&partnerID=MN8TOARS}, DOI={10.1016/j.jmmm.2013.06.031}, abstractNote={The magnetic structure of Fe-doped CoFe2O4 (Co1−xFe2+xO4) grown on MgO (0 0 1) and SrTiO3 (0 0 1) substrates is studied with superconducting quantum interference device magnetometry and soft x-ray magnetic spectroscopies. X-ray and electron diffraction show that the choice of substrate has large effects on the strain, crystal structure and surface morphology of Co1−xFe2+xO4 thin films. Samples grown on MgO have small, coherent strains and surfaces that are nearly atomically flat, whereas films grown on SrTiO3 have large tensile strains and surfaces terminated with islands, which indicate the presence of a large density of misfit dislocations. These differences in structural properties correlate with the large differences seen in the magnetic structure; samples grown on SrTiO3 have larger magnetic moments and increased anisotropies compared to those grown on MgO. Most strikingly, the large magnetic spin and orbital moments found in the films grown on SrTiO3 suggest a suppression of anti-phase boundary formation, which we attribute to the large compressive lattice mismatch and the formation of misfit dislocations during the film growth in order to relieve the epitaxial strain. This results in the films grown on SrTiO3 having magnetic properties that are more similar to bulk Co1−xFe2+xO4 than those grown on MgO, demonstrating that epitaxial strain can result in large changes in the magnetic structure of Co1−xFe2+xO4.}, journal={Journal of Magnetism and Magnetic Materials}, author={Moyer, J. A. and Kumah, D. P. and Vaz, C. A. F. and Arena, D. A. and Henrich, V. E.}, year={2013}, pages={180–189} }
 @article{moyer_vaz_kumah_arena_henrich_2012, title={Enhanced magnetic moment in ultrathin Fe-doped CoFe2O4 films}, volume={86}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84869025735&partnerID=MN8TOARS}, DOI={10.1103/PhysRevB.86.174404}, abstractNote={The effect of film thickness on the magnetic properties of ultrathin Fe-doped cobalt ferrite (Co${}_{1\ensuremath{-}x}$Fe${}_{2+x}$O${}_{4}$) grown on MgO (001) substrates is investigated by superconducting quantum interference device magnetometry and x-ray magnetic linear dichroism, while the distribution of the Co${}^{2+}$ cations between the octahedral and tetrahedral lattice sites is studied with x-ray absorption spectroscopy. For films thinner than 10 nm, there is a large enhancement of the magnetic moment; conversely, the remanent magnetization and coercive fields both decrease, while the magnetic spin axes of all the cations become less aligned with the [001] crystal direction. In particular, at 300 K the coercive fields of the thinnest films vanish. The spectroscopy data show that no changes occur in the cation distribution as a function of film thickness, ruling this out as the origin of the enhanced magnetic moment. However, the magnetic measurements all support the possibility that these ultrathin Fe-doped CoFe${}_{2}$O${}_{4}$ films are transitioning into a superparamagnetic state, as has been seen in ultrathin Fe${}_{3}$O${}_{4}$. A weakening of the magnetic interactions at the antiphase boundaries, leading to magnetically independent domains within the film, could explain the enhanced magnetic moment in ultrathin Fe-doped CoFe${}_{2}$O${}_{4}$ and the onset of superparamagnetism at room temperature.}, number={17}, journal={Physical Review B}, author={Moyer, J. A. and Vaz, C. A. F. and Kumah, D. P. and Arena, D. A. and Henrich, V. E.}, year={2012} }
 @article{moyer_kumah_vaz_arena_henrich_2012, title={Epitaxial strain-induced changes in the cation distribution and resistivity of Fe-doped CoFe2O4}, volume={101}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84863923550&partnerID=MN8TOARS}, DOI={10.1063/1.4735233}, abstractNote={The distribution of cations in Fe-doped cobalt ferrite (Co1−xFe2+xO4) is investigated as a function of epitaxial strain through x-ray absorption measurements of samples grown on SrTiO3, MgO, and CoCr2O4-buffered MgAl2O4 (001). In agreement with recent theoretical calculations, compressive (tensile) strain results in the films having a larger (smaller) degree of cation inversion for iron doping levels up to x = 0.62. Measurements of the resistivity further conclude that the degree of cation inversion has a direct effect on the size of the bandgap for stoichiometric CoFe2O4, an effect that is reduced as the iron doping level is increased.}, number={2}, journal={Applied Physics Letters}, author={Moyer, J. A. and Kumah, D. P. and Vaz, C. A. F. and Arena, D. A. and Henrich, V. E.}, year={2012} }
 @article{marshall_kumah_reiner_baddorf_ahn_walker_2012, title={Piezoelectric force microscopy of crystalline oxide-semiconductor heterostructures}, volume={101}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84866020634&partnerID=MN8TOARS}, DOI={10.1063/1.4750243}, abstractNote={Thin films of epitaxial SrTiO3 grown on silicon exhibit compressive in-plane strain that may stabilize ferroelectricity in this normally non-ferroelectric material. We investigate this possibility by using an ultra-high vacuum atomic force microscope to measure the local force response of coherently strained SrTiO3 films on silicon to an applied ac electric field. The observed cantilever response is different in regions that were previously written with positive and negative voltages, but the frequency dependence of this response indicates that the dominant forces are related to electrostatic charging rather than ferroelectricity.}, number={10}, journal={Applied Physics Letters}, author={Marshall, M. S. J. and Kumah, D. P. and Reiner, J. W. and Baddorf, A. P. and Ahn, C. H. and Walker, F. J.}, year={2012} }
 @article{kumah_wu_husseini_dasika_goldman_yacoby_clarke_2011, title={Correlating structure, strain, and morphology of self-assembled InAs quantum dots on GaAs}, volume={98}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78751558945&partnerID=MN8TOARS}, DOI={10.1063/1.3535984}, abstractNote={We report on the use of a direct x-ray phase retrieval method, coherent Bragg rod analysis, to characterize self-assembled InAs quantum dots (QDs) grown epitaxially on GaAs substrates. Electron density maps obtained close to the x-ray absorption edges of the constituent elements are compared to deconvolute composition and atomic spacing information. Our measurements show no evidence of a wetting layer and reveal bowing of the atomic layers throughout the QD, extending from the QD-substrate interface. This leads to a half-layer stacking shift which may act to partially decouple the QDs electronically from the substrate.}, number={2}, journal={Applied Physics Letters}, author={Kumah, D. P. and Wu, J. H. and Husseini, N. S. and Dasika, V. D. and Goldman, R. S. and Yacoby, Y. and Clarke, R.}, year={2011} }
 @article{moyer_vaz_arena_kumah_negusse_henrich_2011, title={Magnetic structure of Fe-doped CoFe(2)O(4) probed by x-ray magnetic spectroscopies}, volume={84}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80052369670&partnerID=MN8TOARS}, DOI={10.1103/PhysRevB.84.054447}, abstractNote={The magnetic properties of iron-doped cobalt ferrite (Co${}_{1\ensuremath{-}x}$Fe${}_{2+x}$O${}_{4}$) (001) thin films grown epitaxially on MgO (001) substrates are investigated by superconducting quantum interference device magnetometry and soft x-ray magnetic linear and circular dichroisms. All Co${}_{1\ensuremath{-}x}$Fe${}_{2+x}$O${}_{4}$ (0.01 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 0.63) samples have out-of-plane magnetic easy axes and large coercive fields, unlike Fe${}_{3}$O${}_{4}$, due to a large Co${}^{2+}$ orbital moment. The magnetic moments for those samples are significantly reduced from their bulk values; however, as $x$ increases, the magnetic moments tend nearer to their bulk values and increase more rapidly as $x$ approaches 1. This reduction in magnetic moment is attributed to spin canting among the Co${}^{2+}$ cations, owing to a small in-plane tensile strain in the film and to an increased antiferromagnetic alignment among all the cations caused by a partially inverse spinel cubic structure and the likely presence of antiphase boundaries. Our results show that small changes in stoichiometry can lead to significant changes in the magnetic moment of Co${}_{1\ensuremath{-}x}$Fe${}_{2+x}$O${}_{4}$, especially at large values of $x$.}, number={5}, journal={Physical Review B}, author={Moyer, J. A. and Vaz, C. A. F. and Arena, D. A. and Kumah, D. and Negusse, E. and Henrich, V. E.}, year={2011} }
 @article{cohen_yochelis_westreich_shusterman_kumah_clarke_yacoby_paltiel_2011, title={Structure of droplet-epitaxy-grown InAs/GaAs quantum dots}, volume={98}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79960572785&partnerID=MN8TOARS}, DOI={10.1063/1.3599063}, abstractNote={We have used a direct x-ray phasing method, coherent Bragg rod analysis, to obtain sub-angstrom resolution electron density maps of the InAs/GaAs dot system. The dots were grown by the droplet heteroepitaxy (DHE) technique and their structural and compositional properties are compared with those of dots grown by the strain-driven Stranski–Krastanov method. Our results show that the Ga diffusion into the DHE-grown dots is somewhat larger; however, other characteristics such as the composition of the dots’ uppermost layers, the interlayer spacing, and the bowing of the atomic layers are similar.}, number={24}, journal={Applied Physics Letters}, author={Cohen, Eyal and Yochelis, Shira and Westreich, Ohad and Shusterman, Sergey and Kumah, Divine P. and Clarke, Roy and Yacoby, Yizhak and Paltiel, Yossi}, year={2011} }
 @article{cohen_elfassy_koplovitz_yochelis_shusterman_kumah_yacoby_clarke_paltiel_2011, title={Surface X-Ray Diffraction Results on the III-V Droplet Heteroepitaxy Growth Process for Quantum Dots: Recent Understanding and Open Questions}, volume={11}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-82055191681&partnerID=MN8TOARS}, DOI={10.3390/s111110624}, abstractNote={In recent years, epitaxial growth of self-assembled quantum dots has offered a way to incorporate new properties into existing solid state devices. Although the droplet heteroepitaxy method is relatively complex, it is quite relaxed with respect to the material combinations that can be used. This offers great flexibility in the systems that can be achieved. In this paper we review the structure and composition of a number of quantum dot systems grown by the droplet heteroepitaxy method, emphasizing the insights that these experiments provide with respect to the growth process. Detailed structural and composition information has been obtained using surface X-ray diffraction analyzed by the COBRA phase retrieval method. A number of interesting phenomena have been observed: penetration of the dots into the substrate ("nano-drilling") is often encountered; interdiffusion and intermixing already start when the group III droplets are deposited, and structure and composition may be very different from the one initially intended.}, number={11}, journal={Sensors}, author={Cohen, Eyal and Elfassy, Naomi and Koplovitz, Guy and Yochelis, Shira and Shusterman, Sergey and Kumah, Divine P. and Yacoby, Yizhak and Clarke, Roy and Paltiel, Yossi}, year={2011}, pages={10624–10637} }
 @article{kumah_reiner_segal_kolpak_zhang_su_zhu_sawicki_broadbridge_ahn_et al._2010, title={The atomic structure and polarization of strained SrTiO(3)/Si}, volume={97}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78650739173&partnerID=MN8TOARS}, DOI={10.1063/1.3529460}, abstractNote={For thin film devices based on coupling ferroelectric polarization to charge carriers in semiconductors, the role of the interface is critical. To elucidate this role, we use synchrotron x-ray diffraction to determine the interface structure of epitaxial SrTiO3 grown on the (001) surface of Si. The average displacement of the O octahedral sublattice relative to the Sr sublattice determines the film polarization and is measured to be about 0.05 nm toward the Si, with Ti off-center displacements 0.009 nm away from the substrate. Measurements of films with different boundary conditions on the top of the SrTiO3 show that the polarization at the SrTiO3/Si interface is dominated by oxide-Si chemical interactions.}, number={25}, journal={Applied Physics Letters}, author={Kumah, D. P. and Reiner, J. W. and Segal, Y. and Kolpak, A. M. and Zhang, Z. and Su, D. and Zhu, Y. and Sawicki, M. S. and Broadbridge, C. C. and Ahn, C. H. and et al.}, year={2010} }
 @article{kumah_shusterman_paltiel_yacoby_clarke_2009, title={Atomic-scale mapping of quantum dots formed by droplet epitaxy}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-72549110483&partnerID=MN8TOARS}, DOI={10.1038/NNANO.2009.271}, number={12}, journal={Nature Nanotechnology}, author={Kumah, DP and Shusterman, S and Paltiel, Y and Yacoby, Y and Clarke, R}, year={2009}, pages={835–838} }
 @inproceedings{schlepütz_willmott_pauli_herger_martoccia_björck_kumah_clarke_yacoby_2009, title={Surface x-ray diffraction of complex metal oxide surfaces and interfaces - a new era}, volume={1092}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-64849085270&partnerID=MN8TOARS}, DOI={10.1063/1.3086245}, abstractNote={The availability of high‐brilliance hard x‐ray synchrotron radiation and the advent of novel photon counting area detectors have brought surface x‐ray diffraction (SXRD) into a new era. It is now possible to record large numbers of structure factors with much improved reliability within reasonable beamtime durations. As a result, structural determination of the surfaces and interfaces of complex crystallographic systems and heterostructures has now become feasible, especially in conjunction with phase‐retrieval methods. It is thereby hoped that detailed structural information will shed light on the unusual physical properties of these systems. Complex metal oxide systems investigated at the Materials Science beamline of the Swiss Light Source, including the surface of SrTiO3, the interface between LaAlO3 and SrTiO3, and the structure of YBa2Cu3O7 grown on NdGaO3, SrTiO3, and (LaSr)(AlTa)O3 will be presented as examples of what is now possible using SXRD.}, booktitle={AIP Conference Proceedings}, author={Schlepütz, C.M. and Willmott, P.R. and Pauli, S.A. and Herger, R. and Martoccia, D. and Björck, M. and Kumah, D. and Clarke, R. and Yacoby, Y.}, year={2009}, pages={9–12} }
 @book{surface x-ray diffraction of complex metal oxide surfaces and interfaces - a new era_2009, volume={1092}, journal={Synchrotron Radiation in Materials Science}, year={2009}, pages={9–12} }
 @article{liu_husseini_torbet_kumah_clarke_pollock_jones_2008, title={In situ imaging of high cycle fatigue crack growth in single crystal nickel-base superalloys by synchrotron X-radiation}, volume={130}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-47149111212&partnerID=MN8TOARS}, DOI={10.1115/1.2840966}, abstractNote={A novel X-ray synchrotron radiation approach is described for real-time imaging of the initiation and growth of fatigue cracks during ultrasonic fatigue (f=20kHz). We report here on new insights on single crystal nickel-base superalloys gained with this approach. A portable ultrasonic fatigue instrument has been designed that can be installed at a high-brilliance X-ray beamline. With a load line and fatigue specimen configuration, this instrument produces stable fatigue crack propagation for specimens as thin as 150μm. The in situ cyclic loading/imaging system has been used initially to image real-time crystallographic fatigue and crack growth under positive mean axial stress in the turbine blade alloy CMSX-4.}, number={2}, journal={Journal of Engineering Materials and Technology-Transactions of the Asme}, author={Liu, L and Husseini, NS and Torbet, CJ and Kumah, DP and Clarke, R and Pollock, TM and Jones, JW}, year={2008}, pages={0210081–0210086} }
 @article{husseini_kumah_yi_torbet_arms_dufresne_pollock_jones_clarke_2008, title={Mapping single-crystal dendritic microstructure and defects in nickel-base superalloys with synchrotron radiation}, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51449107913&partnerID=MN8TOARS}, DOI={10.1016/j.actamat.2008.05.041}, abstractNote={Solidification of single-crystal nickel-base superalloys introduces large-scale segregation of constituent elements and defects such as dislocations and mosaicity. By exploiting the energy tunability and interference capabilities of high-brilliance X-ray radiation, key structural features of the dendritic single crystals were mapped over large areas. Interference and diffraction of synchrotron X-rays revealed significant misorientations between individual dendrites in the as-solidified state. For the first time this mosaic structure was quantified for an array of dendrites and correlated with the density of “grown-in” dislocations whose density ranged from 107 to 108 cm−2. Absorption contrast permitted simultaneous mapping of the distribution of refractory metal additives (e.g. rhenium and tungsten), which segregated preferentially to the dendrite cores with a linear composition gradient toward the interdendritic regions. The results demonstrate that synchrotron X-ray imaging is promising for in situ studies of single-crystal structure and defects in nickel-base superalloys.}, number={17}, journal={Acta Materialia}, author={Husseini, NS and Kumah, DP and Yi, JZ and Torbet, CJ and Arms, DA and Dufresne, EM and Pollock, TM and Jones, JW and Clarke, R}, year={2008}, pages={4715–4723} }
 @article{resonant coherent bragg rod analysis of strained epitaxial heterostructures_2008, volume={93}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51349103419&partnerID=MN8TOARS}, DOI={10.1063/1.2975835}, abstractNote={The resonant response of the complex x-ray scattering factor has been used in conjunction with the coherent Bragg rod analysis phase-retrieval algorithm to determine the composition and strain profiles of ultrathin layers of GaAs grown on InGaAs buffers. The buffer layers are nominally latticed matched with the InP substrate and the subsequent GaAs growth is compared at two different temperatures: 480 and 520°C. We show that electron density maps extracted from Bragg rod scans measured close to the Ga and As K-edges can be used to deconvolute roughness and intermixing. It is found that indium incorporation and roughening lead to a significant reduction of the strain in this system.}, number={8}, journal={Applied Physics Letters}, year={2008} }
 @article{cionca_riposan_kumah_husseini_walko_yacoby_millunchick_clarke_2008, title={Strain and composition mapping of epitaxial nanostructures}, volume={92}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-42349115693&partnerID=MN8TOARS}, DOI={10.1063/1.2908214}, abstractNote={We have used surface x-ray diffraction and a direct method of phase reconstruction to obtain subangström resolution maps of an ion-beam milled In0.27Ga0.73As∕GaAs(001) thin film exhibiting three-dimensional (3D) epitaxial nanostructures. The 3D electron density was calculated based on the diffraction pattern along the Bragg rods measured with synchrotron radiation, from which the chemical composition, strain profile, and average nanostructure shape were extracted. The film maintained a wetting layer exhibiting a sharp strain gradient, which extended into the substrate. Atop the wetting layer, the ion-beam milled islands possessed an apical shape and were depleted in indium.}, number={15}, journal={Applied Physics Letters}, author={Cionca, CN and Riposan, A and Kumah, DP and Husseini, NS and Walko, DA and Yacoby, Y and Millunchick, JM and Clarke, R}, year={2008} }
 @article{structure determination of monolayer-by-monolayer grown la1-xsrxmno3 thin films and the onset of magnetoresistance_2008, volume={77}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-38849209039&partnerID=MN8TOARS}, DOI={10.1103/PhysRevB.77.085401}, abstractNote={Surface x-ray diffraction was used to determine the atomic structures of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ thin films, grown monolayer by monolayer on ${\mathrm{SrTiO}}_{3}$ by pulsed laser deposition. Structures for one-, two-, three-, four-, six-, and nine-monolayer-thick films were solved using the Coherent Bragg rod analysis phase-retrieval method and subsequent structural refinement. Four important results were found. First, the out-of-plane lattice constant is elongated across the substrate-film interface. Second, the transition from substrate to film is not abrupt, but proceeds gradually over approximately three unit cells. Third, Sr segregates towards the topmost monolayer of the film: we determined a Sr-segregation enthalpy of $\ensuremath{-}15\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}$ from the occupation parameters. Finally, the electronic bandwidth $W$ was used to explain the onset of magnetoresistance for films of nine or more monolayers thickness. Resistivity measurements of the nine monolayer-thick film confirm magnetoresistance and the presence of a dead layer with mostly insulating properties.}, number={8}, journal={Physical Review B}, year={2008} }
 @article{kumah_cebollada_clavero_garcia-martin_skuza_lukaszew_clarke_2007, title={Optimizing the planar structure of (111) Au/Co/Au trilayers}, volume={40}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34247503737&partnerID=MN8TOARS}, DOI={10.1088/0022-3727/40/9/003}, abstractNote={Au/Co/Au trilayers are interesting for a range of applications which exploit their unusual optical and electronic transport behaviour in a magnetic field. Here we present a comprehensive structural and morphological study of a series of trilayers with 0–7 nm Co layer thickness fabricated on glass by ultrahigh vacuum vapour deposition. We use a combination of in situ electron diffraction, atomic force microscopy and x-ray scattering to determine the optimum deposition conditions for highly textured, flat and continuous layered structures. The 16 nm Au-on-glass buffer layer, deposited at ambient temperature, is found to develop a smooth (1 1 1) texture on annealing at 350 °C for 10 min. Subsequent growth of the Co layer at 150 °C produces a (1 1 1) textured film with lateral grain size of ~150 nm in the 7 nm-thick Co layer. A simultaneous in-plane and out-of-plane Co lattice expansion is observed for the thinnest Co layers, converging to bulk values for the thickest films. The roughness of the Co layer is similar to that of the Au buffer layer, indicative of conformal growth. The 6 nm Au capping layer smoothens the trilayer surface, resulting in a surface roughness independent of the Co layer thickness.}, number={9}, journal={Journal of Physics D-Applied Physics}, author={Kumah, DP and Cebollada, A and Clavero, C and Garcia-Martin, JM and Skuza, JR and Lukaszew, RA and Clarke, R}, year={2007}, pages={2699–2704} }
 @article{boschetto_mourou_rousse_mordovanakis_hou_nees_kumah_clarke_2007, title={Spatial coherence properties of a compact and ultrafast laser-produced plasma keV x-ray source}, volume={90}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33846119743&partnerID=MN8TOARS}, DOI={10.1063/1.2426941}, abstractNote={The authors use Fresnel diffraction from knife-edges to demonstrate the spatial coherence of a tabletop ultrafast x-ray source produced by laser-plasma interaction. Spatial coherence is achieved in the far field by producing micrometer-scale x-ray spot dimensions. The results show an x-ray source size of 6μm that leads to a transversal coherence length of 20μm at a distance of 60cm from the source. Moreover, they show that the source size is limited by the spatial spread of the absorbed laser energy.}, number={1}, journal={Applied Physics Letters}, author={Boschetto, D and Mourou, G and Rousse, A and Mordovanakis, A and Hou, BX and Nees, J and Kumah, D and Clarke, R}, year={2007} }
 @article{willmott_pauli_herger_schlepütz_martoccia_patterson_delley_clarke_kumah_cionca_et al._2007, title={Structural basis for the conducting interface between LaAlO3 and SrTiO3}, volume={99}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-35148901504&partnerID=MN8TOARS}, DOI={10.1103/PhysRevLett.99.155502}, abstractNote={The complete atomic structure of a five-monolayer film of ${\mathrm{LaAlO}}_{3}$ on ${\mathrm{SrTiO}}_{3}$ has been determined for the first time by surface x-ray diffraction in conjunction with the coherent Bragg rod analysis phase-retrieval method and further structural refinement. Cationic mixing at the interface results in dilatory distortions and the formation of metallic ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{TiO}}_{3}$. By invoking electrostatic potential minimization, the ratio of ${\mathrm{Ti}}^{4+}/{\mathrm{Ti}}^{3+}$ across the interface was determined, from which the lattice dilation could be quantitatively explained using ionic radii considerations. The correctness of this model is supported by density functional theory calculations. Thus, the formation of a quasi-two-dimensional electron gas in this system is explained, based on structural considerations.}, number={15}, journal={Physical Review Letters}, author={Willmott, P.R. and Pauli, S.A. and Herger, R. and Schlepütz, C.M. and Martoccia, D. and Patterson, B.D. and Delley, B. and Clarke, R. and Kumah, D. and Cionca, C. and et al.}, year={2007} }
 @inproceedings{surface plasmon resonance effects in the magneto optical activity of noble metal-ferro magnet ultrathin films_2007, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-51249083884&partnerID=MN8TOARS}, DOI={10.1109/CLEOE-IQEC.2007.4386554}, abstractNote={We present a combined experimental and theoretical study elucidating the role of surface plasmon resonances in the enhancement of magneto optical activity. Au/Co/Au trilayers were grown on glass substrates by ultra-high-vacuum deposition and the thickness of the ferromagnetic layer was systematically varied between 0.3 and 10 nm. A comprehensive structural, magnetic and magneto-optical characterization of the different layers is provided. Simulations of both Kerr spectra and reflectivity were carried out using a transfer matrix algorithm applied to the multilayers.}, booktitle={Conference on Lasers and Electro-Optics Europe - Technical Digest}, year={2007} }
 @inproceedings{gonzalez-diaz_garcia-martin_armelles_garcia-martin_clavero_cebollada_clarke_kumah_lukaszew_skuza_2007, title={Surface plasmon resonance effects in the Magneto Optical activity of noble metal-ferromagnet ultrathin films}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84899093598&partnerID=MN8TOARS}, booktitle={Optics InfoBase Conference Papers}, author={Gonzalez-Diaz, J.B. and Garcia-Martin, A. and Armelles, G. and Garcia-Martin, J.M. and Clavero, C. and Cebollada, A. and Clarke, R. and Kumah, D. and Lukaszew, R.A. and Skuza, J.}, year={2007} }
 @article{gonzalez-diaz_garcia-martin_armelles_garcia-martin_clavero_cebollada_lukaszew_skuza_kumah_clarke_2007, title={Surface-magnetoplasmon nonreciprocity effects in noble-metal/ferromagnetic heterostructures}, volume={76}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84888319538&partnerID=MN8TOARS}, DOI={10.1103/PhysRevB.76.153402}, abstractNote={We demonstrate magnetic field control of surface plasmon excitations in noble-metal/ferromagnetic/noble metal trilayers, analogous to the effects previously observed in semiconductor structures. We show that the coupling of an external magnetic field to the surface plasmon-polariton wave vector is greatly enhanced in the metallic structure due to the ferromagnetic nature of one of its constituents. The observed coupling could be used to modulate the surface plasmon response in ultrasensitive spectroscopic applications.}, number={15}, journal={Physical Review B}, author={Gonzalez-Diaz, JB and Garcia-Martin, A and Armelles, G and Garcia-Martin, JM and Clavero, C and Cebollada, A and Lukaszew, RA and Skuza, JR and Kumah, DP and Clarke, R}, year={2007} }
 @inproceedings{boschetto_mordovanakis_nees_hou_kumah_clarke_mourou_rousse_2006, title={Demonstration of spatial coherence with a compact and ultrafast x-ray source}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-55649117529&partnerID=MN8TOARS}, DOI={10.1109/CLEO.2006.4627599}, abstractNote={We used Fresnel diffraction from knife-edges to demonstrate spatial coherence of tabletop and ultrafast X-ray sources produced by laser-plasma interaction, and show that source size is limited by the spatial spread of absorbed laser energy.}, booktitle={Conference on Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference, CLEO/QELS 2006}, author={Boschetto, D. and Mordovanakis, A. and Nees, J. and Hou, B. and Kumah, D. and Clarke, R. and Mourou, G. and Rousse, A.}, year={2006} }