@article{harbola_pesquera_xu_ashby_martin_hwang_2024, title={Flexoelectric Enhancement of Strain Gradient Elasticity Across a Ferroelectric-to-Paraelectric Phase Transition}, volume={8}, ISSN={["1530-6992"]}, DOI={10.1021/acs.nanolett.4c02946}, abstractNote={We study the temperature dependent elastic properties of Ba}, journal={NANO LETTERS}, author={Harbola, Varun and Pesquera, David and Xu, Ruijuan and Ashby, Paul D. and Martin, Lane W. and Hwang, Harold Y.}, year={2024}, month={Aug} }
 @article{xu_crassee_bechtel_zhou_bercher_korosec_rischau_teyssier_crust_lee_et al._2024, title={Highly confined epsilon-near-zero and surface phonon polaritons in SrTiO<sub>3</sub> membranes}, volume={15}, ISSN={["2041-1723"]}, url={http://dx.doi.org/10.1038/s41467-024-47917-x}, DOI={10.1038/s41467-024-47917-x}, abstractNote={Abstract Recent theoretical studies have suggested that transition metal perovskite oxide membranes can enable surface phonon polaritons in the infrared range with low loss and much stronger subwavelength confinement than bulk crystals. Such modes, however, have not been experimentally observed so far. Here, using a combination of far-field Fourier-transform infrared (FTIR) spectroscopy and near-field synchrotron infrared nanospectroscopy (SINS) imaging, we study the phonon polaritons in a 100 nm thick freestanding crystalline membrane of SrTiO 3 transferred on metallic and dielectric substrates. We observe a symmetric-antisymmetric mode splitting giving rise to epsilon-near-zero and Berreman modes as well as highly confined (by a factor of 10) propagating phonon polaritons, both of which result from the deep-subwavelength thickness of the membranes. Theoretical modeling based on the analytical finite-dipole model and numerical finite-difference methods fully corroborate the experimental results. Our work reveals the potential of oxide membranes as a promising platform for infrared photonics and polaritonics.}, number={1}, journal={NATURE COMMUNICATIONS}, author={Xu, Ruijuan and Crassee, Iris and Bechtel, Hans A. and Zhou, Yixi and Bercher, Adrien and Korosec, Lukas and Rischau, Carl Willem and Teyssier, Jeremie and Crust, Kevin J. and Lee, Yonghun and et al.}, year={2024}, month={Jun} }
 @article{shao_das_hong_xu_chandrika_gomez-ortiz_garcia-fernandez_chen_hwang_junquera_et al._2023, title={Emergent chirality in a polar meron to skyrmion phase transition}, volume={14}, ISSN={["2041-1723"]}, url={http://dx.doi.org/10.1038/s41467-023-36950-x}, DOI={10.1038/s41467-023-36950-x}, abstractNote={AbstractPolar skyrmions are predicted to emerge from the interplay of elastic, electrostatic and gradient energies, in contrast to the key role of the anti-symmetric Dzyalozhinskii-Moriya interaction in magnetic skyrmions. Here, we explore the reversible transition from a skyrmion state (topological charge of −1) to a two-dimensional, tetratic lattice of merons (with topological charge of −1/2) upon varying the temperature and elastic boundary conditions in [(PbTiO3)16/(SrTiO3)16]8 membranes. This topological phase transition is accompanied by a change in chirality, from zero-net chirality (in meronic phase) to net-handedness (in skyrmionic phase). We show how scanning electron diffraction provides a robust measure of the local polarization simultaneously with the strain state at sub-nm resolution, while also directly mapping the chirality of each skyrmion. Using this, we demonstrate strain as a crucial order parameter to drive isotropic-to-anisotropic structural transitions of chiral polar skyrmions to non-chiral merons, validated with X-ray reciprocal space mapping and phase-field simulations.}, number={1}, journal={NATURE COMMUNICATIONS}, publisher={Springer Science and Business Media LLC}, author={Shao, Yu-Tsun and Das, Sujit and Hong, Zijian and Xu, Ruijuan and Chandrika, Swathi and Gomez-Ortiz, Fernando and Garcia-Fernandez, Pablo and Chen, Long-Qing and Hwang, Harold Y. and Junquera, Javier and et al.}, year={2023}, month={Mar} }
 @article{xu_crust_harbola_arras_patel_prosandeev_cao_shao_behera_caretta_et al._2023, title={Size-Induced Ferroelectricity in Antiferroelectric Oxide Membranes}, volume={2}, ISSN={["1521-4095"]}, url={http://dx.doi.org/10.1002/adma.202210562}, DOI={10.1002/adma.202210562}, abstractNote={AbstractDespite extensive studies on size effects in ferroelectrics, how structures and properties evolve in antiferroelectrics with reduced dimensions still remains elusive. Given the enormous potential of utilizing antiferroelectrics for high‐energy‐density storage applications, understanding their size effects will provide key information for optimizing device performances at small scales. Here, the fundamental intrinsic size dependence of antiferroelectricity in lead‐free NaNbO3 membranes is investigated. Via a wide range of experimental and theoretical approaches, an intriguing antiferroelectric‐to‐ferroelectric transition upon reducing membrane thickness is probed. This size effect leads to a ferroelectric single‐phase below 40 nm, as well as a mixed‐phase state with ferroelectric and antiferroelectric orders coexisting above this critical thickness. Furthermore, it is shown that the antiferroelectric and ferroelectric orders are electrically switchable. First‐principle calculations further reveal that the observed transition is driven by the structural distortion arising from the membrane surface. This work provides direct experimental evidence for intrinsic size‐driven scaling in antiferroelectrics and demonstrates enormous potential of utilizing size effects to drive emergent properties in environmentally benign lead‐free oxides with the membrane platform.}, journal={ADVANCED MATERIALS}, publisher={Wiley}, author={Xu, Ruijuan and Crust, Kevin J. and Harbola, Varun and Arras, Remi and Patel, Kinnary Y. and Prosandeev, Sergey and Cao, Hui and Shao, Yu-Tsun and Behera, Piush and Caretta, Lucas and et al.}, year={2023}, month={Mar} }
 @article{shao_das_hong_xu_chandrika_gómez-ortiz_garcía-fernández_chen_hwang_junquera_et al._2021, title={Emergent chirality in a polar meron to skyrmion transition revealed by 4D-STEM}, volume={27}, DOI={10.1017/S1431927621001793}, abstractNote={particle-like swirling arrangements of the}, number={S1}, journal={Microscopy and Microanalysis}, author={Shao, Y.T. and Das, S. and Hong, Z. and Xu, R. and Chandrika, S. and Gómez-Ortiz, F. and García-Fernández, P/ and Chen, L.-Q. and Hwang, H.Y. and Junquera, J. and et al.}, year={2021}, month={Aug}, pages={348–350} }
 @article{harbola_xu_crossley_singh_hwang_2021, title={Fracture and fatigue of thin crystalline SrTiO3 membranes}, volume={119}, DOI={10.1063/5.0060465}, abstractNote={The increasing availability of a variety of two-dimensional materials has generated enormous growth in the field of nanoengineering and nanomechanics. Recent developments in thin film synthesis have enabled the fabrication of freestanding functional oxide membranes that can be readily incorporated in nanomechanical devices. While many oxides are extremely brittle in bulk, recent studies have shown that, in thin membrane form, they can be much more robust to fracture as compared to their bulk counterparts. Here, we investigate the ultimate tensile strength of SrTiO3 membranes by probing freestanding SrTiO3 drumheads using an atomic force microscope. We demonstrate that SrTiO3 membranes can withstand an elastic deformation with an average strain of ∼6% in the sub-20 nm thickness regime, which is more than an order of magnitude beyond the bulk limit. We also show that these membranes are highly resilient upon a high cycle fatigue test, surviving up to a billion cycles of force modulation at 85% of their fracture strain, demonstrating their high potential for use in nanomechanical applications.}, journal={Applied Physics Letters}, author={Harbola, V. and Xu, R. and Crossley, S. and Singh, P. and Hwang, H.Y.}, year={2021}, pages={053102} }
 @article{shao_nahas_sergei_das_xu_chandrika_harikrishnan_hwang_ramesh_bellaiche_et al._2021, title={Probing the dynamics of ferroelectric topological oscillators with the electron beam}, volume={27}, DOI={10.1017/S143192762100283X}, abstractNote={Skyrmions and merons in ferroic materials are topological structures with real-space swirling arrangements of order parameters. While current-driven motion of spin textures in magnets as nanoscale information carriers very little demonstrated with their electric dipolar counterparts, which in ferroelectric heterostructures In part, this is because their motion does not couple to uniform external fields as for the magnetic case, but instead couples to field gradients. As the polar textures are intrinsically nm-scale, developing imaging and control methods for exploring the stability and response of such particle-like objects to external stimuli at these length scales is of importance for the realization of logic devices. topological dipolar textures created by using the electric-field of a localized current, by the electron of scanning transmission}, number={S1}, journal={Microscopy and Microanalysis}, author={Shao, Y.T. and Nahas, Y. and Sergei, P. and Das, S. and Xu, R. and Chandrika, S. and Harikrishnan, K.P. and Hwang, H.Y. and Ramesh, R. and Bellaiche, L. and et al.}, year={2021}, month={Aug}, pages={690–692} }
 @article{nguyen_guo_qin_frew_xu_agar_2021, title={Symmetry-aware recursive image similarity exploration for materials microscopy}, volume={7}, DOI={10.1038/s41524-021-00637-y}, abstractNote={AbstractIn pursuit of scientific discovery, vast collections of unstructured structural and functional images are acquired; however, only an infinitesimally small fraction of this data is rigorously analyzed, with an even smaller fraction ever being published. One method to accelerate scientific discovery is to extract more insight from costly scientific experiments already conducted. Unfortunately, data from scientific experiments tend only to be accessible by the originator who knows the experiments and directives. Moreover, there are no robust methods to search unstructured databases of images to deduce correlations and insight. Here, we develop a machine learning approach to create image similarity projections to search unstructured image databases. To improve these projections, we develop and train a model to include symmetry-aware features. As an exemplar, we use a set of 25,133 piezoresponse force microscopy images collected on diverse materials systems over five years. We demonstrate how this tool can be used for interactive recursive image searching and exploration, highlighting structural similarities at various length scales. This tool justifies continued investment in federated scientific databases with standardized metadata schemas where the combination of filtering and recursive interactive searching can uncover synthesis-structure-property relations. We provide a customizable open-source package (https://github.com/m3-learning/Recursive_Symmetry_Aware_Materials_Microstructure_Explorer) of this interactive tool for researchers to use with their data.}, journal={npj Computational Materials}, author={Nguyen, T.N.M. and Guo, Y. and Qin, S. and Frew, K.S. and Xu, R. and Agar, J.C.}, year={2021}, pages={166} }
 @article{pesquera_parsonnet_qualls_xu_gubser_kim_jiang_velarde_huang_hwang_et al._2020, title={Beyond substrates: Strain engineering of ferroelectric membranes}, volume={32}, DOI={10.1002/adma.202003780}, abstractNote={AbstractStrain engineering in perovskite oxides provides for dramatic control over material structure, phase, and properties, but is restricted by the discrete strain states produced by available high‐quality substrates. Here, using the ferroelectric BaTiO3, production of precisely strain‐engineered, substrate‐released nanoscale membranes is demonstrated via an epitaxial lift‐off process that allows the high crystalline quality of films grown on substrates to be replicated. In turn, fine structural tuning is achieved using interlayer stress in symmetric trilayer oxide‐metal/ferroelectric/oxide‐metal structures fabricated from the released membranes. In devices integrated on silicon, the interlayer stress provides deterministic control of ordering temperature (from 75 to 425 °C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to <10 kV cm−1 and improving switching times to <5 ns for a 20 µm diameter capacitor in a 100‐nm‐thick film). In devices integrated on flexible polymers, enhanced room‐temperature dielectric permittivity with large mechanical tunability (a 90% change upon ±0.1% strain application) is demonstrated. This approach paves the way toward the fabrication of ultrafast CMOS‐compatible ferroelectric memories and ultrasensitive flexible nanosensor devices, and it may also be leveraged for the stabilization of novel phases and functionalities not achievable via direct epitaxial growth.}, number={43}, journal={Advanced Materials}, author={Pesquera, D. and Parsonnet, E. and Qualls, A. and Xu, R. and Gubser, A. and Kim, J. and Jiang, Y. and Velarde, G. and Huang, Y.-L. and Hwang, H.Y. and et al.}, year={2020}, month={Oct}, pages={2003780} }
 @article{taz_prasad_huang_chen_hsu_xu_thakare_sakthivel_liu_hettick_et al._2020, title={Integration of amorphous ferromagnetic oxides with multiferroic materials for room temperature magnetoelectric spintronics}, volume={10}, DOI={10.1038/s41598-020-58592-5}, abstractNote={AbstractA room temperature amorphous ferromagnetic oxide semiconductor can substantially reduce the cost and complexity associated with utilizing crystalline materials for spintronic devices. We report a new material (Fe0.66Dy0.24Tb0.1)3O7-x (FDTO), which shows semiconducting behavior with reasonable electrical conductivity (~500 mOhm-cm), an optical band-gap (2.4 eV), and a large enough magnetic moment (~200 emu/cc), all of which can be tuned by varying the oxygen content during deposition. Magnetoelectric devices were made by integrating ultrathin FDTO with multiferroic BiFeO3. A strong enhancement in the magnetic coercive field of FDTO grown on BiFeO3 validated a large exchange coupling between them. Additionally, FDTO served as an excellent top electrode for ferroelectric switching in BiFeO3 with no sign of degradation after ~1010 switching cycles. RT magneto-electric coupling was demonstrated by modulating the resistance states of spin-valve structures using electric fields.}, journal={Scientific Reports}, author={Taz, H. and Prasad, B. and Huang, Y.-L. and Chen, Z.H. and Hsu, S.-L. and Xu, R. and Thakare, V. and Sakthivel, T. and Liu, C. and Hettick, M. and et al.}, year={2020}, pages={3583} }
 @article{xu_huang_barnard_hong_singh_wong_jansen_harbola_xiao_wang_et al._2020, title={Strain-induced room-temperature ferroelectricity in SrTiO3 membranes}, volume={11}, DOI={10.1038/s41467-020-16912-3}, abstractNote={AbstractAdvances in complex oxide heteroepitaxy have highlighted the enormous potential of utilizing strain engineering via lattice mismatch to control ferroelectricity in thin-film heterostructures. This approach, however, lacks the ability to produce large and continuously variable strain states, thus limiting the potential for designing and tuning the desired properties of ferroelectric films. Here, we observe and explore dynamic strain-induced ferroelectricity in SrTiO3 by laminating freestanding oxide films onto a stretchable polymer substrate. Using a combination of scanning probe microscopy, optical second harmonic generation measurements, and atomistic modeling, we demonstrate robust room-temperature ferroelectricity in SrTiO3 with 2.0% uniaxial tensile strain, corroborated by the notable features of 180° ferroelectric domains and an extrapolated transition temperature of 400 K. Our work reveals the enormous potential of employing oxide membranes to create and enhance ferroelectricity in environmentally benign lead-free oxides, which hold great promise for applications ranging from non-volatile memories and microwave electronics.}, journal={Nature Communications}, author={Xu, R. and Huang, J. and Barnard, E. and Hong, S.S. and Singh, P. and Wong, E. and Jansen, T. and Harbola, V. and Xiao, J. and Wang, B.Y. and et al.}, year={2020}, pages={3141} }
 @article{gao_jain_pandya_dong_yuan_zhou_dedon_thoreton_saremi_xu_et al._2019, title={Designing optimal perovskite structure for high ionic conduction}, volume={32}, DOI={10.1002/adma.201905178}, abstractNote={Solid-oxide fuel/electrolyzer cells are limited by a dearth of electrolyte materials with low ohmic loss and an incomplete understanding of the structure-property relationships that would enable the rational design of better materials. Here, using epitaxial thin-film growth, synchrotron radiation, impedance spectroscopy, and density-functional theory, the impact of structural parameters (i.e., unit-cell volume and octahedral rotations) on ionic conductivity is delineated in La0.9 Sr0.1 Ga0.95 Mg0.05 O3-δ . As compared to the zero-strain state, compressive strain reduces the unit-cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit-cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit-cell volumes and octahedral rotations decrease migration barriers and create low-energy migration pathways, respectively. The desired combination of large unit-cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures both expanded unit-cell volume and large octahedral rotations are experimentally realized, which result in an enhancement of the ionic conductivity. All told, the potential to tune ionic conductivity with structure alone by a factor of ≈2.5 at around 600 °C is observed, which sheds new light on the rational design of ion-conducting perovskite electrolytes.}, number={1}, journal={Advanced Materials}, author={Gao, R. and Jain, A. and Pandya, S. and Dong, Y. and Yuan, Y. and Zhou, H. and Dedon, L. and Thoreton, V. and Saremi, S. and Xu, R. and et al.}, year={2019}, month={Jan}, pages={1905178} }
 @article{lu_crossley_xu_hikita_hwang_2019, title={Freestanding oxide ferroelectric tunnel junction memories transferred onto silicon}, volume={19}, DOI={10.1021/acs.nanolett.9b01327}, abstractNote={Crystalline oxide ferroelectric tunnel junctions enable persistent encoding of information in electric polarization, featuring nondestructive readout and scalability that can exceed current commercial high-speed, nonvolatile ferroelectric memories. However, the well-established fabrication of epitaxial devices on oxide substrates is difficult to adapt to silicon substrates for integration into complementary metal-oxide-semiconductor electronics. In this work, we report ferroelectric tunnel junctions based on 2.8 nm-thick BaTiO3 films grown epitaxially on SrTiO3 growth substrates, released, and relaminated onto silicon. The performance of the transferred devices is comparable to devices characterized on the oxide substrate, suggesting a viable route toward next-generation nonvolatile memories broadly integrable with different materials platforms.}, number={6}, journal={Nano Letters}, author={Lu, D. and Crossley, S. and Xu, R. and Hikita, Y. and Hwang, H.Y.}, year={2019}, pages={3999–4003} }
 @article{xu_liu_saremi_gao_wang_hong_lu_ghosh_pandya_bonturim_et al._2019, title={Kinetic control of tunable, multi-state switching in ferroelectric thin films}, volume={10}, DOI={10.1038/s41467-019-09207-9}, abstractNote={AbstractDeterministic creation of multiple ferroelectric states with intermediate values of polarization remains challenging due to the inherent bi-stability of ferroelectric switching. Here we show the ability to select any desired intermediate polarization value via control of the switching pathway in (111)-oriented PbZr0.2Ti0.8O3 films. Such switching phenomena are driven by kinetic control of the volume fraction of two geometrically different domain structures which are generated by two distinct switching pathways: one direct, bipolar-like switching and another multi-step switching process with the formation of a thermodynamically-stable intermediate twinning structure. Such control of switching pathways is enabled by the competition between elastic and electrostatic energies which favors different types of ferroelastic switching that can occur. Overall, our work demonstrates an alternative approach that transcends the inherent bi-stability of ferroelectrics to create non-volatile, deterministic, and repeatedly obtainable multi-state polarization without compromising other important properties, and holds promise for non-volatile multi-state functional applications.}, journal={Nature Communications}, author={Xu, R. and Liu, S. and Saremi, S. and Gao, R. and Wang, J.J. and Hong, Z.J. and Lu, H. and Ghosh, A. and Pandya, S. and Bonturim, E. and et al.}, year={2019}, pages={1282} }
 @article{lu_chen_cao_tang_xu_saremi_zhang_you_dong_das_et al._2019, title={Mechanical-force-induced non-local collective ferroelastic switching in epitaxial lead-titanate thin films}, volume={10}, DOI={10.1038/s41467-019-11825-2}, abstractNote={AbstractFerroelastic switching in ferroelectric/multiferroic oxides plays a crucial role in determining their dielectric, piezoelectric, and magnetoelectric properties. In thin films of these materials, however, substrate clamping is generally thought to limit the electric-field- or mechanical-force-driven responses to the local scale. Here, we report mechanical-force-induced large-area, non-local, collective ferroelastic domain switching in PbTiO3 epitaxial thin films by tuning the misfit-strain to be near a phase boundary wherein c/a and a1/a2 nanodomains coexist. Phenomenological models suggest that the collective, c-a-c-a ferroelastic switching arises from the small potential barrier between the degenerate domain structures, and the large anisotropy of a and c domains, which collectively generates much larger response and large-area domain propagation. Large-area, non-local response under small stimuli, unlike traditional local response to external field, provides an opportunity of unique response to local stimuli, which has potential for use in high-sensitivity pressure sensors and switches.}, journal={Nature Communications}, author={Lu, X. and Chen, Z. and Cao, Y. and Tang, Y. and Xu, R. and Saremi, S. and Zhang, Z. and You, L. and Dong, Y. and Das, S. and et al.}, year={2019}, pages={3951} }
 @article{saremi_xu_allen_maher_agar_gao_hosemann_martin_2018, title={Local control of defects and switching properties in ferroelectric thin films}, volume={2}, DOI={10.1103/PhysRevMaterials.2.084414}, abstractNote={Electric-field switching of polarization is the building block of a wide variety of ferroelectric devices. In turn, understanding the factors affecting ferroelectric switching and developing routes to control it are of great technological significance. This work provides systematic experimental evidence of the role of defects in affecting ferroelectric-polarization switching and utilizes the ability to deterministically create and spatially locate point defects in $\mathrm{PbZ}{\mathrm{r}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.8}{\mathrm{O}}_{3}$ thin films via focused-helium-ion bombardment and the subsequent defect-polarization coupling as a knob for on-demand control of ferroelectric switching (e.g., coercivity and imprint). At intermediate ion doses ($0.22--2.2\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.16em}{0ex}}\mathrm{ions}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$), the dominant defects (isolated point defects and small clusters) show a weak interaction with domain walls (pinning potentials from $200--500\phantom{\rule{0.16em}{0ex}}\mathrm{K}\phantom{\rule{0.16em}{0ex}}\mathrm{MV}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$), resulting in small and symmetric changes in the coercive field. At high doses ($0.22--1\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.16em}{0ex}}\mathrm{ions}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$), on the other hand, the dominant defects (larger defect complexes and clusters) strongly pin domain-wall motion (pinning potentials from 500 to $1600\phantom{\rule{0.16em}{0ex}}\mathrm{K}\phantom{\rule{0.16em}{0ex}}\mathrm{MV}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$), resulting in a large increase in the coercivity and imprint, and a reduction in the polarization. This local control of ferroelectric switching provides a route to produce novel functions; namely, tunable multiple polarization states, rewritable pre-determined 180\ifmmode^\circ\else\textdegree\fi{} domain patterns, and multiple zero-field piezoresponse and permittivity states. Such an approach opens up pathways to achieve multilevel data storage and logic, nonvolatile self-sensing shape-memory devices, and nonvolatile ferroelectric field-effect transistors.}, number={8}, journal={Physical Review Materials}, author={Saremi, S. and Xu, R. and Allen, F. and Maher, J. and Agar, J.C. and Gao, R. and Hosemann, P. and Martin, L.W.}, year={2018}, month={Aug}, pages={084414} }
 @article{dasgupta_saremi_xu_dedon_pandya_damodaran_martin_2018, title={Nonstoichiometry, structure, and properties of Ba1−xTiOy thin films}, volume={6}, DOI={10.1039/C8TC02725K}, abstractNote={Sometimes worse is better – variations in the chemistry of BaTiO3can have unexpected effects on properties.}, journal={Journal of Materials Chemistry C}, author={Dasgupta, A. and Saremi, S. and Xu, R. and Dedon, L. and Pandya, S. and Damodaran, A.R. and Martin, L.W.}, year={2018}, pages={10751–10759} }
 @article{xu_gao_saremi_reyes-lillo_dong_qi_lu_chen_lu_hsu_et al._2018, title={Reducing coercive-field scaling in ferroelectric thin films via orientation control}, volume={12}, DOI={10.1021/acsnano.8b01399}, abstractNote={The desire for low-power/voltage operation of devices is driving renewed interest in understanding scaling effects in ferroelectric thin films. As the dimensions of ferroelectrics are reduced, the properties can vary dramatically, including the robust scaling relationship between coercive field ( Ec) and thickness ( d), also referred to as the Janovec-Kay-Dunn (JKD) law, wherein Ec ∝ d-2/3. Here, we report that whereas (001)-oriented heterostructures follow JKD scaling across the thicknesses range of 20-330 nm, (111)-oriented heterostructures of the canonical tetragonal ferroelectric PbZr0.2Ti0.8O3 exhibit a deviation from JKD scaling wherein a smaller scaling exponent for the evolution of Ec is observed in films of thickness ≲ 165 nm. X-ray diffraction reveals that whereas (001)-oriented heterostructures remain tetragonal for all thicknesses, (111)-oriented heterostructures exhibit a transition from tetragonal-to-monoclinic symmetry in films of thickness ≲ 165 nm as a result of the compressive strain. First-principles calculations suggest that this symmetry change contributes to the deviation from the expected scaling, as the monoclinic phase has a lower energy barrier for switching. This structural evolution also gives rise to changes in the c/ a lattice parameter ratio, wherein this ratio increases and decreases in (001)- and (111)-oriented heterostructures, respectively, as the films are made thinner. In (111)-oriented heterostructures, this reduced tetragonality drives a reduction of the remanent polarization and, therefore, a reduction of the domain-wall energy and overall energy barrier to switching, which further exacerbates the deviation from the expected scaling. Overall, this work demonstrates a route toward reducing coercive fields in ferroelectric thin films and provides a possible mechanism to understand the deviation from JKD scaling.}, number={5}, journal={ACS Nano}, author={Xu, R. and Gao, R. and Saremi, S. and Reyes-Lillo, S.E. and Dong, Y. and Qi, Y. and Lu, H. and Chen, Z. and Lu, X. and Hsu, S.-L. and et al.}, year={2018}, pages={4736–4743} }
 @article{pandya_velarde_gao_everhardt_wilbur_xu_maher_agar_dames_martin_2019, title={Understanding the role of ferroelastic domains on the pyroelectric and electrocaloric effects in ferroelectric thin films}, volume={31}, DOI={10.1002/adma.201803312}, abstractNote={AbstractTemperature‐ and electric‐field‐induced structural transitions in a polydomain ferroelectric can have profound effects on its electrothermal susceptibilities. Here, the role of such ferroelastic domains on the pyroelectric and electrocaloric response is experimentally investigated in thin films of the tetragonal ferroelectric PbZr0.2Ti0.8O3. By utilizing epitaxial strain, a rich set of ferroelastic polydomain states spanning a broad thermodynamic phase space are stabilized. Using temperature‐dependent scanning‐probe microscopy, X‐ray diffraction, and high‐frequency phase‐sensitive pyroelectric measurements, the propensity of domains to reconfigure under a temperature perturbation is quantitatively studied. In turn, the “extrinsic” contributions to pyroelectricity exclusively due to changes between the ferroelastic domain population is elucidated as a function of epitaxial strain. Further, using highly sensitive thin‐film resistive thermometry, direct electrocaloric temperature changes are measured on these polydomain thin films for the first time. The results demonstrate that temperature‐ and electric‐field‐driven domain interconversion under compressive strain diminish both the pyroelectric and the electrocaloric effects, while both these susceptibilities are enhanced due to the exact‐opposite effect from the extrinsic contributions under tensile strain.}, number={5}, journal={Advanced Materials}, author={Pandya, S. and Velarde, G.A. and Gao, R. and Everhardt, A.S. and Wilbur, J.D. and Xu, R. and Maher, J.T. and Agar, J.C. and Dames, C. and Martin, L.W.}, year={2019}, month={Feb}, pages={1803312} }
 @article{khan_hoffmann_chatterjee_lu_xu_serrao_smith_martin_hu_ramesh_et al._2017, title={Differential voltage amplification from ferroelectric negative capacitance}, volume={111}, DOI={10.1063/1.5006958}, abstractNote={We demonstrate that a ferroelectric can cause a differential voltage amplification without needing an external energy source. As the ferroelectric switches from one polarization state to the other, a transfer of energy takes place from the ferroelectric to the dielectric, determined by the ratio of their capacitances, which, in turn, leads to the differential amplification. This amplification is very different in nature from conventional inductor-capacitor based circuits where an oscillatory amplification can be observed. The demonstration of differential voltage amplification from completely passive capacitor elements only has fundamental ramifications for next generation electronics.}, journal={Applied Physics Letters}, author={Khan, A.I. and Hoffmann, M. and Chatterjee, K. and Lu, Z. and Xu, R. and Serrao, C. and Smith, S. and Martin, L.W. and Hu, C.C. and Ramesh, R. and et al.}, year={2017}, pages={253501} }
 @article{chen_chen_liu_holtz_li_renshaw wang_lü_motapothula_fan_turcaud_et al._2017, title={Electron Accumulation and Emergent Magnetism in LaMnO3 / SrTiO3 Heterostructures}, volume={119}, DOI={10.1103/PhysRevLett.119.156801}, abstractNote={Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched LaMnO_{3}/SrTiO_{3} (001) heterostructures. Using a combination of element-specific x-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation, and ferromagnetism have been observed within the polar, antiferromagnetic insulator LaMnO_{3}. Our results show that the critical thickness for the onset of electron accumulation is as thin as 2 unit cells (UC), significantly thinner than the observed critical thickness for ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely induced by electron overaccumulation. In turn, by controlling the doping of the LaMnO_{3}, we are able to neutralize the excessive electrons from the polar mismatch in ultrathin LaMnO_{3} films and thus enable ferromagnetism in films as thin as 3 UC, extending the limits of our ability to synthesize and tailor emergent phenomena at interfaces and demonstrating manipulation of the electronic and magnetic structures of materials at the shortest length scales.}, number={15}, journal={Physical Review Letters}, author={Chen, Z. and Chen, Z. and Liu, Z.Q. and Holtz, M.E. and Li, C.J. and Renshaw Wang, X. and Lü, W.M. and Motapothula, M. and Fan, L.S. and Turcaud, J.A. and et al.}, year={2017}, month={Oct}, pages={156801} }
 @article{saremi_xu_dedon_gao_ghosh_dasgupta_martin_2018, title={Electronic Transport and Ferroelectric Switching in Ion-Bombarded, Defect-Engineered BiFeO3 Thin Films}, volume={5}, DOI={10.1002/admi.201700991}, abstractNote={AbstractDespite continued interest in the multiferroic BiFeO3 for a diverse range of applications, use of this material is limited by its poor electrical leakage. This work demonstrates some of the most resistive BiFeO3 thin films reported to date via defect engineering achieved via high‐energy ion bombardment. High leakage in as‐grown BiFeO3 thin films is shown to be due to the presence of moderately shallow isolated trap states, which form during growth. Ion bombardment is shown to be an effective way to reduce this free carrier transport (by up to ≈4 orders of magnitude) by trapping the charge carriers in bombardment‐induced, deep‐lying defect complexes and clusters. The ion bombardment is also found to give rise to an increased resistance to switching as a result of an increase in defect concentration. This study demonstrates a systematic ion‐dose‐dependent increase in the coercivity, extension of the defect‐related creep regime, increase in the pinning activation energy, decrease in the switching speed, and broadening of the field distribution of switching. Ultimately, the use of such defect‐engineering routes to control materials will require identification of an optimum range of ion dosage to achieve maximum enhancement in resistivity with minimum impact on ferroelectric switching.}, number={3}, journal={Advanced Materials Interfaces}, author={Saremi, S. and Xu, R. and Dedon, L.R. and Gao, R. and Ghosh, A. and Dasgupta, A. and Martin, L.W.}, year={2018}, month={Feb}, pages={1700991} }
 @article{gao_reyes-lillo_xu_dasgupta_dong_dedon_kim_saremi_chen_serrao_et al._2017, title={Ferroelectricity in Pb1+δZrO3 Thin Films}, volume={29}, DOI={10.1021/acs.chemmater.7b02506}, abstractNote={Antiferroelectric PbZrO3 is being considered for a wide range of applications where the competition between centrosymmetric and noncentrosymmetric phases is important to the response. Here, we focus on the epitaxial growth of PbZrO3 thin films and understanding the chemistry–structure coupling in Pb1+δZrO3 (δ = 0, 0.1, 0.2). High-quality, single-phase Pb1+δZrO3 films are synthesized via pulsed-laser deposition. Although no significant lattice parameter change is observed in X-ray studies, electrical characterization reveals that while the PbZrO3 and Pb1.1ZrO3 heterostructures remain intrinsically antiferroelectric, the Pb1.2ZrO3 heterostructures exhibit a hysteresis loop indicative of ferroelectric response. Further X-ray scattering studies reveal strong quarter-order diffraction peaks in PbZrO3 and Pb1.1ZrO3 heterostructures indicative of antiferroelectricity, while no such peaks are observed for Pb1.2ZrO3 heterostructures. Density functional theory calculations suggest the large cation nonstoichiometry ...}, number={15}, journal={Chemistry of Materials}, author={Gao, R. and Reyes-Lillo, S.E. and Xu, R. and Dasgupta, A. and Dong, Y. and Dedon, L.R. and Kim, J. and Saremi, S. and Chen, Z. and Serrao, C.R. and et al.}, year={2017}, pages={6544–6551} }
 @article{rogers_xu_pandya_martin_shim_2017, title={Slow Conductance Relaxation in Graphene–Ferroelectric Field-Effect Transistors}, volume={121}, DOI={10.1021/acs.jpcc.7b00753}, abstractNote={Tuning graphene conduction states with the remnant polarization of ferroelectric oxides holds much promise for a range of low-power transistor and memory applications. However, understanding how the ferroelectric polarization affects the electronic properties of graphene remains challenging because of a variety of intricate and dynamic screening processes that complicate the interaction. Here, we report on a range of slow electrical conductance relaxation behavior in graphene–ferroelectric field-effect transistors with the extreme case leading to the convergence of two polarization-induced states. Piezoresponse force microscopy through the graphene channel reveals that the ferroelectric polarization remains essentially unchanged during this conductance relaxation. When measured in vacuum, the conductance relaxation is significantly reduced, suggesting equilibration with adsorbates from the ambient atmosphere that can cause charge transfer to and from graphene to be the origin of the slow relaxation.}, number={13}, journal={Journal of Physical Chemistry C}, author={Rogers, S.P. and Xu, R. and Pandya, S. and Martin, L.W. and Shim, M.}, year={2017}, pages={7542–7548} }
 @article{damodaran_pandya_agar_cao_vasudevan_xu_saremi_li_kim_mccarter_et al._2017, title={Three-State Ferroelastic Switching and Large Electromechanical Responses in PbTiO3 Thin Films}, volume={29}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000412184100007&KeyUID=WOS:000412184100007}, DOI={10.1002/adma.201702069}, abstractNote={Leveraging competition between energetically degenerate states to achieve large field‐driven responses is a hallmark of functional materials, but routes to such competition are limited. Here, a new route to such effects involving domain‐structure competition is demonstrated, which arises from strain‐induced spontaneous partitioning of PbTiO3 thin films into nearly energetically degenerate, hierarchical domain architectures of coexisting c/a and a1/a2 domain structures. Using band‐excitation piezoresponse force microscopy, this study manipulates and acoustically detects a facile interconversion of different ferroelastic variants via a two‐step, three‐state ferroelastic switching process (out‐of‐plane polarized c+ → in‐plane polarized a → out‐of‐plane polarized c− state), which is concomitant with large nonvolatile electromechanical strains (≈1.25%) and tunability of the local piezoresponse and elastic modulus (>23%). It is further demonstrated that deterministic, nonvolatile writing/erasure of large‐area patterns of this electromechanical response is possible, thus showing a new pathway to improved function and properties.}, number={37}, journal={Advanced Materials}, author={Damodaran, A.R. and Pandya, S. and Agar, J.C. and Cao, Y. and Vasudevan, R. and Xu, R. and Saremi, S. and Li, Q. and Kim, J. and McCarter, M.R. and et al.}, year={2017}, month={Oct}, pages={1702069} }
 @article{saremi_xu_dedon_mundy_hsu_chen_damodaran_chapman_evans_martin_2016, title={Enhanced electrical resistivity and properties via ion bombardment of ferroelectric thin films}, volume={28}, DOI={10.1002/adma.201603968}, abstractNote={A novel approach to on-demand improvement of electronic properties in complex-oxide ferroelectrics is demonstrated whereby ion bombardment - commonly used in classic semiconductor materials - is applied to the PbTiO3 system. The result is deterministic reduction in leakage currents by 5 orders of magnitude, improved ferroelectric switching, and unprecedented insights into the nature of defects and intergap state evolution in these materials.}, number={48}, journal={Advanced Materials}, author={Saremi, S. and Xu, R. and Dedon, L.R. and Mundy, J.M. and Hsu, S.-L. and Chen, Z.H. and Damodaran, A.R. and Chapman, S.P. and Evans, J.T. and Martin, L.W.}, year={2016}, month={Dec}, pages={10750–10756} }
 @article{agar_pandya_xu_yadav_liu_angsten_saremi_asta_ramamoorthy_martin_2016, title={Frontiers in strain-engineered multifunctional ferroic materials}, volume={6}, DOI={10.1557/mrc.2016.29}, abstractNote={Multifunctional, complex oxides capable of exhibiting highly-coupled electrical, mechanical, thermal, and magnetic susceptibilities have been pursued to address a range of salient technological challenges. Today, efforts are focused on addressing the pressing needs of a range of applications and identifying, understanding, and controlling materials with the potential for enhanced or novel responses. In this prospective, we highlight important developments in theoretical and computational techniques, materials synthesis, and characterization techniques. We explore how these new approaches could revolutionize our ability to discover, probe, and engineer these materials and provide a context for new arenas where these materials might make an impact.}, journal={MRS Communications}, author={Agar, J.C. and Pandya, S. and Xu, R. and Yadav, A. and Liu, Z. and Angsten, T. and Saremi, S. and Asta, M. and Ramamoorthy, R. and Martin, L.W.}, year={2016}, pages={151–166} }
 @article{lu_wang_li_lipatov_lee_rajapitamahuni_xu_hong_farokhipoor_martin_et al._2016, title={Nanodomain Engineering in Ferroelectric Capacitors with Graphene Electrodes}, volume={16}, DOI={10.1021/acs.nanolett.6b02963}, abstractNote={Polarization switching in ferroelectric capacitors is typically realized by application of an electrical bias to the capacitor electrodes and occurs via a complex process of domain structure reorganization. As the domain evolution in real devices is governed by the distribution of the nucleation centers, obtaining a domain structure of a desired configuration by electrical pulsing is challenging, if not impossible. Recent discovery of polarization reversal via the flexoelectric effect has opened a possibility for deterministic control of polarization in ferroelectric capacitors. In this paper, we demonstrate mechanical writing of arbitrary-shaped nanoscale domains in thin-film ferroelectric capacitors with graphene electrodes facilitated by a strain gradient induced by a tip of an atomic force microscope (AFM). A phase-field modeling prediction of a strong effect of graphene thickness on the threshold load required to initiate mechanical switching has been confirmed experimentally. Deliberate voltage-free domain writing represents a viable approach for development of functional devices based on domain topology and electronic properties of the domains and domain walls.}, number={10}, journal={Nano Letters}, author={Lu, H. and Wang, B. and Li, T. and Lipatov, A. and Lee, H. and Rajapitamahuni, A. and Xu, R. and Hong, X. and Farokhipoor, S. and Martin, L.W. and et al.}, year={2016}, pages={6460–6466} }
 @article{damodaran_agar_pandya_chen_dedon_xu_apgar_saremi_martin_2016, title={New modalities of strain-control of ferroelectric thin films}, volume={28}, DOI={10.1088/0953-8984/28/26/263001}, abstractNote={Ferroelectrics, with their spontaneous switchable electric polarization and strong coupling between their electrical, mechanical, thermal, and optical responses, provide functionalities crucial for a diverse range of applications. Over the past decade, there has been significant progress in epitaxial strain engineering of oxide ferroelectric thin films to control and enhance the nature of ferroelectric order, alter ferroelectric susceptibilities, and to create new modes of response which can be harnessed for various applications. This review aims to cover some of the most important discoveries in strain engineering over the past decade and highlight some of the new and emerging approaches for strain control of ferroelectrics. We discuss how these new approaches to strain engineering provide promising routes to control and decouple ferroelectric susceptibilities and create new modes of response not possible in the confines of conventional strain engineering. To conclude, we will provide an overview and prospectus of these new and interesting modalities of strain engineering helping to accelerate their widespread development and implementation in future functional devices.}, number={26}, journal={Journal of Physics: Condensed Matter}, author={Damodaran, A.R. and Agar, J.C. and Pandya, S. and Chen, Z. and Dedon, L. and Xu, R. and Apgar, B. and Saremi, S. and Martin, L.W.}, year={2016}, pages={263001} }
 @article{chen_wang_qi_yang_soares_apgar_gao_xu_lee_zhang_et al._2016, title={Self-Assembled, Nanostructured, Tunable Metamaterials via Spinodal Decomposition}, volume={10}, DOI={10.1021/acsnano.6b05736}, abstractNote={Self-assembly via nanoscale phase separation offers an elegant route to fabricate nanocomposites with physical properties unattainable in single-component systems. One important class of nanocomposites are optical metamaterials which exhibit exotic properties and lead to opportunities for agile control of light propagation. Such metamaterials are typically fabricated via expensive and hard-to-scale top-down processes requiring precise integration of dissimilar materials. In turn, there is a need for alternative, more efficient routes to fabricate large-scale metamaterials for practical applications with deep-subwavelength resolution. Here, we demonstrate a bottom-up approach to fabricate scalable nanostructured metamaterials via spinodal decomposition. To demonstrate the potential of such an approach, we leverage the innate spinodal decomposition of the VO2-TiO2 system, the metal-to-insulator transition in VO2, and thin-film epitaxy, to produce self-organized nanostructures with coherent interfaces and a structural unit cell down to 15 nm (tunable between horizontally and vertically aligned lamellae) wherein the iso-frequency surface is temperature-tunable from elliptic to hyperbolic dispersion producing metamaterial behavior. These results provide an efficient route for the fabrication of nanostructured metamaterials and other nanocomposites for desired functionalities.}, number={11}, journal={ACS Nano}, author={Chen, Z.H. and Wang, X. and Qi, Y. and Yang, S. and Soares, J.A.N.T. and Apgar, B.A. and Gao, R. and Xu, R. and Lee, Y. and Zhang, X. and et al.}, year={2016}, pages={10237–10244} }
 @article{hinnefeld_xu_rogers_pandya_shim_martin_mason_2016, title={Single gate P-N junctions in graphene-ferroelectric devices}, volume={108}, DOI={10.1063/1.4950975}, abstractNote={Graphene's linear dispersion relation and the attendant implications for bipolar electronics applications have motivated a range of experimental efforts aimed at producing p-n junctions in graphene. Here we report electrical transport measurements of graphene p-n junctions formed via simple modifications to a PbZr0.2Ti0.8O3 substrate, combined with a self-assembled layer of ambient environmental dopants. We show that the substrate configuration controls the local doping region, and that the p-n junction behavior can be controlled with a single gate. Finally, we show that the ferroelectric substrate induces a hysteresis in the environmental doping which can be utilized to activate and deactivate the doping, yielding an “on-demand” p-n junction in graphene controlled by a single, universal backgate.}, journal={Applied Physics Letters}, author={Hinnefeld, J.H. and Xu, R. and Rogers, S. and Pandya, S. and Shim, M. and Martin, L.W. and Mason, N.}, year={2016}, pages={203109} }
 @article{pandya_damodaran_xu_hsu_agar_martin_2016, title={Strain-induced growth instability and nanoscale surface patterning in perovskite thin films}, volume={6}, DOI={10.1038/srep26075}, abstractNote={AbstractDespite extensive studies on the effects of epitaxial strain on the evolution of the lattice and properties of materials, considerably less work has explored the impact of strain on growth dynamics. In this work, we demonstrate a growth-mode transition from 2D-step flow to self-organized, nanoscale 3D-island formation in PbZr0.2Ti0.8O3/SrRuO3/SrTiO3 (001) heterostructures as the kinetics of the growth process respond to the evolution of strain. With increasing heterostructure thickness and misfit dislocation formation at the buried interface, a periodic, modulated strain field is generated that alters the adatom binding energy and, in turn, leads to a kinetic instability that drives a transition from 2D growth to ordered, 3D-island formation. The results suggest that the periodically varying binding energy can lead to inhomogeneous adsorption kinetics causing preferential growth at certain sites. This, in conjunction with the presence of an Ehrlich-Schwoebel barrier, gives rise to long-range, periodically-ordered arrays of so-called “wedding cake” 3D nanostructures which self-assemble along the [100] and [010].}, journal={Scientific Reports}, author={Pandya, S. and Damodaran, A.R. and Xu, R. and Hsu, S.-L. and Agar, J.C. and Martin, L.W.}, year={2016}, pages={26075} }
 @article{xu_zhang_chen_martin_2015, title={Orientation-dependent structural phase diagrams and dielectric properties of PbZr1−xTixO3 polydomain thin films}, volume={91}, DOI={10.1103/PhysRevB.91.144106}, abstractNote={© 2015 American Physical Society. The orientation-dependent equilibrium ferroelectric domain structures and dielectric properties of polydomain PbZr 1-x Ti x O 3 thin films are investigated using a phenomenological Ginzburg-Landau-Devonshire thermodynamic model. We develop and describe three-dimensional polydomain models for (001)-, (101)-, and (111)-oriented films and explore the evolution of the structure and dielectric permittivity of the system as a function of epitaxial strain across the composition range 0.5 ≤ x ≤ 1.0. Our studies reveal that the film orientation, epitaxial strain, and composition can combine in unexpected ways to drive exotic phase stability and transformations which have intriguing implications for the properties. In particular, in (101)- and (111)-oriented films, the application of epitaxial strains along non-〈001〉-type crystallographic directions significantly reduces the stability range of the parent tetragonal phase [which is dominant in (001)-oriented films] and results in a variety of new symmetries. We also observe that the film orientation can be used to tune the relative fraction of intrinsic (i.e., within a domain) and extrinsic (i.e., from domain wall motion) contributions to the dielectric permittivity. Ultimately these studies reveal how composition, epitaxial strain, and film orientation provide for comprehensive control of the structure and properties of ferroelectrics.}, number={14}, journal={Physical Review B}, author={Xu, R. and Zhang, J. and Chen, Z.H. and Martin, L.W.}, year={2015}, month={Apr}, pages={144106} }
 @article{islam_saldana-greco_gu_wang_breckenfeld_lei_xu_hawley_xi_martin_et al._2016, title={Surface Chemically Switchable Ultraviolet Luminescence from Interfacial Two-Dimensional Electron Gas}, volume={16}, DOI={10.1021/acs.nanolett.5b04461}, abstractNote={We report intense, narrow line-width, surface chemisorption-activated and reversible ultraviolet (UV) photoluminescence from radiative recombination of the two-dimensional electron gas (2DEG) with photoexcited holes at LaAlO3/SrTiO3. The switchable luminescence arises from an electron transfer-driven modification of the electronic structure via H-chemisorption onto the AlO2-terminated surface of LaAlO3, at least 2 nm away from the interface. The control of the onset of emission and its intensity are functionalities that go beyond the luminescence of compound semiconductor quantum wells. Connections between reversible chemisorption, fast electron transfer, and quantum-well luminescence suggest a new model for surface chemically reconfigurable solid-state UV optoelectronics and molecular sensing.}, number={1}, journal={Nano Letters}, author={Islam, M.A. and Saldana-Greco, D. and Gu, Z. and Wang, F. and Breckenfeld, E. and Lei, Q. and Xu, R. and Hawley, C.J. and Xi, X.X. and Martin, L.W. and et al.}, year={2016}, pages={681–687} }
 @article{chen_damodaran_xu_lee_martin_2014, title={Effect of “symmetry mismatch” on the domain structure of rhombohedral BiFeO3 thin films}, volume={104}, DOI={10.1063/1.4875801}, abstractNote={Considerable work has focused on the use of epitaxial strain to engineer domain structures in ferroic materials. Here, we revisit the observed reduction of domain variants in rhombohedral BiFeO3 films on rare-earth scandate substrates. Prior work has attributed the reduction of domain variants to anisotropic in-plane strain, but our findings suggest that the monoclinic distortion of the substrate, resulting from oxygen octahedral rotation, is the driving force for variant selection. We study epitaxial BiFeO3/DyScO3 (110)O heterostructures with and without ultrathin, cubic SrTiO3 buffer layers as a means to isolate the effect of “symmetry mismatch” on the domain formation. Two-variant stripe domains are observed in films grown directly on DyScO3, while four-variant domains are observed in films grown on SrTiO3-buffered DyScO3 when the buffer layer is >2 nm thick. This work provides insights into the role of the substrate—beyond just lattice mismatch—in manipulating and controlling domain structure evolution in materials.}, journal={Applied Physics Letters}, author={Chen, Z.H. and Damodaran, A.R. and Xu, R. and Lee, S. and Martin, L.W.}, year={2014}, pages={182908} }
 @article{xu_liu_karthik_damodaran_grinberg_rappe_martin_2015, title={Ferroelectric polarization reversal via successive ferroelastic transitions}, volume={14}, DOI={10.1038/nmat4119}, abstractNote={Switchable polarization makes ferroelectrics a critical component in memories, actuators and electro-optic devices, and potential candidates for nanoelectronics. Although many studies of ferroelectric switching have been undertaken, much remains to be understood about switching in complex domain structures and in devices. In this work, a combination of thin-film epitaxy, macro- and nanoscale property and switching characterization, and molecular dynamics simulations are used to elucidate the nature of switching in PbZr0.2Ti0.8O3 thin films. Differences are demonstrated between (001)-/(101)- and (111)-oriented films, with the latter exhibiting complex, nanotwinned ferroelectric domain structures with high densities of 90° domain walls and considerably broadened switching characteristics. Molecular dynamics simulations predict both 180° (for (001)-/(101)-oriented films) and 90° multi-step switching (for (111)-oriented films) and these processes are subsequently observed in stroboscopic piezoresponse force microscopy. These results have implications for our understanding of ferroelectric switching and offer opportunities to change domain reversal speed. Ferroelectric switching is studied in PbZr0.2Ti0.8O3 thin films. Nanotwinned ferroelectric domains with broadened switching characteristics are observed and control over ferroelectric switching is demonstrated.}, journal={Nature Materials}, author={Xu, R. and Liu, S. and Karthik, J. and Damodaran, A.R. and Grinberg, I. and Rappe, A.M. and Martin, L.W.}, year={2015}, pages={79–86} }
 @article{xu_karthik_damodaran_martin_2014, title={Stationary domain wall contribution to enhanced ferroelectric susceptibility}, volume={5}, DOI={10.1038/ncomms4120}, abstractNote={In ferroelectrics, the effect of domain wall motion on properties has been widely studied, but non-motional or stationary contributions from the volume of material within the domain wall itself has received less attention. Here we report the measurement of stationary domain wall contributions to permittivity in PbZr0.2Ti0.8O3 films. Studies of (001)-, (101)- and (111)-oriented epitaxial films reveal that (111)-oriented films, in which the motional domain wall contributions are frozen out, exhibit permittivity values approximately three times larger than the intrinsic response alone. This discrepancy can only be accounted for by considering a stationary contribution from the domain wall volume of the material that is 6–78 times larger than the bulk response, and is consistent with predictions of the enhancement of susceptibilities within 90° domain walls. This work offers new insights into the microscopic origin of dielectric enhancement and provides a pathway to engineer the dielectric response of materials. The motion of ferroelectric domain walls is critical in determining the response of ferroelectrics to an applied stimulus. Here, the authors directly measure the effect of an additional non-motional or stationary domain wall contribution to dielectric susceptibility in nanodomain ferroelectric films.}, journal={Nature Communications}, author={Xu, R. and Karthik, J. and Damodaran, A.R. and Martin, L.W.}, year={2014}, pages={3120} }
 @article{zhang_xu_damodaran_chen_martin_2014, title={Understanding order in compositionally graded ferroelectrics: Flexoelectricity, gradient, and depolarization field effects}, volume={89}, DOI={10.1103/PhysRevB.89.224101}, abstractNote={A nonlinear thermodynamic formalism based on Ginzburg-Landau-Devonshire theory is developed to describe the total free energy density in (001)-oriented, compositionally graded, and monodomain ferroelectric films including the relative contributions and importance of flexoelectric, gradient, and depolarization energy terms. The effects of these energies on the evolution of the spontaneous polarization, dielectric permittivity, and the pyroelectric coefficient as a function of position throughout the film thickness, temperature, and epitaxial strain state are explored. In general, the presence of a compositional gradient and the three energy terms tend to stabilize a polar, ferroelectric state even in compositions that should be paraelectric in the bulk. Flexoelectric effects produce large built-in fields which diminish the temperature dependence of the polarization and susceptibilities. Gradient energy terms, here used to describe short-scale correlation between dipoles, have minimal impact on the polarization and susceptibilities. Finally, depolarization energy significantly impacts the temperature and strain dependence, as well as the magnitude, of the susceptibilities. This approach provides guidance on how to more accurately model compositionally graded films and presents experimental approaches that could enable differentiation and determination of the constitutive coefficients of interest.}, number={22}, journal={Physical Review B}, author={Zhang, J. and Xu, R. and Damodaran, A.R. and Chen, Z.H. and Martin, L.W.}, year={2014}, month={Jun}, pages={224101} }
 @article{baeumer_rogers_xu_martin_shim_2013, title={Tunable Carrier Type and Density in Graphene/PbZr0.2Ti0.8O3 Hybrid Structures through Ferroelectric Switching}, volume={13}, DOI={10.1021/nl4002052}, abstractNote={Bidirectional interdependency between graphene doping level and ferroelectric polarization is demonstrated in graphene/PbZr0.2Ti0.8O3 hybrid structures. The polarization of the PbZr0.2Ti0.8O3 can be effectively switched with graphene electrodes and can in turn alter carrier type and density in the graphene. A complete reversal of the current-voltage hysteresis direction is observed in the graphene when external environmental factors are minimized, converting p-type graphene into n-type with an estimated carrier density change as large as ~10(13) cm(-2). Nonvolatility and reversibility are also demonstrated.}, number={4}, journal={Nano Letters}, author={Baeumer, C. and Rogers, S.P. and Xu, R. and Martin, L.W. and Shim, M.}, year={2013}, pages={1693–1698} }