Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session Y55: Topological Phenomena and Ferroelectric Oxide InterfacesFocus Live

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Sponsoring Units: DMP Chair: Guru Khalsa, Cornell University 
Friday, March 19, 2021 11:30AM  11:42AM Live 
Y55.00001: Topological phase transition in polar skyrmions: a perspective from second principles simulations Mauro Gonçalves, Fernando GómezOrtiz, Pablo GarciaFernandez, Jorge Iniguez, Javier Junquera When ultrathin ferroelectric layers of PbTiO_{3} are embedded in superlattices with SrTiO_{3}, the interplay between elastic, electrostatic, and gradient energies produces complex patterns of the electrical polarization. In particular, depending on the mechanical boundary conditions, nanometer scale of vortex arrays^{1} and topologically nontrivial polar skyrmion bubbles have been recently detected^{2}. Using secondprinciples methods^{3} as implemented in the SCALEUP code^{4} we study the response of these topologically protected structures to electric field and temperature. We show a reversible phase transition from the skyrmion state to a trivial uniform ferroelectric state, accompanied by large tunability of the dielectric permittivity^{5}. 
Friday, March 19, 2021 11:42AM  11:54AM Live 
Y55.00002: Emergent Z_{2} topological phases in (001) oxide superlattices Hongquan Liu, Jiaji Ma, Gang Li, Hanghui Chen Complex oxides exhibit many exotic properties, including hightemperature superconductivity, colossal magnetoresistance, metalinsulator transition, etc. However, the intensively studied Z_{2} topological insulating state has yet to be realized in complex oxides. In this work, we combine firstprinciple calculations and model analysis to show that, in a number of (001) (ABO_{3})_{1}/(AB’O_{3})_{1 }oxide superlattices, a proper band alignment can induce a parity inversion between Bd and B’d states. Spinorbit coupling opens a topologically nontrivial gap in the superlattice and results in a threedimensional topological insulating state with Z_{2} index 1(001). Our work shows that, while two bulk constituent oxides are topologically trivial, the superlattices they compose can have nontrivial topological properties  a demonstration of emergent phenomena in oxide heterostructures. 
Friday, March 19, 2021 11:54AM  12:06PM Live 
Y55.00003: Emergent topological phase transition dynamics of polar skyrmions Elizabeth Donoway, YuTsun Shao, Sujit Das, zijian hong, Lucas Caretta, Margaret McCarter, David Anthony Muller, Ramamoorthy Ramesh Polar topological textures—skyrmions, vortices, and merons—have uniquely been observed and robustly stabilized in epitaxial confined ferroelectrics. Therein, interfacial constraints favor the nonuniform rotation of polarization within the polar layers, leading to the formation of emergent topological structures due to the lowdimensionality of these systems. These topologies characterize phases concomitant with exotic functional properties, motivating exploration into ways to extrinsically manipulate these features. Here, using scanning convergent beam electron diffraction, Xray holography, and phasefield simulations, we demonstrate that polar skyrmions in (PbTiO_{3})_{n}/(SrTiO_{3})_{n} superlattices form a phase characterized by interactions that are extrinsically tunable via temperature and strain, enabling control over phase transitions between distinct ordering configurations and topological states. I will discuss several emerging techniques we have employed to examine the interplay between emergent phenomena—namely topology, ordering, and chirality—and extrinsic parameters, to describe the topological phase diagram and phenomenology of polar oxide superlattices. 
Friday, March 19, 2021 12:06PM  12:18PM Live 
Y55.00004: Chiral polar textures probed with resonant Xray diffraction Margaret McCarter, Christoph Klewe, Elizabeth Donoway, Sujit Das, Vladimir A Stoica, Dong Ryeol Lee, John Freeland, Padraic Shafer, Gerrit Van der Laan, Ramamoorthy Ramesh The formation of nonuniform polar vortices and skyrmions in complex oxide heterostructures opens up many intriguing areas of study, such as chirality, negative capacitance, collective dynamics, and topological phase transitions. Because these are inherently dependent on the 3dimensional nature of such polar textures, a complete picture of the nanoscale structure is required. A lot can be gained from hard Xray diffraction and transmission electron microscopy studies, but resonant soft Xray diffraction coupled with circular dichroism can give additional insights. Combining soft Xray reciprocal space scans, circular dichroism, and scattering calculations, our picture of the 3dimensional structures can be refined. I will discuss the nonuniform polar vortices and skyrmions that occur in (PbTiO3)/(SrTiO3) heterostructures, and how scattering studies at the Ti Ledges can be used to understand their 3dimensional, chiral structures. 
Friday, March 19, 2021 12:18PM  12:54PM Live 
Y55.00005: Optical creation of a supercrystal with nanoscale periodicity Invited Speaker: Venkatraman Gopalan

Friday, March 19, 2021 12:54PM  1:06PM Live 
Y55.00006: Polar distortions and electron states in Scdoped ZnSnO3 from first principles Limin Wang, Karin M Rabe Abstract: The ternary oxide ZnSnO3 with LiNbO3type structure has a large spontaneous polarization and a large bandgap, and through its similarity with BaSnO3 is expected to have high mobility when the electron is doped. Using firstprinciples methods, we investigate the crystal structure and electron states in ZnSnO3 and Scdoped ZnSnO3 to explore the effects of Sc doping, with a focus on the change in polar distortion and the predicted conductivity of the doped polar system. 
Friday, March 19, 2021 1:06PM  1:18PM Live 
Y55.00007: Reversal of Tunneling Electroresistance in Ferroelectric Tunnel Junctions by Interface Engineering Ming Li, Lingling Tao, Evgeny Y Tsymbal The tunneling electroresistance (TER) effect is a sizable change in resistance upon the reversal of ferroelectric polarization in a ferroelectric tunnel junction (FTJ), which consists of two metal electrodes separated by a nmthick ferroelectric barrier. Usually, for TER, the resistance is expected to be low (high) when polarization points to the electrode with a longer (shorter) screening length. We show, however, that this observation is not always valid, and TER can be reversed by changing the interface termination. Using density functional theory calculations, we demonstrate that the sign of TER in a La_{1x}Sr_{x}MnO_{3}/BaTiO_{3}/Pt FTJ depends on the interface termination, either La_{1x}Sr_{x}O/TiO_{2} or MnO_{2}/BaO. Due to the opposite sign of interfacial ionic charges, these interfaces shift the Fermi energy of La_{1x}Sr_{x}MnO_{3} closer to either the conduction band minimum (La_{1x}Sr_{x}O/TiO_{2}) or the valence band maximum (MnO_{2}/BaO) of BaTiO_{3} resulting in the crossover between electron and hole tunneling. Due to the opposite energy dependence of the tunneling probability, these two tunneling regimes exhibit a reversed TER. 
Friday, March 19, 2021 1:18PM  1:54PM Live 
Y55.00008: Topological Order Parameters in Oxides Invited Speaker: Sinead Griffin With the now vast collection of materials hosting topological phases, the next goal is to provide design routes for their control with strain and fields. In particular, oxides are an enticing class of materials in which to investigate ondemand topological phases since they already host highlytunable orders such as ferroelectricity, ferromagnetic and superconductivity. In this talk, I will discuss how the conventional order parameters in oxides such as ferroelectric mode distortions and octahedral rotations can be used tune topological order. I will provide two cases studies looking at higherorder topological phases in perovskite oxides, and the interplay between real and reciprocal space topological in hexagonal manganites using first principles calculations and phenomenology theory. Finally, I will discuss the prospects of these materials as hosts for Majorana fermions and their potential as robust elements in topological quantum computers. 
Friday, March 19, 2021 1:54PM  2:06PM Live 
Y55.00009: Lattice Modes and Polarization Switching in ChargeOrderDriven Ferroelectrics Yubo Qi, Karin M Rabe Charge ordering (CO) can occur in mixedvalence compounds when ordering of the charge states on different sites breaks symmetry and produce ferroelectricity. Several examples of chargeorder driven ferroelectricity have been recently discussed, with experimental observations in magnetite Fe_{3}O_{4} and firstprinciples predictions in systems including LuFe_{2}O_{4}, LaVO3/SrVO3 superlattices and LSFO. However, the switchability and precise mechanism of polarization switching in CO materials is still an open question. In this work, we use firstprinciples methods to investigate the ferroelectricity in SrVO_{3}LaVO_{3} superlattices, and demonstrate that the coupling of the charge order to the lattice has a crucial influence on the stabilities of various polar and nonpolar chargeordered states. We discuss the application of this mechanism to explain the different electrical behaviors in LuFe_{2}O_{4} and Fe_{3}O_{4}. 
Friday, March 19, 2021 2:06PM  2:18PM Live 
Y55.00010: Ferroelectric domain studies in freestanding PbZr_{0.2}Ti_{0.8}O_{3} membranes Qiuchen Wu, Kun Wang, Dawei Li, Xia Hong We report a comprehensive ferroelectric domain study of freestanding PbZr_{0.2}Ti_{0.8}O_{3} (PZT) and PZT/(La,Sr)MnO_{3} (LSMO) membranes. We deposit 5 to 50 nm epitaxial PZT and PZT/LSMO thin films on Sr_{3}Al_{2}O_{6} (SAO) buffered SrTiO_{3} (STO) substrates using offaxis RF magnetron sputtering. The asdeposited thin films are single crystalline with no impurity phases, as confirmed by xray diffraction (XRD) measurements. Atomic force microscopy (AFM) studies show smooth surfaces with the RMS roughness of 0.5 nm for PZT and 0.8 nm for PZT/LSMO. We then immerse the samples in water to dissolve the SAO buffer layer and transfer the suspended oxide membranes to different conductive substrates, including Au and LSMO/STO. The RMS roughness of the membranes increases to 0.9 nm for PZT and 1.6 nm for PZT/LSMO, while XRD studies show they maintain the crystallinity. Piezoresponse force microscopy (PFM) measurements reveal a uniform polarization down state for both samples, while the switching field of the PZT membranes is significantly higher than that of PZT/LSMO. We also discuss the effect of PZT thickness and the bottom electrode type on the domain wall roughness. 
Friday, March 19, 2021 2:18PM  2:30PM On Demand 
Y55.00011: Berezinskii– Kosterlitz–Thouless phase transitions in PbTiO$_3$/SrTiO$_3$ superlattices Fernando GómezOrtiz, Pablo GarciaFernandez, Javier Junquera It is well known that two dimensional systems with continuous symmetries and shortrange interactions, in the absence of symmetry breaking mechanisms, can undergo topological phase transitions driven by the unbinding of local defects pairs [1]. 
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