Session Y55: Topological Phenomena and Ferroelectric Oxide Interfaces

Live

When ultrathin ferroelectric layers of PbTiO₃ are embedded in superlattices with SrTiO₃, 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 arrays¹ and topologically non-trivial polar skyrmion bubbles have been recently detected². Using second-principles methods³ as implemented in the SCALE-UP code⁴ 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 permittivity⁵.

¹ A. K. Yadav et al., Nature 530, 198 (2016)
² S. Das et al., Nature 568, 368 (2019)
³ J. Wojdel et al., J. Phys.:Condens. Matter 25, 305401 (2013)
⁴ P. García-Fernández et al., Phys. Rev. B 93 195137 (2016)
⁵ S. Das et al., Nat. Mater. (2020)   

Live

Complex oxides exhibit many exotic properties, including high-temperature superconductivity, colossal magnetoresistance, metal-insulator transition, etc. However, the intensively studied Z₂ topological insulating state has yet to be realized in complex oxides. In this work, we combine first-principle calculations and model analysis to show that, in a number of (001) (ABO₃)₁/(AB’O₃)₁ oxide superlattices, a proper band alignment can induce a parity inversion between B-d and B’-d states. Spin-orbit coupling opens a topologically nontrivial gap in the superlattice and results in a three-dimensional topological insulating state with Z₂ index 1(001). Our work shows that, while two bulk constituent oxides are topologically trivial, the superlattices they compose can have non-trivial topological properties -- a demonstration of emergent phenomena in oxide heterostructures.   

Live

Polar topological textures—skyrmions, vortices, and merons—have uniquely been observed and robustly stabilized in epitaxial confined ferroelectrics. Therein, interfacial constraints favor the non-uniform rotation of polarization within the polar layers, leading to the formation of emergent topological structures due to the low-dimensionality 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, X-ray holography, and phase-field simulations, we demonstrate that polar skyrmions in (PbTiO₃)_n/(SrTiO₃)_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.   

Live

The formation of non-uniform 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 3-dimensional nature of such polar textures, a complete picture of the nanoscale structure is required. A lot can be gained from hard X-ray diffraction and transmission electron microscopy studies, but resonant soft X-ray diffraction coupled with circular dichroism can give additional insights. Combining soft X-ray reciprocal space scans, circular dichroism, and scattering calculations, our picture of the 3-dimensional structures can be refined. I will discuss the non-uniform polar vortices and skyrmions that occur in (PbTiO3)/(SrTiO3) heterostructures, and how scattering studies at the Ti L-edges can be used to understand their 3-dimensional, chiral structures.   

Live

Stimulation with ultrafast light pulses can realize and manipulate states of matter with emergent structural, electronic and magnetic phenomena. However, these non-equilibrium phases are often transient and the challenge is to stabilize them as persistent states. Here, we show that atomic-scale PbTiO3/SrTiO3 superlattices, counterpoising strain and polarization states in alternate layers, are converted by sub-picosecond optical pulses to a supercrystal phase. This phase persists indefinitely under ambient conditions, has not been created via equilibrium routes, and can be erased by heating. X-ray scattering and microscopy show this unusual phase consists of a coherent three-dimensional structure with polar, strain and charge-ordering periodicities of up to 30 nm. By adjusting only dielectric properties, the phase-field model describes this emergent phase as a photo-induced charge-stabilized supercrystal formed from a two-phase equilibrium state. Progress on understanding the formation mechanism for creation of the supercrystal will be presented. Our results demonstrate opportunities for light- activated pathways to thermally inaccessible and emergent metastable states.   

Live

Abstract: The ternary oxide ZnSnO3 with LiNbO3-type 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 first-principles methods, we investigate the crystal structure and electron states in ZnSnO3 and Sc-doped 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.   

Live

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 nm-thick 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_1-xSr_xMnO₃/BaTiO₃/Pt FTJ depends on the interface termination, either La_1-xSr_xO/TiO₂ or MnO₂/BaO. Due to the opposite sign of interfacial ionic charges, these interfaces shift the Fermi energy of La_1-xSr_xMnO₃ closer to either the conduction band minimum (La_1-xSr_xO/TiO₂) or the valence band maximum (MnO₂/BaO) of BaTiO₃ 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.   

Live

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 on-demand topological phases since they already host highly-tunable 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 higher-order 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.   

Live

Charge ordering (CO) can occur in mixed-valence compounds when ordering of the charge states on different sites breaks symmetry and produce ferroelectricity. Several examples of charge-order driven ferroelectricity have been recently discussed, with experimental observations in magnetite Fe₃O₄ and first-principles predictions in systems including LuFe₂O₄, 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 first-principles methods to investigate the ferroelectricity in SrVO₃-LaVO₃ 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 charge-ordered states. We discuss the application of this mechanism to explain the different electrical behaviors in LuFe₂O₄ and Fe₃O₄.   

Live

We report a comprehensive ferroelectric domain study of free-standing PbZr_0.2Ti_0.8O₃ (PZT) and PZT/(La,Sr)MnO₃ (LSMO) membranes. We deposit 5 to 50 nm epitaxial PZT and PZT/LSMO thin films on Sr₃Al₂O₆ (SAO) buffered SrTiO₃ (STO) substrates using off-axis RF magnetron sputtering. The as-deposited thin films are single crystalline with no impurity phases, as confirmed by x-ray 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. Piezo-response 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.   

On Demand

It is well known that two dimensional systems with continuous symmetries and short-range interactions, in the absence of symmetry breaking mechanisms, can undergo topological phase transitions driven by the unbinding of local defects pairs [1].

Moreover, it has been proven that these type of transitions can appear in ferroelectrics mediating between their low-symmetry and high-symmetry phases when the order parameter is restricted within a plane [2,3].

Using second-principles methods as implemented in the SCALE-UP code [4] we study the possibility of finding these type of transitions in PbTiO$_3$/SrTiO$_3$ superlattices when the order parameter is allowed to escape to the third dimension forming vortices in the polar structures [5].
[1] J. M. Kosterlitz and D. J. Thouless, J. Phys. C 5, L124 (1972)
[2] Y. Nahas et al., Phys. Rev. Lett. 119, 117601 (2017)
[3] C. Xu et al., Phys. Rev. B 101 241402 (2020)
[4] P. García-Fernández et al., Phys. Rev. B 93 195137 (2016)
[5] P. Shafer et al., PNAS 115, 915 (2018)