Session K71: Ferroelectric and Multiferroic Phases in Complex Oxide Heterostructures

Freestanding Ferroelectric bubble domains

Ferroelectric bubble domains in complex oxide superlattices can be grown only on select substrates. This is a key bottleneck for their heterogeneous integration. Using the epitaxial layer transfer technique, we create freestanding superlattices. We find structural ripples appear in a freestanding state, but bubble domains remain intact. Using atomic and piezoresponse force microscopy, microwave impedance microscopy, and numerical calculations combining atomistic simulation and circuit model we study piezo and capacitive responses of the freestanding bubble domains. We anticipate these results will initiate a new thrust to explore non-trivial ferroelectric polar orders with arbitrary boundaries.   

Complex oxides under simulated electric field: Determinants of defect polarization in ABO3 perovskites

Polarization of ionic and electronic defects in response to high electric fields plays an essential role in determining properties of materials in applications such as memristive devices. However, isolating the polarization response of individual defects has been challenging for both models and measurements. Here we quantify the nonlinear dielectric response of neutral oxygen vacancies, comprised of strongly localized electrons at an oxygen vacancy site, in perovskite oxides of the form ABO₃. Our approach implements a computationally efficient local Hubbard U correction in density functional theory simulations. These calculations indicate that the electric dipole moment of this defect is correlated positively with the lattice volume, which we varied by elastic strain and by A-site cation species. In addition, the dipole of the neutral oxygen vacancy under electric field increased with increasing reducibility of the B-site cation. The predicted relationship among point defect polarization, mechanical strain, and transition metal chemistry provides insights for properties of memristive materials and devices under high electric fields.   

Nonvolatile Electric-Field Control of Inversion Symmetry

In condensed-matter systems, competition between ground states at phase boundaries can lead to significant changes in material properties under external stimuli, particularly when these ground states have different crystal symmetries. A key scientific and technological challenge is to stabilize and control coexistence of symmetry-distinct phases with external stimuli. Using BiFeO₃ (BFO) layers confined between layers of the dielectric TbScO₃ as a model system, we stabilize the phase coexistence of centrosymmetric and non-centrosymmetric BFO phases with antipolar, insulating and polar, semiconducting behavior, respectively at room temperature. Application of in-plane electric fields results in reversible, nonvolatile interconversion between the two phases. This interconversion between the centrosymmetric insulating and non-centrosymmetric semiconducting phases results in a change in the non-linear optical response of over three orders of magnitude and a change in resistivity of over five orders of magnitude. Moreover, this work establishes a mechanism by which to stabilize an antipolar state with an electric field. Our work establishes a materials platform allowing for novel cross-functional devices which take advantage of changes in optical, electrical, and ferroic responses.   

Multiferroic band structure at the monolayer Fe/BaTiO3 interface

In this work, we study the electronic structure of the multiferroic behavior that has been predicted and observed at the interface of Fe and BaTiO₃ (BTO). To examine the influence of Fe on the BTO electronic structure, we synthesize and measure the band structure of monolayer Fe-BTO interfaces. We observe a two-dimensional electron gas (2DEG) that emerges at the interface of Fe and BTO. This electronic structure is measured using a facility developed on the floor of the National Synchrotron Light Source II (NSLS-II), which combines an oxide MBE growth chamber and a UHV suitcase for sample transfer to an angle-resolved photoemission spectrometer. A comparison of the band structure of the BTO-vacuum interface and the BTO-Fe interface reveals a transfer of spectral weight from the BTO 2DEG to an incoherent band 0.5 eV below the Fermi level. These states are expected to play a role in the tunnel transport from Fe through ferroelectric BTO tunnel barriers.   

Microstructural analysis and electro-optic properties of thick epitaxial BaTiO3 films integrated on silicon (001)

Integrating ferroelectric materials onto Si is a critical part of the design new photonic devices such as optical modulators and switches. One property used to make optical modulators is the electro-optic effect. Most work in electro-optic modulators is directly related to analyzing the devices themselves, whereas less work has been doing on investigating the properties of the thin films. Here, we analyze thick BTO films integrated on Si via multiple experimental techniques and compare with modelling to study their crystalline properties as a function of thickness. We use this to more carefully analyze electro-optic measurements of BTO on Si. This work can be used to compliment and improve the BTO integrated electro-optic device making process.   

Antisite defects stabilized by antiphase boundaries in multiferroic YFeO3 thin films

Controlling the response of multiferroic materials to applied magnetic and electric fields necessitates knowledge of the defects present and their potential role in modifying behavior. In this presentation, we will report on the structure and chemistry of antiphase boundaries in Y-rich multiferroic YFeO₃ thin films using aberration corrected scanning transmission electron microscopy and theory. Combining imaging with atomic resolution energy dispersive X-ray spectroscopy, we find that Fe_Y antisites, which are not stable in the film bulk, periodically arrange along antiphase boundaries due to changes in the local structural environment. The impact that the point and planar defects will also be explored through theory. For example, using density functional theory, we will show that the antiphase boundaries are polar and bi-stable, where the presence of Fe_Y antisites significantly decreases the switching barrier. Finally, we will discuss how these results highlight that planar defects, such as antiphase boundaries, can stabilize point defects that would otherwise not be expected to form within the structure.   

Multi-state Switching Dynamics in the Polar Vortex Phase

The recent discovery of ferroelectric vortices in PbTiO₃/SrTiO₃ heterostructures has opened the door to many novel phenomena such as control and manipulation of negative capacitance, chirality, toroidal moments, and collective dynamics. Here, we examined the electric field switching dynamics of the polar vortices; revealing the existence of multiple switching events which are correlated to a hierarchy of structural and polar switching events. We elucidate the nanosecond-scale dynamics and mechanism of the various switching events using traditional and time-resolved ferroelectric capacitance measurements in combination with in-situ imaging techniques. We also reveal how such multistate switching can potentially be used for non-destructive readout of the ferroelectric state.      

Crystal-Liquid phase transition of polarization curl density in PbTiO$_3$/SrTiO$_3$ superlattices

Recent studies of the temperature evolution of the polarization patterns in oxide superlattices shows complex behaviour [1-3] different from the classical ferro/paraelectric phase transitions where the polarization is the relevant order parameter to track.

Using second-principles methods as implemented in the Scale-UP [4] code we report a first-order crystal-liquid like temperature-driven phase transition in (PbTiO$_3$)$_n$/(SrTiO$_3$)$_n$ superlattices in which the relevant order parameter is related with the curl density of the polarization pattern. Different values of the perioditiy $n=8,~10,~14$ were studied showing good scaling for the phase transition. 

[1] Y. Nahas, et al., Phys. Rev. Lett. 119, 117601 (2017)

[2] C. Xu, et al., Phys. Rev. B. 101, 241402 (2020)

[3] Y. Nahas, et al., Nature 577, 47–51 (2020)

[4] P. Garc\'ia-Fern\'andez, et al., Phys. Rev. B 93 195137 (2016)   

Electric-Field Control of Chirality of vortices in PbTiO3/SrTiO3 superlattices

Dipolar textures in ferroelectrics, such as polar vortices and skyrmions have been extensively studied and several interesting physical phenomena have been discovered. Perhaps the most exotic and unexpected among them is the emergence of chirality that arise from a sequence of achiral materials [1]. Strain engineering or electric-field control of such chiral response would be of fundamental and technological interest. Extensive theoretical work has been devoted to accomplish this objective [2-4] but has remained elusive form the experimental point of view. A recent work [5] addresses this issue, however, the presence of random fields during the switching precludes a deterministic control of the chirality. Using second-principles methods as implemented in the SCALE-UP code [6] we proved deterministic and reversible control of chirality over mesoscale regions by means of an homogeneous electric field [7].

[1] P. Shafer, et al. Proc. Natl. Acad. Sci. U.S.A. 115, 915 (2018) [2] W. J. Chen, et al. RSC Adv. 8, 4434 (2018) [3] L. L. Ma, et al. Acta Mater. 158, 23 (2018) [4] Y. Tikhonov, et al. Sci. Rep. 10, 8657 (2020) [5] P. Chen, et al. arXiv:2104.12310 (2021) [6] P. García-Fernández, et al., Phys. Rev. B 93 195137 (2016) [7] P. Behera, et al. arXiv:2105.14109 (2021)