Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C37a: Dielectric and Ferroelectric Oxides IFocus
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Sponsoring Units: DMP Chair: Mutgar Ahart, Carnegie Institute of Washington Room: 383 |
Monday, March 13, 2017 2:30PM - 3:06PM |
C37a.00001: Ferroelectric oxide thin films for advanced energy applications Invited Speaker: Sanghan Lee Ferroelectrics are considered as promising photoanode materials because their high built-in-potential due to their spontaneous polarizations can largely enhance the separation and drift of photo-generated carriers. Especially, among ferroelectrics, BiFeO$_{\mathrm{3}}$ has different spontaneous polarizations and ferroelectric domain structure depending on the crystallographic orientations, so it is of great significance to clarify the direct relationship between photocatalytic properties, spontaneous polarizations and ferroelectric domain structures. However, the photocatalytic properties of epitaxial BiFeO$_{\mathrm{3}}$ thin film photoanodes with different crystallographic orientations and subsequently different ferroelastic domain structures have not been systematically studied yet. Furthermore, the effect of ferroelectric switching on the PEC properties of epitaxial BiFeO$_{\mathrm{3}}$ thin film photoanodes has not been identified. Considering the above, in this study, the most enhanced photocatalytic performances of our BiFeO$_{\mathrm{3}}$ thin film photoanodes showed in the (111)$_{\mathrm{pc}}$ BiFeO$_{\mathrm{3}}$ thin film photoanode, due to its high spontaneous polarization and mono-variant domain structure, which was approximately a 5.3 times larger photocurrent density at 0 V \textit{vs.} Ag/AgCl and a 0.180 V shift in the onset potential in comparison with the (001)$_{\mathrm{pc}}$ BiFeO$_{\mathrm{3}}$ thin film photoanode in the downward polarization. Furthermore, ferroelectric polarization switching in the (111)$_{\mathrm{pc}}$ BiFeO$_{\mathrm{3}}$ thin film photoanode caused an approximate change of 8,000{\%} in the photocurrent density at 0 V \textit{vs.} Ag/AgCl and 0.330 V shift in the onset potential. We believe that domain-engineered ferroelectric materials can be used as an effective charge separation and collection layer for effective solar water splitting photoanodes. [Preview Abstract] |
Monday, March 13, 2017 3:06PM - 3:18PM |
C37a.00002: Visualization of GHz Acoustic Wave in LiNbO3 by Microwave Impedance Microscopy Lu Zheng, Hui Dong, Xiaoyu Wu, Yen-Lin Huang, Weida Wu, Zheng Wang, Keji Lai Acoustic wave devices based on piezoelectric materials play a key role in the modern information technology and the research field of phononic metamaterials. High-resolution real-space mapping of the phononic modes is therefore of fundamental importance for the understanding of scattering, diffraction, and localization of the acoustic waves. To date, however, it has been challenging to directly image the GHz-range acoustic properties in piezoelectrics. Using a microwave impedance microscope (MIM), we demonstrate the ability to visualize the interference pattern of GHz acoustic waves in periodically poled lithium niobate (PPLN) samples, where the domain walls serve as good reflectors of the elastic deformation. The constructive and destructive interference regions exhibit different loss in the microwave images, which can be simulated by finite-element analysis of the PPLN samples. Our results pave the way to locally probe various phenomena of sound waves in phononic materials by nanoscale electromagnetic imaging. [Preview Abstract] |
Monday, March 13, 2017 3:18PM - 3:30PM |
C37a.00003: Measurement of Surface Acoustic Wave Resonances in Ferroelctric Domains by Microwave Microscopy Scott Johnston, Yongliang Yang, Yongtao Cui, Eric Yue Ma, Jian Zhou, Minghui Lu, Yan-Feng Chen, Zhi-Xun Shen Surface acoustic wave (SAW) devices made from ferroelectric materials are commonly used as radio frequency filters and delay lines in modern wireless devices. These devices can make use of the reflection of SAWs from ferroelectric domain walls. Here, we report on the measurement of SAW resonances within a closed domain in the ferroelectric LiTaO$_3$ via scanning Microwave Impedance Microscopy (sMIM). The sMIM probe is used for both SAW generation and measurement, allowing contact-less measurement within a mesoscopic structure. Measurements taken at different microwave frequencies demonstrate the expected wavelength dependence of the SAW patterns and agree with literature values for SAW velocity. This ability to map the surface acoustic wave patterns could be coupled with the well established ability to pattern ferrolectric domains via applied voltage on the same instrument to allow for rapid prototyping of novel SAW devices such as domain wall based wave-guides and couplers. [Preview Abstract] |
Monday, March 13, 2017 3:30PM - 3:42PM |
C37a.00004: Room Temperature Ferroelectricity and Photovoltaic Effect in Atomic Layer Deposited SnTiOX Thin Films R. Agarwal, Y. Sharma, S. Chang, S. Nakhmanson, C. Takoudis, R. Katiyar, S. Hong We have studied ferroelectricity and photovoltaic effects in atomic layer deposited 40 nm thick SnTiOX films. These films showed well-defined and repeatable polarization hysteresis loops at room temperature, as detected by polarization versus electric field (P-E) and field cycling measurements. A photo-induced enhancement in ferroelectricity was also observed as the spontaneous polarization increased under white-light illumination, indicating photoferroelectric nature of SnTiOX films. Interestingly, we observed ferroelectric photovoltaic behavior in these films under the illumination of wide spectrum of light, from visible to ultraviolet regions. A short circuit current of 3 micro Amp. and open circuit voltage of 0.12 V were observed under visible light, while these values were found to be slightly lower in ultraviolet illumination. Though, the origin of ferroelectricity is not very clear yet, but we believe that either the formation of non-centrosymmetric crystalline phases in the film matrix during the growth or presence of charged defects in non-stoichiometric SnTiOX could be the possible reasons. Our study provides a way to develop green ferroelectric SnTiOx thin films, which are compatible to semiconducting processes, and can be used for various ferroelectric applications. [Preview Abstract] |
Monday, March 13, 2017 3:42PM - 3:54PM |
C37a.00005: Lifshitz invariants from ab initio lattice dynamics Andrea Schiaffino, Massimiliano Stengel The interaction between different order parameters is vital to explain the emergence of new functionalities (hybrid improper ferroelectricity, magnetoelectricity) in multiferroic systems. While considerable theoretical efforts have been directed in the past at studying couplings (e.g. trilinear or biquadratic) that occur in a homogeneous sample, recent research has revealed an increasing number of cases where the interesting physics emerges from inhomogeneities in some order parameter (e.g. flexoelectricity, domain walls), rather than the uniform bulk phase itself. These are usually described in phenomenological theories via symmetry-allowed gradient-mediated terms, the so-called Lifshitz invariants. Here I will present a general method to calculate such couplings ab initio, within the framework of density-functional perturbation theory. I will start with a brief overview on the most challenging aspects of these calculations, i.e. how to deal with the breakdown of the translational symmetry, and with the unusual electrostatic effects that occur in such a regime. Next, I will demonstrate this strategy in practice by presenting calculations of the most relevant gradient coefficients involving strain, octahedral tilts and polarization in ferroelastic SrTiO$_3$. [Preview Abstract] |
Monday, March 13, 2017 3:54PM - 4:06PM |
C37a.00006: Weak ferromagnetism and short range polar order in NaMnF$_{\mathrm{3}}$ thin films Amit KC, Pavel Borisov, Vladimir Shvartsman, David Lederman The orthorhombically distorted perovskite NaMnF3 has been predicted to become ferroelectric if an a $=$ c distortion of the bulk \textit{Pnma} structure is imposed. In order to test this prediction, NaMnF$_{\mathrm{3}}$ thin films were grown on SrTiO$_{\mathrm{3}}$ (100) single crystal substrates via molecular beam epitaxy. The best films were smooth and single phase with four different twin domains. In-plane magnetization measurements revealed the presence of antiferromagnetic ordering with weak ferromagnetism below the N\'{e}el temperature $T_{N} \quad =$ 66 K. For the dielectric studies, NaMnF$_{\mathrm{3}}$ films were grown on a 30 nm SrRuO$_{\mathrm{3}}$ (100) layer used as a bottom electrode grown via pulsed laser deposition. The complex permittivity as a function of frequency indicated a strong Debye-like relaxation contribution characterized by a distribution of relaxation times. A power-law divergence of the characteristic relaxation time revealed an order-disorder phase transition at 8 K. The slow relaxation dynamics indicated the formation of super-dipoles (superparaelectric moments) that extend over several unit cells, similar to polar nanoregions of relaxor ferroelectrics. [Preview Abstract] |
Monday, March 13, 2017 4:06PM - 4:18PM |
C37a.00007: Lack of ferroelectricity in PbTiO$_3$ at high pressures R.E. Cohen, Muhtar Ahart, Russell J. Hemley The classic ferroelectric PbTiO$_3$ continues to surprise. It was believed that ferroelectrics would become paraelectric under pressure, and this was observed in Raman experiments on PbTiO$_3$. [1] We predicted a morphotropic phase transition under pressure [2] and verified it experimentally. [3] At higher pressures it become paraelectric, but DFT [4, 5] predicted higher pressure ferroelectricity, and this seemed confirmed by experiments. [6] New Second Harmonic Generation (SHG) measurements on PbTiO$_3$ to 100 GPa and down to 10 K find no evidence for ferroelectricity above 20 GPa. Our DFT computations show centrosymmetric I4mcm as most stable from 20-90 GPa; I4mcm is the ground state of SrTiO$_3$, and the rotations quench the polar instability. We predict a polar I4cm structure above 90 GPa, but the double well depth is very small. [1] J. Sanjurjo et al., PRB 28, 7260 (1983). [2] Z. Wu and R. Cohen, PRL, 037601 95 (2005). [3] M. Ahart et al., Nature 451, 545 (2008). [4] I. A. Kornev and L. Bellaiche, Phase Transitions 80, 385 (2007). [5] I. A. Kornev et al., PRL 95 196804 (2005). [6] P. E. Janolin et al., PRL 101 237601 (2008). [Preview Abstract] |
Monday, March 13, 2017 4:18PM - 4:30PM |
C37a.00008: Evidence for a Berezinskii-Kosterlitz-Thouless phase in ferroelectric thin-films Yousra Nahas, Sergei Prokhorenko, Igor Kornev, Laurent Bellaiche The Berezinskii-Kosterlitz-Thouless ($\mathsf{BKT}$) theory, discovered more than 40 years ago, has propelled the notion of topological phase transition, and has a venerable history and a seminal impact upon condensed matter physics and other areas in physics. So far, the question of whether low-dimensional ferroelectrics would manifest $\mathsf{BKT}$ physics has been eluded. Our work aims at bridging this gap as it focuses on the investigation of the critical properties of ferroelectric thin-films, namely BaTiO$_3$ under tensile strain. Using Monte Carlo simulations of a first-principles-based effective Hamiltonian scheme as well as scaling, symmetry, and topological arguments, we find that an intermediate critical $\mathsf{BKT}$ phase underlain by quasi-continuous symmetry emerges between the ferroelectric phase and the disordered paraelectric one. This overlooked intermediate phase supports quasi-long-range order reflected in the algebraic decay of the correlation function and sustained by the existence of neutral bound pairs of vortices and antivortices, in accordance with defining characteristics of a $\mathsf{BKT}$ phase. Our results therefore highlight the subtle, fundamental richness of low-dimensional ferroelectrics and widen the realm of $\mathsf{BKT}$ transitions. [Preview Abstract] |
Monday, March 13, 2017 4:30PM - 4:42PM |
C37a.00009: Fluctuations and topological defects in proper ferroelectric crystals Sergei Prokhorenko, Yousra Nahas, Laurent Bellaiche In this work we use a combination of homotopy theory and first-principles-based effective Hamiltonian simulations to investigate the stability of topological defects in proper ferroelectric crystals. Taking BaTiO$_3$ as a model example, we show that, despite a nearly trivial topology of the order parameter space, these materials can exhibit stable topological point defects in their tetragonal polar phase and stable topological line defects in their orthorhombic polar phase. Stability of such defects originate from a novel mechanism of topological protection related to finite- temperature fluctuations of local dipoles. [Preview Abstract] |
Monday, March 13, 2017 4:42PM - 4:54PM |
C37a.00010: Structural Phase Diagram and Size-Dependent Electrical Polarization in Freestanding SrTiO$_{\mathrm{3\thinspace }}$Nanoparticles Han Zhang, Sizhan Liu, Megan Scofield, Stanislaus Wong, Xinguo Hong, Vitali Prakapenka, Eran Greenberg, Trevor Tyson The bulk phase of the classic perovskite SrTiO$_{\mathrm{3}}$ (STO) is paraelectric and exhibits a structural phase transition at a pressure P\textasciitilde 6 GPa at room temperature. Understanding the structural phase diagram of nano scale STO has important implications on the basic physics and applications of the general class of oxide perovskites. Previous research indicated a stable polar state in STO over a wide pressure range with small particle size. We have established a size-dependent phase diagram of STO. Meanwhile, we have further investigated polarization properties with freestanding STO nanoparticles with experimental methods and modeling. This work is supported by DOE Grant DE-FG02-07ER46402. [Preview Abstract] |
Monday, March 13, 2017 4:54PM - 5:06PM |
C37a.00011: Temperature dependence of Ti 1s near-edge spectra in Ti-based perovskites: theory and experiment Eric Shirley, Eric Cockayne, Bruce Ravel, Joseph Woicik Ti 1s near-edge spectra (around 4970~eV) in SrTiO$_3$ and PbTiO$_3$ reveal electric-dipole and quadrupole transitions to Ti 3d, 4p and mixed 3d-4p states. Crystal field-split “pre-edge” features attributed to 1s$\rightarrow$3d transitions are small compared to the main “edge jump” at the onset of the Ti 4s/4p continuum. Pre-edge and subsequent near-edge features are predicted to be weaker than what is observed, unless one accounts for ferroelectric polarization in PbTiO$_3$ and thermal motion in both compounds. Using density-functional theory molecular dynamics simulations at various temperatures (including sampling two phases of PbTiO$_3$), we capture the statistically averaged root-mean-square deviations of Ti$^{4+}$ ions from the centers of their oxygen cages. By sampling appropriate snapshots of atomic configurations and averaging Ti 1s absorption spectra computed within a Bethe-Salpeter Equation framework, we obtain absorption spectra that agree well with experiment~[ R.V. Vedrinskii et al., J. Phys. IV France 7, C2-107 (1997), J.C. Woicik et al., Phys. Rev. B 75, 140103(R) (2007)], including details related to ferroelectric polarization, phase transitions, and fluctuations of atomic coordinates. [Preview Abstract] |
Monday, March 13, 2017 5:06PM - 5:18PM |
C37a.00012: Scaling of SrTiO$_{\mathrm{3}}$ growth rates of using hybrid molecular beam epitaxy Jason Lapano, Matthew Brahlek, Lei Zhang, Roman Engel-Herbert Perovskites exhibit a wide range of desirable properties, including ferroelectricity, ferromagnetism, as well as transport properties ranging from superconductivity to Mott-like behavior. However, deposition is plagued by notoriously slow growth rates, as well as a high sensitivity to nonstoichiometric defects. In this work, we have been able to mitigate these barriers for SrTiO$_{\mathrm{3}}$ films grown using hybrid molecular beam epitaxy (HMBE). In HMBE, one of the cations is supplied via a volatile metalorganic precursor. This allows for the development of a stoichiometric ``growth window'', similar to those seen in GaAs and to replicate the fast deposition rates achievable in GaAs growth.$^{\mathrm{\thinspace }}$ In-situ reflection high energy electron diffraction was used to assess film stoichiometry and efficiently determine the limits of the growth window. A series of SrTiO$_{\mathrm{3}}$ films were grown on LSAT substrates at rates ranging from \textasciitilde 25 nm/h to 500 nm/h. I will present x-ray diffraction, atomic force microscopy, and electron microscopy images to show homoepitaxial SrTiO$_{\mathrm{3}}$ films are indistinguishable from the bulk substrate, even at these accelerated growth rates. [Preview Abstract] |
Monday, March 13, 2017 5:18PM - 5:30PM |
C37a.00013: A unified view of the substitution-dependent antiferrodistortive phase transition in SrTiO$_{\mathrm{3}}$ Eric McCalla, Jeff Walter, Chris Leighton The cubic-to-tetragonal antiferrodistortive transition at 105 K in the most widely studied perovskite, SrTiO$_{\mathrm{3}}$, is perhaps the preeminent example of a second-order structural phase transition. Extensive investigations since the 1960's have tracked the softening of the phonon mode associated with this transition, and the interplay with incipient ferroelectricity and superconductivity. Lesser known is that modest ionic substitutions vary the transition temperature ($T_{\mathrm{a}})$ over a remarkable range in SrTiO$_{\mathrm{3}}$, from 0 K to above ambient, the trends being difficult to understand based on simple ionic radii or tolerance factor ideas. In addition to providing new data on the thermodynamics of the transition \textit{via} specific heat, we present here the first quantitative rationalization of the trends in $T_{\mathrm{a}}$ with substitution in SrTiO$_{\mathrm{3}}$. We emphasize the importance of ionic valence mismatch, using bond valence concepts to establish a new parameter, \textless $\varepsilon^{\mathrm{4}}$\textgreater , exhibiting a universal linear dependence with $T_{\mathrm{a}}$ for all known substitutions. This provides the first unified view of the substituent-dependent $T_{\mathrm{a}}$ in SrTiO$_{\mathrm{3}}$, deepens our understanding of the phase transition (including a theoretical maximum in the rate of $T_{\mathrm{a}}$ suppression), and demonstrates predictive power \textit{via} a simply computed parameter. [Preview Abstract] |
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