Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B02: Dielectric & Ferroic Oxides -- Flexoelectric, Photovoltaic, and Piezoelectric propertiesFocus
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Sponsoring Units: DMP DCOMP Chair: Daniel Phelan Room: BCEC 107A |
Monday, March 4, 2019 11:15AM - 11:51AM |
B02.00001: Clamped-ion flexoelectricity from first principles Invited Speaker: Cyrus Dreyer The flexoelectric (FxE) effect, where polarization is induced by a strain gradient, is universal in all insulators. As devices shrink to the micro and nano scale, large strain gradients can occur, and therefore the FxE effect can play a significant role in their electrical and mechanical properties. Also, the FxE effect can be exploited for novel device design paradigms such as piezoelectric ``meta-materials'' constructed from nonpiezoelectric constituents, or mechanical switching of ferroelectric polarization. One of the crucial limitations to understanding and exploiting the FxE effect is the lack of an efficient first-principles methodology to calculate all of the components of the bulk FxE tensor; the clamped-ion transverse and shear components in particular are problematic. In this work we develop such a methodology based on density functional perturbation theory to calculate the full bulk, clamped-ion FxE tensor from a single unit cell, by calculating the current-density response to the adiabatic displacement of atoms from a long wavelength acoustic phonon. In this talk I will outline our methodology, including recent developments relating to the implementation of a “metric wave” formalism, and apply it to calculate the clamped-ion flexoelectric constants in the context of cubic and distorted perovskite oxides. |
Monday, March 4, 2019 11:51AM - 12:03PM |
B02.00002: Spatial dispersion effects in ferroic oxides: Dynamical quadrupoles and flexoelectric tensor Massimiliano Stengel, Miquel Royo In condensed-matter physics, spatial dispersion refers to the dependence of many material properties on the wavevector q at which they are probed, and is ultimately due to the nonlocality of the response to a given external field (electric, magnetic, strain). A remarkable example is the flexoelectric tensor, describing the polarization response to a gradient of applied strain, or equivalently the electrical current that is produced by an acoustic phonon at second-order in q. Density-functional perturbation theory (DFPT) appears as the ideal framework to compute these effects from first principles, but the general computational tools to deal with the long-wavelength limit are currently missing. Here we present a general formalism, based on the analytical long-wavelength expansion of the second-order DFPT energies, that enables the direct calculation of spatial dispersion quantities at a computational cost that is comparable to that of a uniform-field response calculation. We present results for the clamped-ion flexoelectric tensor in SrTiO3 and the dynamical quadrupoles (the higher-order multipolar counterpart of the Born effective charges) in tetragonal PbTiO3. The quadrupoles relate to the clamped-ion piezoelectric tensor as predicted by R. Martin in his 1972 seminal paper. |
Monday, March 4, 2019 12:03PM - 12:15PM |
B02.00003: Flexoelectricity and Twins in LaAlO3 Christopher Mizzi, Binghao Guo, Laurence Marks Flexoelectricity in complex oxides has recently garnered much interest due to its ubiquity, intrinsic scaling at the nanoscale, and device applications. Although this has resulted in a dramatic increase in the number of publications on this phenomenon, an abundance of fundamental questions persists. In particular, the role of microstructure on the flexoelectric response of oxides is scantly considered in the existing literature. Twinned oxides are well-poised to elucidate this question because they naturally bridge the gap between single crystals and polycrystalline ceramics. In this work, a dynamic mechanical analyzer in a three-point bending configuration was used to study the flexoelectric response of {100}pc-LaAlO3 with lamellar twin microstructures as a function of temperature. We found the flexoelectric response of LaAlO3 is dominated by its twinned microstructure and tunable by two orders of magnitude via changing the number of twins, domain wall orientation, and domain wall mobility. |
Monday, March 4, 2019 12:15PM - 12:27PM |
B02.00004: Ferroelectric Materials for Photocatalytic Water-Splitting – Strained Mixed Anion Perovskites Nathalie Vonrüti, Ulrich Aschauer Polarity, for example in ferroelectric materials, can significantly increase a catalyst’s performance by improving charge-carrier separation. However, polar distortions also increase the band gap as shown for epitaxially strained SrTiO3 (1). While this band-gap increase is small for oxides, our density functional theory calculations show a much larger increase for oxynitrides: The enhanced covalency due to reduced electronegativity of nitrogen compared to oxygen results in larger strain-induced polar distortions and therefore more strongly increased band gaps by up to 1.5 eV. The reduced electronegativity, which leads to a higher valence band in oxynitrides and therefore a band gap in the visible that is attractive for photocatalysis, thus also has a detrimental effect on photo absorption when polar distortions are present. This results in a trade-off between small band gaps and polarity. We will discuss different strategies on how to overcome this trade-off with mixed anion perovskite compounds, which have not yet been considered for photocatalytic water-splitting. |
Monday, March 4, 2019 12:27PM - 1:03PM |
B02.00005: Bulk- and Flexo-Photovoltaic effects Invited Speaker: Marin Alexe Two years after the invention of modern prototype solar cells, it was found that BaTiO3exhibits a photovoltaic effect distinct from that of p-n junctions, later called the bulk photovoltaic (BPV) effect. Under uniform illumination, a homogeneous ferroelectric material gives rise to a short-circuit current and produces an anomalously large photo-voltage well exceeding the bandgap energy. The microscopic origins of this effect supposed to originate from the asymmetric distribution of photoexcited non-equilibrium carriers in k-space, caused by absence of centrosymmetry. The talk will present a short history and the basics of the bulk photovoltaic effect, tip enhancement, as well as the electronic origin of the anomalous BPV in some materials such as BiFeO3. Potential applications such as energy harvesting or light-induced reversible switching of ferroelectric polarization at room temperature will be adressed. I will show how the BPV effect may be used in optical switching tunnel junctions or other similar devices. Finally, I will discuss a new photovoltaic effect which turns the BPV effect into a universal effect allowed in all semiconductors by mediation of the flexoelectric effect. [M.-M. Yang D. J. Kim, & M. Alexe, Flexo-Photovoltaic Effect, Science 360, 904 (2018)] |
Monday, March 4, 2019 1:03PM - 1:15PM |
B02.00006: Complex chalcogenides as highly-polarizable semiconductors Stephen Filippone, Shanyuan Niu, Kristopher Williams, William A Tisdale, Yi-Yang Sun, Jayakanth Ravichandran, Rafael Jaramillo Ternary sulfides and selenides in the distorted-perovskite and related structures (“complex chalcogenides”) are predicted to be semiconductors with band gap in the visible-to-infrared, and may be useful for optical, electronic, and energy conversion technologies. The crystal structures familiar to complex oxides, together with larger chalcogenide anions, suggest that complex chalcogenides will be highly polarizable semiconductors. |
Monday, March 4, 2019 1:15PM - 1:27PM |
B02.00007: Understanding Negative Thermal Expansion in Layered Perovskites Chris Ablitt, Nicholas C Bristowe, Mark S Senn, Arash A Mostofi Negative thermal expansion (NTE) is an unusual phenomenon where a material shrinks rather than expands with increasing temperature. We will present recent results showing that in layered perovskites there is a significant enhancement of elastic anisotropy due to symmetry breaking that results from the combined effect of layering and frozen rotations of oxygen octahedra. This feature, unique to layered perovskites of certain symmetry, is what allows uniaxial NTE to persist over a large temperature range [1]. Since the structure of this phase facilitates cooperative strains in-plane and along the layering axis without necessitating the deformation of stiff nearest neighbour bonds, it has been possible to derive a mathematical description of this mechanism in an idealised system using simple geometrical models [2]. This insight has allowed us to investigate how changing structural features, such as the layer thickness [3]; physical features, such as temperature [1]; and chemistry, by substitutional doping [4] may all be used to control the thermal expansion of the material. |
Monday, March 4, 2019 1:27PM - 1:39PM |
B02.00008: Reduction of the thermal conductivity in PbTiO3 thin-films by ferroelectric domain walls. Eric Langenberg, Davig Bugallo-Ferrón, Dipanjan Saha, El�as Ferreiro-Vila, Jonathan A Malen, Darrell G. Schlom, Francisco Rivadulla The development of materials with switchable thermal conductivity have been limited to small low/high thermal conductivity ratios. We experimentally demonstrate that the thermal conductivity of PbTiO3 films can be significantly reduced through ferroelectric domain wall engineering. Ferroelectric domain walls (DWs) are effective phonon scattering sites, whose density and spacing can be controlled by an electric field, and therefore offer a unique opportunity to modulate the thermal conductivity by an external stimulus. In our experiments, we significantly reduced thermal conductivity by engineering the type and configuration of the DWs, as well as the DW density, through epitaxial strain and impurity doping. |
Monday, March 4, 2019 1:39PM - 1:51PM |
B02.00009: ABSTRACT WITHDRAWN
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Monday, March 4, 2019 1:51PM - 2:03PM |
B02.00010: First-principles study on piezoelectricity in Bi(Fe,Co)O3 Hiroshi Katsumoto, Kunihiko Yamauchi, Tamio Oguchi The perovskite solid solution BiFe1-xCoxO3 (BFCO) is composed of BiFeO3 and BiCoO3, which have rhombohedral and tetragonal structure, respectively. BFCO is a promissing lead-free piezoelectric material, since the morphotropic phase boundary (MPB) was discovered around 0.2 < x < 0.4 region at room temperature, where monoclinic phase is present [1]. A typical multiferroic material, BiFeO3 shows ferroelectricity and G-type antiferromagnetic order at room temperature. On the other hand, BiCoO3 shows high tetragonality and C-type antiferromagnetic order. The enhancement of piezoelectric coefficients and polarization rotation can be expected in a vicinity of the MPB as in PZT. The monoclinic crystal structure is √2×√2×1 perovskite cell which accommodates the antiferromagnetic configuration. The spontaneous polarization and the piezoelectric coefficients have not been measured in the bulk system so far, owing to the large leakage current. In this context, we performed first-principles calculations of the spontaneous polarization and the piezoelectric response of a Fe/Co chemically ordered model of BFCO. Based on the results, we discuss the ferroelectric distortion and the microscopic mechanism of the piezoelectric effect in BFCO. |
Monday, March 4, 2019 2:03PM - 2:15PM |
B02.00011: ABSTRACT WITHDRAWN
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