Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G41: Focus Session: Flexoelectrics, Electrocalorics and Perovskites on Semiconductors |
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Sponsoring Units: DMP DCOMP Chair: Alexander Tagantsev, EPFL Room: Mile High Ballroom 3C |
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G41.00001: Electrocaloric energy efficiency and cooling power Invited Speaker: Neil Mathur How much energy is required to drive electrocaloric effects near ferroelectric phase transitions? I will compare electrocaloric ceramic and polymer films with each other, with magnetocaloric materials (exploited in over 40 prototype refrigerators), and with elastocaloric materials. I will also discuss the cooling power that could be achieved in electrocaloric heat pumps based on multilayer capacitors in which heat flow is modelled using finite element analysis. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G41.00002: Thermodynamics of Multicaloric Effects in Multiferroics Antoni Planes, Teresa Castan, Avadh Saxena Ferroic and multiferoic materials thermally respond to externally driven changes of ferroic properties. Usually these changes are induced by application or removal of the field thermodynamically conjugated to a specific property. The isothermal change of entropy and the adiabatic change of temperature are commonly used in order to quantify the caloric response of a given material. From this perspective we provide a general thermodynamic framework to study multicaloric effects in multiferroic materials. This is applied to the case of a magnetoelectric multiferroic, which is described by means of a Landau free energy with a biquadratic coupling between polarization and magnetization. We obtain a phase diagram, the isothermal entropy change and the adiabatic temperature change across different continuous and first order transitions as the applied electric and magnetic fields are varied. The obtained multicaloric effects are suitably decomposed into the corresponding electrocaloric and magnetocaloric contributions. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G41.00003: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G41.00004: First principles investigation of the electro-caloric effect in BaTiO$_3$ Claude Ederer, Madhura Marathe The electro-caloric effect, a change of temperature or entropy under the application of an electric field, is very promising for future applications in solid state cooling devices [1]. It has been shown that temperature changes of several Kelvin can be achieved in thin films close to the ferroelectric transition temperature [2]. However, to utilize this effect within an actual device, a good control over the caloric properties at different operating temperatures as well as a good understanding of materials-specific trends is very important. Here, we use first principles-based effective Hamiltonians [3] to study the electro-caloric effect in the prototypical ferroelectric material BaTiO$_3$. In particular, we assess the effect of epitaxial strain, which is likely to occur in thin film devices, on the caloric properties, and we show that the electro-caloric effect is quite sensitive to such epitaxial strain. We also compare direct and indirect determination of the adiabatic temperature change. The latter uses a Maxwell relation that relates the electro-caloric and pyroelectric effects. [1] J. F. Scott, Annu. Rev. Mater. Res. 41, 229 (2011). [2] Mischenko et al., Science 311, 1270 (2006). [3] Nishimatsu et al., Phys Rev. B 78, 104104 (2008). [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G41.00005: Flexoelectricity via coordinate transformations Invited Speaker: Massimiliano Stengel Flexoelectricity describes the electric polarization that is linearly induced by a strain gradient, and is being intensely investigated as a tantalizing new route to converting mechanical stimulation into electrical signals and vice versa [1]. While several breakthough experiments have been reported in the past few years, progress on the theoretical front has been comparatively slow, especially in the context of first-principles electronic-structure theory. The main difficulty with calculating the flexoelectric response of a material is the inherent breakdown of translational periodicity that a strain gradient entails, which at first sight questions the very applicability of traditional plane-wave pseudopotential methods. In this talk I will show how these obstacles can be overcome by combining density-functional perturbation theory with generalized coordinate transformations [2,3], gaining access to the full microscopic response (in terms of electronic charge density, polarization and atomic displacements) of a crystal or nanostructure to an arbitrary deformation field. As a practical demonstration, I will present results on the full flexoelectric response of a SrTiO$_3$ film, including atomic relaxations and surface effects. \\[4pt] [1] P. Zubko, G. Catalan, and A. K. Tagantsev, Annu. Rev. Mater. Res. {\bf 43}, 387-421 (2013).\\[0pt] [2] M. Stengel, Phys. Rev. B, in press. (arXiv:1306.4240).\\[0pt] [3] M. Stengel, Nature Communications {\bf 4}, 2693 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G41.00006: Contribution of polar nanorregions to the giant flexoelectricity of relaxor ferroelectrics Gustau Catalan, Jackeline Narvaez We have studied the bending-induced polarization of single crystal relaxor ferroelectrics close to a morphotropic phase boundary. Anomalously large flexoelectric and flexocoupling coefficients were registered, with values well in excess (up to 10 times bigger) than theoretical expectations based on Kogan's theory [1] below critical temperature. The temperature dependence of the effective flexoelectric coefficients shows that this anomalous enhancement persists in the temperatures up to T* $\sim$ 250$^{\circ}$C, above which the values fall back in line with theoretical expectation for pure flexoelectricity. Cross-correlation between flexoelectric and elastic measurements indicates that the anomalous enhancement of bending-induced polarization is caused by the flexoelectric reorientation of non-percollating polar nanotwins that exist in the temperature range between Tc and T*. \\[4pt] [1] S. M. Kogan, Soviet Physics Solid State, 5, (1964) 2069. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G41.00007: Structural and Magnetotransport Study of SrTiO$_{3-\delta}$/Si Films Grown by Molecular Beam Epitaxy Alex Currie, Ryan Cottier, Oscar Villarreal, Jesus Cantu, Arturo Ponce, Nikoleta Theodoropoulou SrTiO$_{3}$ (STO) films were grown on p-Si (001) substrates using molecular beam epitaxy (MBE). Oxygen vacancies were introduced by controlling the Oxygen resulting in SrTiO$_{3-\delta}$ with $\delta$ $\sim$ 0.02{\%} for the lowest pressure. The single phase STO/Si films were of high crystalline quality as verified by x-ray diffraction, transmission electron microscopy, and had an rms roughness of less than 0.5nm measured by atomic force microscopy. Transport measurements were performed on the STO/Si structures in a Van der Pauw configuration. We measured resistance as a function of temperature, T $=$ 3K-300K and as a function of an applied magnetic field , H$=$0 to $\pm$ 9T. The resistivity decreased from 1 Ohm cm to 3x10$^{-2}$ Ohm cm as the film thickness increased (3nm-60nm) for all temperatures. The magnetoresistance (MR) shows a reproducible trend for all films, the MR is positive at 300K, becomes negative between 200K and 100K and at low temperatures T$=$3-20K the MR is positive at low H$=$0 to $\pm$ 2T but at high fields, it starts decreasing again. The MR behavior combined with the Hall effect data indicates the presence of localized electrons that delocalize with H and T. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G41.00008: BaTiO$_3$/GaAs heterostructures: a possible route to reconfgurable III-V nanoelectronics Dongyue Yang, Giriraj Jnawali, Lu Cheng, Feng Bi, Patrick Irvin, Jeremy Levy, Borzoyeh Shojaei, Chris Palmstrom, Rocio Contreras-Guerrero, Ravi Droopad Ferroelectric field effect device concepts have existed since the early days of the transistor.\footnote{J. F. Scott , \textit {Ferroelectric Memories}, Springer-Verlag , Berlin \textbf {2000}} The challenges have been more materials-based rather than conceptual. Recent advances in oxide-molecular-beam epitaxy have allowed high quality interfaces between complex oxides and compound semiconductors. Here we focus on heterostructures between GaAs and BaTiO$_3$, which are well lattice-matched and have atomically sharp interfaces. These structures can be configured both in the III-V layer, by growing GaAs/AlGaAs heterostructures and quantum wells; and they can be ferroelectrically patterned in the BaTiO$_3$ layer using a scanning probe microscope. We discuss current efforts to develop this material for both optical and transport-based experiments. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G41.00009: Ab initio study of the epitaxial BaTiO3/Ge interface Mehmet Dogan, Divine Kumah, Charles Ahn, Frederick Walker, Sohrab Ismail-Beigi Growing thin films of crystalline metal oxides on silicon or germanium has been of great research interest for decades because of the possible applications of such systems in electronic devices. An example is provided by the ferroelectric oxide BaTiO$_{3}$: if it remains ferroelectric on a semiconductor, one may be able to realize non-volatile electronic devices based on the interfacial field effect where the state of the system is encoded in the polarization direction of the oxide. Thanks to recent advances in epitaxial growth methods, one can explore such interfaces in parallel with the experiment. Here, we use density functional theory to study the interface between BaTiO$_{3}$ and Ge. We describe how the structure of the interface depends on the oxygen content of the interface and compare to X-ray diffraction experiments. We show how the polarization of the BaTiO$_{3}$ thin film changes compared to the bulk. We analyze the electronic structure of the interface and illustrate how valence and conductance bands are aligned. We explore the energetics of oxygen vacancies in BaTiO$_{3}$ both in terms of positional and concentration dependence. We also discuss dynamics of oxygen vacancies by computing the energy barriers for oxygen vacancy diffusion. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G41.00010: Atomic and electronic structure of the BaTiO3-Ge (001) interface Kurt Fredrickson, Patrick Ponath, Agham Posadas, Martha McCartney, Toshihiro Aoki, David Smith, Alexander Demkov There is tremendous interest in putting perovskite oxides, such as SrTiO$_{\mathrm{3}}$ (STO) or BaTiO$_{\mathrm{3}}$ (BTO), on semiconductors due to their very high permitivitties. BTO can be grown directly on Ge using an approach similar to the growth of STO in Si. To date, very little is known about the atomic and electronic structure of the BTO-Ge interface. We use molecular beam epitaxy to grow BTO with in-plane polarization directly on Ge(001) using a Sr Zintl buffer layer. This results in an atomically flat, oxygen- and carbon-free Ge surface with very sharp (2x1) reconstruction as observed with reflection high energy electron diffraction. Using scanning transmission electron microscopy, we are able to precisely determine the atomic geometry of the interface, with the exception of the exact positions of the oxygen atoms. \textit{In situ }x-ray photoemission spectroscopy is used to analyze the oxidation state of the interfacial Ge and to determine the valence band offset at the interface. We use density functional theory to determine placement of interfacial O and calculate the valence band offset. The theoretical valence band offset is in good agreement with the photoemission data, strongly suggesting the correctness of the interface geometry. We calculate the effect of O vacancies and ionic substitution at the interfacial layer on the valence band offsets. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G41.00011: Epitaxial BaTiO$_{3}$ on Ge (001) Patrick Ponath, Agham Posadas, Keji Lai, Dave Smith, Alex Demkov Germanium, with its higher hole and electron mobility, might become an attractive candidate to replace silicon as a channel material in a field effect transistor. The ferroelectric high-k dielectric barium titanate (BTO) can be integrated on germanium (001) due to the small lattice mismatch between BTO and Ge and could therefore be a potential candidate for a ferroelectric memory if the problem of relatively high leakage could be solved. We report the epitaxial growth of BTO on a germanium (001) substrate with a thin buffer layer, which causes the BTO to be out of plane polarized. The BTO film crystallizes as-deposited which is monitored by reflection high energy electron diffraction. X-ray diffraction measurements of the BTO film indicate an out of plane ferroelectric polarization. [Preview Abstract] |
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