Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Electronic Conduction and Defects |
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Sponsoring Units: DMP DCOMP Chair: Ichiro Takeuchi, University of Maryland Room: C143/149 |
Monday, March 21, 2011 11:15AM - 11:27AM |
B33.00001: Conduction at different ferroelectric domain walls in BiFeO$_{3}$ Saeedeh Farokhipoor, Christophe Daumont, Beatriz Noheda BiFeO$_{3}$ (BFO) is, at room temperature, a rhombohedrally distorted, ferroelectric perovskite. There are eight possible polarization directions (or domains) and three different types of domain walls, namely, 180\r{ } (ferroelectric) and 109\r{ } and 71\r{ } (ferroelectric and ferroelastic) domain walls. Recent works have shown that the domain walls of BFO can display functionalities different from those of the domains, generating photocurrents [1], inducing exchange bias [2] and displaying conductivity at room temperature [3]. Conduction has been reported at 180\r{ } and 109\r{ } domain walls[3] and it was proposed that the reduction of the band gap, associated with the suppression of ferroelectric distortions, at domain walls was responsible for the observed conduction[3]. It is, however, not yet clear if and how other (extrinsic) mechanisms affect the conductivity at the walls. In order to help clarifying the origin of domain wall conductivity, we have performed temperature, thickness and orientation dependent local conductivity measurements in BFO thin films. The results will be discussed in this presentation. \\[4pt] [1] S.Y. Yang, Nature Nanotech. 5, 143 (2010); [2] L.W. Martin et al. Nano Letters 8, 2050 (2008);[3] J. Seidel et al., Nature Mat. 8, 229 (2009). [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B33.00002: Electronic Transport properties of ultra-thin BiFeO$_{3}$ Dipanjan Mazumdar, Oleg Mryasov, Vilas Shelke, Stephen Jesse, Arthur Baddorf, Sergei Kalinin, Arunava Gupta We have investigated the electronic transport properties of rhombohedral (R) and the nearly-tetragonal (T) phase of BiFeO$_{3}$ using beyond density functional techniques, and combined with nanoscale I-V transport measurements. Using Quasi-particle GW approximation, we show the R and T phase to have significantly different electronic structures. We find that the T phase has significantly lower effective mass at the conduction band edge compared to the R phase leading to a lower effective barrier height for tunnel electrons (0.38 eV vs 3.6 eV). We therefore anticipate that tunnel devices with T phase BFO to have significantly lower resistances. Local transport measurements performed on ultra-thin BFO R phase are consistent with this inference. Tunneling measurements on the tetragonal phase films are also presented. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B33.00003: Oxygen vacancies in lanthanum aluminate (LaAlO$_3$) Joshua Sayre, Nicola Spaldin Oxygen vacancies can affect the properties of an oxide in various manners such as increasing its ion or electronic conductivity, changing its lattice constant or causing dielectric breakdown. The aim of this research is to investigate structural changes and consequent changes in properties caused by oxygen vacancies in a model complex oxide, lanthanum aluminate, LaAlO$_3$. We use density functional theory with the generalized gradiant approximation (GGA) and within the VASP package to calculate the structure and properties of representative oxygen vacancy profiles. We find that the presence of oxygen vacancies modifies the pattern of rotations of the oxygen octahedra. We discuss the implications of our results for understanding the correlation between epitaxial strain in oxide thin films and intrinsic defect profiles. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B33.00004: Defect-Induced Electronic Structures and Formation Energies of Vacancy Complexes in SrTiO$_{3}$ Jiyeon Kim, Choong H. Kim, Rokyeon Kim, Jaejun Yu Recently defect induced ferroelectricity in SrTiO$_{3}$ has been reported at room temperature. Strontium-oxygen vacancies were suggested as a possible source of electric polarization regarding to the existence of mid-gap states. To understand the detailed electronic structures induced by defects and their formation energies, we carried out density-functional-theory calculations for various defects such as Sr, Ti, O, Sr-O, Sr-O- O vacancies. We employed the LDA+$U$ method as implemented in the VASP code to describe the d-orbital occupation at the Ti- site due to the presence of oxygen vacancy. A complex of Sr-O-O vacancies is found to contribute to the localized electronic states in the band gap and its formation energy is small enough to form easily under the poor oxygen limit. We conclude that the vacancy-complex defects play a crucial role in determination of the physical properties of SrTiO$_{3}$ thin films. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B33.00005: Quasiparticle electronic structure calculations of F centers in SrTiO$_{3}$ perovskite Chandrima Mitra, Alex Demkov Among the broad class of oxides, ABO$_{3}$ perovskites have attracted a lot of attention in the recent past due to its beneficial material properties. SrTiO$_{3}$ is one such example of this class of compounds. It shows a wide range of properties from being ferroelectric to exhibiting superconducting properties in doped SrTiO$_{3}$. The anomalous dielectric properties in this material make it a potential candidate for technological applications. However, being a semicovalent oxide, the complexities in its electronic structure have hindered a proper characterization of the system. For instance, the intrinsic excitonic luminescence, in this system, is not well understood and there is no general agreement as to whether it is caused by defects or due to self trapped excitons. This calls for an accurate theoretical description of the electronic levels as well as the various defect states in this material. In this work we present results for quasiparticle GW calculations of pure as well as \textit{defective} SrTiO$_{3}$ containing oxygen vacancies which form F centers in these compounds. From a quasiparticle description of the system excitonic properties of SrTiO$_{3}$ will be examined. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B33.00006: Charge transition levels of oxygen vacancies in monoclinic hafnia Manish Jain, James R. Chelikowsky, Steven G. Louie We perform electronic structure calculations on oxygen vacancies in hafnia using a combined density functional theory (DFT) and GW formalism. This formalism corrects for the error in calculating formation energy and charge transition levels using standard DFT. While the formalism is, in principle, exact; in previous calculations of this kind, one makes several approximations to make the calculation tractable. We assess the impact of these approximations on the charge transition levels of the oxygen vacancy in hafnia. In particular, we examine the assumption that the quasiparticle wavefunctions are the same as DFT wavefunctions for the defect states. We show that this assumption can lead to erroneous results in this system and present the charge transition levels without making use of this assumption. We also explore the possibility that these defects are negative U centers. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B33.00007: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 12:39PM - 12:51PM |
B33.00008: Disproportionation and comproportionation reactions of resistive switching in polycrystalline NiOx films Forest S.-S. Chien, Yi-Ta Wu, Guan-Long Lai, Y.H. Lai The NiO$_{x}$ thin film exhibit excellent bistable unipolar resistive switching, which has strong potential in nanoscale nonvolatile-memory applications. The underlying mechanism of NiOx resistive switching is still in debate. We studied the chemical bonding states of Ni 2p and O 1s at high/low resistance spots by focused X-ray photoemission spectroscopy. The disproportionation and comproportionation reactions of 3NiO $\leftrightarrow $ Ni + Ni$_{2}$O$_{3}$, accounts for the resistive switching of NiO$_{x}$. The calculated Gibbs energy of the reaction indicates the reversibility of the reaction thermochemically. The dynamic breathing of the filaments with switching was observed by conducting atomic force microscopy. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B33.00009: The effect of oxygen migration for random resistance access memory in oxide-based devices Zhaoliang Liao, Peng Gao, Yang Meng, Hongwu Zhao, Xuedong Bai, Dongmin Chen The observed electric field induced resistance switching in metal oxide thin films is generally thought to arise from the creation or annihilation of oxygen defects. By depositing different kinds of metal electrodes on Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ and CeO$_{2}$ thin films to construct sandwiched devices, we found that the devices can be categorized into two groups with different switching behaviors, depending on the Gibbs free energy of oxidation of the top electrodes with respect to that of underneath metal oxide\textit{. In-situ} TEM measurements show a structure change with an applied electric field. Our analysis indicates that the structure change is related to the oxygen migration driven by external electric field. Therefore, it suggests that not only the oxygen defects but also their migration play important roles in the functionality of oxide-based devices [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B33.00010: Conduction and Loss Mechanisms in Flexible Oxide-Based Memristors J.L. Tedesco, N. Gergel-Hackett, L. Stephey, A.A. Herzing, M. Hernandez-Mora, C.A. Hacker, J. Obrzut, L.J. Richter, C.A. Richter In order to study the conduction and loss mechanisms behind their operation, flexible sol-gel based memristors were fabricated with differing oxide film thicknesses and device sizes. XPS, TEM, EELS, and VASE measurements indicated the oxide was amorphous TiO$_{2}$, with a significant fraction of organic material. Analysis of the bias and sweep rate dependence of the devices suggested the switching mechanism was induced by charge flow in the memristor and not by the electric field. Further analysis of the I-V curves indicated that once the memristors were switched into the high-current ``ON'' state, conduction through them generally became ohmic. Once such memristors were cut to yield two smaller devices, there was typically only one device that remained ohmic, indicating that localized conduction pathways caused switching in the flexible memristors. There was a shift in the capacitance-frequency and conductance-frequency measurements following switches between the ``ON'' and ``OFF'' states of the devices, indicating that an additional dielectric loss mechanism was present in these films that was not present in ordinary TiO$_{2}$ films. This loss mechanism is attributed to dipoles in the organic constituents of the films that are by-products of the sol-gel process. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B33.00011: Capacitive network near the metal insulator transition in Vanadium Dioxide J.G. Ramirez, E.J. Patino, R. Schmidt, A. Sharoni, M.E. Gomez, I.K. Schuller Recent infrared spectroscopy and transport measurements in nano-scaled junction of VO2 have revealed the existence of phase separation into metallic and insulating phases. Here we present Impedance spectroscopy measurements performed in high quality Vanadium dioxide (VO$_2$) thin films for the first time. This technique allows distinguishing between the resistive and capacitive response of the VO$_2$ films and provides the dielectric properties across the metal-insulator transition (MIT). The film capacitance exhibits an unusual increase close to the MIT which implies the formation of a capacitor network produced by the nanoscale phase separation of metallic and insulating phases. This work has been supported by AFOSR, COLCIENCIAS, CENM and Ramon y Cajal Fellowship. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B33.00012: Substitution site for Zn in LiNbO$_3$ from detailed EXAFS analysis Frank Bridges, Brad Car, Jaime Castillo, Michael Kozina, Scott Medling We report detailed EXAFS studies of Zn doped LiNbO$_3$, at the Zn and Nb K-edges, as a function of dopant and as a function of temperature. For this material there exist several models concerning the substition site(s) for Zn. Our data are only consistent with Zn substitution on the Li site. Any substitution on the Nb site is very small. Further as the Zn concentration changes from 5-9\% the EXAFS r-space function for the Zn K-edge changes very little, a slight amplitude reduction consistent with increased local disorder for increasing Zn concentration. Our detailed analysis shows that the nearest O neighbors to Zn are slightly pulled inward while the nearst metal atoms - Nb - are pushed away. We cannot tell if there are vacancies on the Li sites because Li is a very weak backscatterer, and the amplitude of the rather long Zn-Li peak is very low. We discuss and compare our results with previous proposed models and with recent calculations for other defects that suggest that many +2 dopants substitute at the Li site. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B33.00013: Structure and Magnetic Properties of Electron Doped YMnO$_{3}$ Tian Yu, Peng Gao, Trevor Tyson Combined local and long range structural measurements were conducted on the electron doped ferroelectric Y$_{1-x}$Zr$_{x}$MnO$_{3}$ system. Doping by Zr is found to maintain the hexagonal structure for a large range of x-values. The location of Zr in the lattice is identified and changes in structure with doping are followed. These details of the local structure are examined by x-ray diffraction and x-ray absorption spectroscopy and compared with detailed magnetic studies to correlate the impact of electron doping and atomic structure on the magnetic order in these systems. This work is supported by DOE Grant DE-FG02-07ER46402. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B33.00014: First-principles calculations of Ti and O NMR chemical shift tensors in ferroelectric perovskites Daniel Pechkis, Eric Walter, Henry Krakauer Complementary chemical shift calculations were carried out with embedded clusters, using quantum chemistry methods, and with periodic boundary conditions, using the GIPAW approach\footnote{C.~\ J.~\ Pickard and F.~\ Mauri Phys. Rev. B {\bf 63}, 245101 (2001)} within the Quantum Espresso package.\footnote{P. Giannozzi et al., Journal of Physics: Condensed Matter {\bf 21}, 395502 (2009)} Compared to oxygen chemical shifts, $\hat \delta$(O),\footnote{D.~\ L.~\ Pechkis, E.~\ J.~\ Walter, and H.~\ Krakauer. J.~\ Chem. Phys. {\bf 131}, 184511 (2009)} cluster calculations for $\hat \delta$(Ti) were found to be more sensitive to size effects, termination, and choice of gaussian-type atomic basis set, while GIPAW results were found to be more sensitive to the pseudopotential construction. The two approaches complemented each other in optimizing these factors. We show that the two approaches yield comparable chemical shifts for suitably converged simulations, and results are compared with available experimental measurements. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B33.00015: Electrical Breakdown in Lightning Arrestor Connector (LAC) Devices Harold P. Hjalmarson, Kenneth Kambour, Andrew C. Pineda Lightning arrestor connector (LAC) devices protect electronic devices by providing a conductive path to ground for electrical power surges caused by lightning. Such devices consist of an insulating material between electrodes. This insulation region is composed of an air gap and a high permittivity dielectric. In this presentation, the physics of the phenomena active in the early stages of the flow of transient electrical current will be described. The conditions that lead to thermal breakdown of the dielectric will also be discussed.--Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
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