Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L14: Focus Session: Multifunctional Oxides I |
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Sponsoring Units: FIAP Chair: Jane P. Chang, UCLA Room: LACC 403B |
Tuesday, March 22, 2005 2:30PM - 3:06PM |
L14.00001: Multifunctional Oxides: Growth and Integration Invited Speaker: Recently, the formation of new classes of integrated structures based complex oxides has been pursued. The application of low temperature growth techniques have allowed the development of important oxides, such as lithium niobate, in forms not previously achieved through the processing of traditional bulk materials. The application of epitaxial processes to these systems can generate new function and levels of performance in optical devices. More intriguing applications are being developed through the integration of these materials directly with semiconductors. The deposition and thermal processing of complex oxide materials on semiconductor surfaces are complicated by many issues which impact the long term stability of these interfaces. These oxides are often characterized by a very large phase stability region leading to a broad range of stiochiometry, particularly in comparison to conventional semiconducting materials, a variety of types and concentrations of defects, and the possibility of ionic diffusion and conductivity. The general features of simple reaction systems will be reviewed and applied to the complex oxides. The development of other approaches based on recent advances and applications in materials integration techniques, such as wafer bonding, bypass many of these issues and can lead to a more general technology for the formation of new multifunctional devices. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:18PM |
L14.00002: Second generation photocatalytic materials - anion doped TiO$_2$ James Lewis, Hao Wang TiO$_2$ is a wide-band gap semiconductor with band gap of ~3.0 eV. Recently an effective photoresponse in the visible-light region has been observed in anion doped TiO$_2$, which is a promising second generation photocatalyst. We contribute a theoretical understanding of this phenomenon. Our ab initio density functional theory investigations show that substitutional anion dopants incorporated into TiO$_2$ drastically affect the electronic structure of the material thus improving its photoactivity. The resulting smaller bandgaps (in the visible) predicted in this work agrees with the available experimental observations for larger anion concentrations around 5$\%$. We also address the effects of doping concentration on the photoresponse of this material. [Preview Abstract] |
Tuesday, March 22, 2005 3:18PM - 3:30PM |
L14.00003: Quantum dynamics simulations of interfacial electron transfer in sensitized TiO$_2$ semiconductors Sabas G. Abuabara, Luis G. C. Rego, Victor S. Batista A mixed quantum-classical method combining {\it ab initio}-DFT molecular dynamics simulations with electronic relaxation calculations is used to investigate interfacial electron transfer in catechol/TiO$_2$-anatase nanostructures under vacuum conditions at finite temperature. The calculations demonstrate that the injection mechanism is accelerated by thermal nuclear motion. In particular, electron-phonon scattering leads to ultrafast adsorbate monolayer electron transfer and the disappearance of the anisotropic charge delocalization ({\it i.e.}, carrier diffusion) identified in frozen lattice studies, due to increased coupling between quasistationary molecular orbitals localized on the adsorbates and those orbitals delocalized throughout the semiconductor bulk. The results are particularly relevant to the understanding of surface charge separation in efficient mechanisms of molecular-based photovoltaic devices. [Preview Abstract] |
Tuesday, March 22, 2005 3:30PM - 3:42PM |
L14.00004: Non-bolometric Photoresponse in Thin Films of Perovskite Manganites M.Raj Rajeswari, Mason Overby, Vera Smolyaninova, David Cox, Anthony Davidson We have studied the light induced resistance changes (photoresponse) in the thin films of several manganite systems that undergo insulator- metal transition. While the photoresponse in materials such as La $_ {1-x}$ Ca $_{x}$ MnO$_{3}$ is purely thermal (bolometric) in origin, we find that photoresponse of low Tc manganites that exhibit first order percolative transitions (e.g.La$_{0.7-y}$Pr$_{y}$CaMnO$_{3})$ is remarkably different. In these latter type of materials, we observe photoresponse that cannot be accounted for by thermal effects alone. The temperature dependence of the non-bolometric response suggests a light-induced reduction in the resistivity. We believe that the origin of the non-bolometric response is linked to the co-existence of the charge ordered insulating (COI) regions with the ferromagnetic metallic regions (FMM) in the low T$_{c} $ materials. Reduction in resistivity could arise due to the radiation induced switching of the COI regions into the FMM state. We will discuss the details of the non-bolometric response including its dependence on temperature, radiation intensity and frequency, its relaxational dynamics and the possible correlation with magnetoresistance. This work is supported by the NSF grants DMR-0116619 and DMR- 0348939, Undergraduate Research Grant from the College of Science and Mathematics, Towson University and the Federal work- study support for M. Overby. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L14.00005: Multiferroic thin films Wilma Eerenstein, Neil Mathur, Finlay Morrison, Jim Scott Multiferroic materials are both ferroelectric and ferromagnetic. This combination opens up new applications, as the magnetization can be addressed with an electric field and the polarization by a magnetic field. However, most multiferroic materials either have low polarizations and/or low transition temperatures, thus limiting their potential use in novel devices. We report on high quality epitaxial films of two candidate materials, namely BiFeO$_{3}$ and BiMnO$_{3}$. Both films have narrow X-ray rocking curves of around 0.04\r{ } and are highly insulating with resistivities in excess of 10$^{9}$ Ohm cm. The BiMnO$_{3}$ films are ferromagnetic (2.2 $\mu _{B}$/Mn) and the BiFeO$_{3}$ films show a very weak ferromagnetic magnetization of $<$0.05 $\mu _{B}$/Fe, in contrast to recent claims of an epitaxial enhancement to 1 $\mu _{B}$/Fe (Wang \textit{et al}., Science \textbf{299}, 1719 (2003)). In BiFeO$_{3}$ films we record a dielectric constant of 80 at room temperature. Interestingly, this figure is 2500 for BiMnO$_{3}$ suggesting good ferroelectric properties. [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L14.00006: Finite-temperature properties of ferroelectric PZT ultrathin films near the morphotropic phase boundary Igor Kornev, Huaxiang Fu, Laurent Bellaiche Physical properties at nanometer scales in low dimensional ferroelectric structures are attractive fundamentally, as well as technologically. In this talk, a first-principles-based scheme allowing the computation of finite-temperature properties of complex ferroelectric (001) Pb(Zr,Ti)O$_3$ (PZT) thin films under different boundary conditions will be presented. The effects of uncompensated depolarizing fields and mechanical boundary conditions on the properties of films will be revealed. In particular, it will be shown that new ferroelectric phases, including unusual triclinic and monoclinic states can occur depending on the interplay between electrical and mechanical boundary conditions. Finally, multidomains and their formation mechanism and atomic characteristics in ultrathin ferroelectric films will be discussed. This work is supported by ONR grants N 00014-01-1-0365, N 00014-04-1-0413 and N 00014-01-1-0600 and NSF grants DMR-9983678 and DMR-0404335. [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L14.00007: Investigations on multiferroic characteristics of sol-gel synthesized (1-x) Pb (Fe$_{0.5}$Nb$_{0.5}$)O$_3$ – x PbTiO$_3$ thin films S.B. Majumder, V. Rajasekarakumar, R.S. Katiyar, A. Manivannan Pb(Fe$_{0.5}$Nb$_{0.5})$O$_{3}$ (PFN) ceramics exhibit very high dielectric constant and therefore it is an attractive candidate for multilayer capacitors. From our earlier studies on bulk PFN ceramics we have found that the paraelectric to ferroelectric phase transition temperature (T$_{c})$ is diffused in nature with a dielectric constant $\sim $ 58000 (measured at 1 kHz) at T$_{c}$ ($\sim $390K). The process optimized perovskite PFN ceramics also exhibit room temperature ferromagnetic behavior, and therefore, PFN ceramics qualify as typical multiferroic materials. To promote the perovskite phase formation and reduce the loss tangent in PFN, we have synthesized solid solution thin films of PFN with PbTiO$_{3}$ (PT) deposited on platinized silicon and single crystalline strontium titanate substrates. The PT contents were systematically varied up to 40.0 at {\%} and the films were characterized in terms of their phase formation behavior, microstructure evolution, dielectric and ferroelectric properties. Unlike bulk ceramics, PFN thin films on strontium titanate substrates exhibited typical anti-ferromagnetic ordering with a Neel temperature $\sim $ 120K. The effect of PT contents on the magnetic ordering of PFN-PT thin films will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L14.00008: Morphological and Magnetic Properties of Pulse Laser Deposited Barium Hexaferrite Thick Films for Application in Microwave Circulators Yanko Kranov, Tejasvi Prakash, David McIlroy Barium-hexaferrite thick films have been grown by Laser Ablation on MgO(111) and Si(111) substrates. For purpose of the experiment excimer laser was used with adjustable energy up to 1J. As a source of BaM a custom made high density BaM target was used. In order to investigate the crystallographic properties of the films XRD analysis was carried out. To characterize the magnetic properties, both MOKE and VSM were performed. SEM imaging provided the information for the overall topography of the films and their thickness as well. [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L14.00009: Theoretical Study of Gallium Oxide Clusters Gowtham S, Aurora Costales, Mrinalini Deshpande, Ravindra Pandey Gallium oxide is an important semiconducting oxide with applications in the areas of optics and micro-electronics. It is, therefore, not surprising that considerable efforts have been made in the past to understand the structural, optical and electronic properties of gallium oxide. On the other hand, interest in studying the properties of nanostructures and nanoclusters of gallium oxide is relatively recent. Small clusters of gallium oxide can be taken as a prototype to understand the Physics and Chemistry of surfaces and nanostructures. In this talk, we report the equilibrium structure, bonding and electronic properties of small clusters of gallium oxide which are studied under the framework of Density Functional Theory. All the clusters studied so far have shown a preference for planar arrangement of constituent atoms. The ionization induced structural distortions in the neutral clusters are relatively small, except those for Ga$_3$O$_2$. In anionic (cationic) clusters, the added (removed) electron is shared by gallium atoms, except in case of GaO$_3$. All the isomers are dominated by ionic Ga-O bond. The HOMO-LUMO gap in oxygen deficient Ga$_m$O$_n$ clusters depends upon the gallium content, i.e., the gap will change with particle size even if $n/m$ is fixed. Ionization potential and electron affinity of these clusters have been calculated, for the first time, in this study, and both values increase with the increase in oxygen to metal ratio. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L14.00010: Intermittent polaron dynamics: Born-Oppenheimer out of equilibrium Dima Mozyrsky, Matthew Hastings, Ivar Martin We consider the non-equilibrium dynamics of a molecular level interacting with local phonon modes in the case of a strong polaronic shift which prevents a perturbative treatment of the problem. Instead, we find that in an adiabatic regime when the electronic states react faster than the phonon modes it is possible to provide a fully non-perturbative treatment of the phonon dynamics including random noise and dissipation. The result shows intermittent switching between bistable states of the oscillator with an effective random telegraph noise. [Preview Abstract] |
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