Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W42: Excitation and Transport at Complex Interfaces |
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Sponsoring Units: DMP Chair: Patrick Woodward, Ohio State University Room: D138 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W42.00001: Transport at Complex Oxide Interfaces William Cole, Anamitra Mukherjee, Nandini Trivedi, Mohit Randeria, Patrick Woodward We study interface properties and transport in a `generalized' double exchange Hamiltonian with electron-phonon coupling and superexchange, that can be used to model complex oxides such as the manganites and the double perovskites. The junction between different materials is modeled by imposing different Hamiltonian parameters on either side of an interface layer. We treat both the lattice and the localized spins as slow ``classical" degrees of freedom which provide a strongly spatially fluctuating background in which the fast electrons move. We then use classical Monte Carlo to optimize the spin-lattice configuration, minimizing the free energy, while solving the electronic problem through exact diagonalization self consistently with the Poisson equation. We track the charge redistribution, orbital reconstruction and evolution of the magnetic states across the interface and study the transport in the Kubo regime, and discuss implications of our results for manganite and double perovskite interfaces. [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W42.00002: Novel Electronically Conducting Tellurium Oxides Mas Subramanian, Theerunan Siritanon, Arthur Sleight Tellurium oxides seldom show measurable electronic conductivity. Tellurium oxides that appear to have Te$^{5+}$ contain Te$^{4+}$ and Te$^{6+}$ in two distinct crystallographic sites and are electronic insulators. Here we report on the synthesis and characterization of several new tellurium rich oxides of the general formula, CsM$_{x}$Te$_{2-x}$O$_{6}$, crystallizing in modified pyrochlore structure. Most of the compounds reported here are black in color with some exhibiting good electronic conductivities (2 S/cm) and Seebeck measurements indicate all are n-type. The observation of high electronic conductivities in compounds like CsGe$_{0.5}$Te$_{1.5}$O$_{6}$, CsAl$_{0.33}$Te$_{1.67}$O$_{6 }$confirms that observed conductivity is arising from doping of electrons into the empty 5s orbitals of Te$^{6+}$. This reduction is apparently accompanied with some small deviation from the ideal formula: oxygen content and/or ratio of cations on octahedral sites. This is in consistent with single-crystal X-ray as well as powder neutron diffraction structure refinements and the observed sign of the Seebeck coefficient. To our knowledge, this is a first observance of high electrical conductivity in mixed valent tellurium oxides. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W42.00003: Theory of magnetic and transport properties of double perovskites Oinam Nganba Meetei, Anamitra Mukherjee, Mohit Randeria, Nandini Trivedi, Patrick Woodward We map out the finite temperature phase diagram of the generalized double exchange model for double perovskites $A_2BB^\prime O_6$ by self consistently solving the tight-binding Hamiltonian of the ``fast'' electrons moving in a background of ``slowly'' fluctuating classical spins. We investigate the stability of ferromagnetic and various antiferromagnetic phases as a function of electron density, the $B$-$B^\prime$ charge transfer energy, the direct $B^\prime$-$B^\prime$ hopping and various exchange interactions. We compute the temperature and doping dependence of the $B$-site magnetization and conduction electron polarization. We also investigate the effects of antisite ($B/B^\prime$) disorder on magnetism and transport. We thus gain insight into material trends in the properties of $A_2 Fe B^\prime O_6$ and $A_2 Cr B^\prime O_6$ families, with $B^\prime =Mo, W$ and $A=La, Ca, Sr$. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W42.00004: Oxygen-vacancy-induced charge carrier in n-type interface of LaAlO$_{3}$ overlayer on SrTiO$_{3}$ (001): interface vs bulk doping carrier Yun Li, Sutassana Na Phattalung, Jaejun Yu We investigated the role of oxygen vacancy at the $n$-type interface of LaAlO$_{3}$ (LAO) overlayer on SrTiO$_{3}$ (STO) (001) by carrying out density-functional-theory calculations. We found that regardless of the concentration of oxygen vacancies there are always oxygen vacancies formed in LAO surface. The oxygen vacancies in the surface induce a two-dimensional carriers of no more than 0.5 electron per two-dimensional unit cell at the interface, which partially or completely compensate the polar electric field in LAO and lead to band bending at the interface in STO side. In addition, oxygen vacancies may be formed uniformly in STO for the configuration with high concentration of vacancies. Our calculations show that every oxygen vacancy in STO donates two electron carriers. We ascribed the three-dimensional distribution of higher density of carriers found in experiments to the large amount of vacancies in STO. [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W42.00005: Observation of charge redistribution during bond formation in one-dimensional alkali-metal wires Guido Fratesi, Congcong Huang, Donald A. MacLaren, Weidong Luo, William Allison, Gian Paolo Brivio We have tracked the formation of Li wires in the Li/Cu(100) system. Wire formation is driven by the increased in-plane adatom density as Li coverage increases from 0.5 to 0.6 Monolayers (ML). We used a combination of Helium Atom Scattering (HAS) and Density Functional Theory (DFT) to show the emergence of anisotropic, 1-dimensional inter-Li bonding. HAS, which is sensitive to the outermost electronic charge density, shows a transition to an unexpectedly large, 1-dimensional electronic corrugation at a coverage of 0.6ML. DFT calculations identify the corrugation as the formation of a corrugated array of Li wires. Bond formation along the Li wires is enabled by charge donation from the substrate but without significant change to the layer spacing. [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W42.00006: Confinement and dot formation in quantum wires by inhomogeneous magnetic fields Thomas Heinzel, Mihai Cerchez, Hengyi Xu, Alexey Tarasov, Stefan Hugger, Igor Zozoulenko, Dirk Reuter, Andreas Wieck Ballistic quantum wires are exposed to longitudinal profiles of perpendicular magnetic fields composed of a spike (a \emph{magnetic barrier}) and a homogeneous part. An asymmetric magnetoconductance peak as a function of the homogeneous magnetic field is found, comprising quantized conductance steps in the interval where the homogeneous magnetic field and the magnetic barrier have identical polarities, and a characteristic shoulder with several resonances in the interval of opposite polarities. The observations are interpreted in terms of inhomogeneous diamagnetic shifts of the quantum wire modes leading to magnetic confinement. Depending on the offset magnetic field, single or double quantum dots can e formed. A numerical simulation of the conductance based on recursive Green's functions reproduces all aspects of the data qualitatively. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W42.00007: Guided Wave Plasmon Polariton Modes in Trilayer Structures Matthew Oostman, Trevor Morgan, Brad Johnson, Stephen McDowall, Janelle Leger The need to interface optical signals with increasingly small electronic components has led to an interest in subwavelength waveguides. Waveguides based on the excitation of surface plasmon polaritons (SPPs), or optically coupled plasmons, at dielectric-metal boundaries have been demonstrated, but the loss of energy due to Ohmic damping effects in the metal limits propagation length. Guided wave plasmon polariton modes (GWPPMs) are theoretically predicted surface-constructed waves that can exist in structures composed of a central dielectric layer separating two metallic layers. Because the bulk of the energy is confined to the dielectric, losses due to damping may be drastically reduced. Here we report the observation of SPPs in trilayer Au/SrTiO3/Au structures, for which GWPPMs are theoretically predicted to exist for visible light. Experimental observations are in good agreement with theory. Waveguides based on GWPPMs have the potential to improve a broad range of applications such as telecommunications, optical signal processing, and solar concentration which rely on low loss energy transmission or compatibility with nanoscale components. [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W42.00008: Stability and work function of TiC$_{x}$N$_{1-x}$ alloy surfaces: Density functional theory calculations Hong Zhu, Mark Aindow, Rampi Ramprasad Critical factors that control the vacuum work function of the TiC$_{x}$N$_{1-x}$ ternary system surfaces were determined using detailed density functional theory calculations. Although the work function of the most stable surfaces does vary with the alloy composition (i.e., $x $value), surface chemistry (i.e., orientation, stoichiometry, and defect density) was found to play the most important role in determining the work function value, far surpassing the impact of alloy composition on the work function. In general, Ti-deficient surfaces display larger work functions. Work function tuning may thus be effectively accomplished by controlling the surface chemistry rather than the composition. [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W42.00009: Behavior of gradient copolymers at liquid/liquid interfaces Wa Yuan, Michelle Mok, John Torkelson, SonBinh Nguyen, Kenneth Shull The behavior of styrene/acrylic acid gradient and diblock copolymers at liquid/liquid interfaces was investigated using drop shape analysis. Copolymers were dissolved in chloroform, and pendant drops of these solutions were created in water.~ Molecular conformations at the interface were inferred by measuring changes in the interfacial tension as the interface was contracted and expanded through the control of drop volume. In our experiments, we find that gradient copolymers are much more effective interfacial modifiers than diblock copolymers with similar molecular weights and overall compositions, a result that is ascribed to the reduction of kinetic barriers associated with the micellar aggregates that form in the chloroform phase. Structural evolution of the adsorbed layer can be characterized by trajectories on an interfacial map that includes two independent parameters, interfacial pressure and the area modulus.~ The area modulus of the adsorbed layer is dependent on the processing history and not directly related to the interfacial pressure.~ This result is attributed to a local segmental desorption process where portions of the molecules reversibly desorb while the number of copolymer molecules at the interface remains fixed. [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W42.00010: Molecular Dynamics Simulations of Halomethane Adsorption on Two Dissimilar Surfaces Gary Leuty, Chiranjivi Lamsal, Mesfin Tsige In this study, atomic-scale MD simulations were used to study multilayer adsorption of three different halomethane compounds (CF$_4$, CF$_3$Cl and CF$_3$Br) onto two different surfaces (graphite and hydroxylated $\alpha$-quartz) to show how orientation, layer structure, packing and dynamics vary as a function of temperature and substrate surface characteristics. In this way, non-polar CF$_4$ on hydroxylated $\alpha$-quartz is shown to exhibit a highly ordered packing arrangement only weakly dependent on temperature variation. In contrast, interactions between polar adsorbate groups and hydroxyl groups on the surface give CF$_3$Cl and CF$_3$Br a very different layer structure at the surface. Analysis of the orientations of polar molecules in the first adsorbed layer shows a lightly ordered arrangement of dipoles, and residence time and dipole correlation calculations suggest that this ordering is subject to frequent shifts in orientation and position within the film. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W42.00011: Coupled Atomistic-Continuum Calculations of Ionizing Radiation Tracks in Insulators Harold Hjalmarson, Paul Crozier, Rudolph Magyar, Roderick Devine, Carolyn Phillips Irradiation of an insulator by energetic ionizing radiation creates hot electrons and holes. In our work, the resulting radiation damage is computed using coupled atomistic and continuum calculations. The effects of electrons and holes are incorporated into the atomistic calculations by solving for the quasi-equilibrium hot carrier density. Results for silicon dioxide will be presented and discussed in the talk. The contributions of excitonic, Coulomb explosion and thermal spike mechanisms to the damage will be discussed. The transient electrical effects will also be discussed. [Preview Abstract] |
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