Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G20: Focus Session: Complex Oxide Thin Films Surfaces and Interfaces II: Surfaces and Theory |
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Sponsoring Units: DMP GMAG Chair: Maria Varela, Oak Ridge National Laboratory Room: Baltimore Convention Center 317 |
Tuesday, March 14, 2006 8:00AM - 8:12AM |
G20.00001: Loss of Ferromagnetism at the Surface of in-situ cleaved La$_{1.28}$Sr$_{1.72}$Mn$_2$O$_7$ John W. Freeland, Ken Gray, John F. Mitchell, Jerald Kavich, Richard Kodama Instability of surface magnetism in the manganites is currently an area not well understood. In our previous work using exploring loss of ferromagnetic (FM) order at the surfaces of layered manganites, we found clear evidence for an intrinsic insulating non-FM surface layer atop adjacent ferromagnetic subsurface layers.[1] The presence of a non-FM surface layer of one bilayer thickness was observed by x-ray resonant magnetic scattering (XRMS), and point contact tunneling results showed this layer to be insulating. Although that study involved samples cleaved in air, our surface-sensitive spectroscopy indicated they were not chemically degraded. Here we present the XRMS results from samples cleaved under UHV conditions at 75 K. The XRMS shows the same interference effect due to the loss of FM order in the surface layer, thus supporting our earlier claim that the magnetic instability is not related to the preparation conditions. This research was supported by the U.S. Department of Energy, Office of Science, under Contract No. W-31-109-Eng-38. \newline [1] J.W. Freeland, K.E. Gray, L. Ozyuzer, P. Berghuis, Elvira Badica, J. Kavich, H. Zheng and J.F. Mitchell, Nature Materials {\textbf 4}, 62 (2005) [Preview Abstract] |
Tuesday, March 14, 2006 8:12AM - 8:24AM |
G20.00002: Surface Structural Phases of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ R.G. Moore, V.B. Nascimento, Jiandi Zhang, Ismail NLS, R. Jin, D. Mandrus, E.W. Plummer Surface structural phases of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ (CSRO) are investigated using Low Energy Electron Diffraction (LEED-IV). The surface structure and phases are compared to the bulk phases in this layered material. Normally at a crystal surface the reduced atomic coordination enhances electron-electron correlations, if there is not a reconstruction, and stabilizes the Mott-insulating phase resulting in higher MIT temperatures than in the bulk. Surprisingly, Ca$_{1.9}$Sr$_{0.1}$RuO$_{4}$ exhibits an unusual surface MIT 20K lower than the bulk. The properties of CSRO are extremely sensitive to the RuO$_{6}$ octahedral orientation. LEED-IV shows no symmetry change associated with the MIT, but small distortions of the RuO$_{6}$ octahedral that drive the surface MIT. In addition, the tetragonal-to-orthorhombic surface phase transition near the Quantum Critical Point at x=0.5 is investigated and contrasted with the bulk phase transition. [Preview Abstract] |
Tuesday, March 14, 2006 8:24AM - 8:36AM |
G20.00003: LEED study of TMOs surfaces V.B. Nascimento, R.G. Moore, J. Rundgren, Jiandi Zhang , Lei Cai, R. Jin, D. G. Mandrus, E.W. Plummer In contrast with the case of metals and semiconductors, the final theory-experiment agreement obtained for LEED $I-V$ data for TMOs is generally not very satisfactory. One reason is that the calculations of the phase shifts for metal oxides is not a straightforward procedure due to the charge transfer. In this work we address this issue of phase shift generation for TMOs by determining the structure of the (001) surface of Ca$_{1.5}$Sr$_{0.5}$RuO$_{4}$, a prototype layered TMS system. The adoption of an optimized muffin-tin potential approach, as proposed by Rundgren, has enabled reaching an acceptable final theory-experiment agreement. Our results indicate that the determination of an energy dependent inner potential plays a key role in the final agreement. *Work supported jointly by NSF and DOE (DMS) NSF -DMR-0451163, NSF DMR-0346826 and by U.S. DOE under DE-FG02-04ER46125 and DE-AC05-00OR22725. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G20.00004: Orbital ordering at the surface of the layered manganite La$_{0.5}$Sr$_{1.5}$MnO$_{4}$ Y. Wakabayashi, S. Grenier, M.H. Upton, J.P. Hill, C.S. Nelson, J.F. Mitchell, J-W. Kim, P.J. Ryan, A.I. Goldman Surface x-ray diffraction measurements were made on a (001) cleaved surface of the orbitally ordered layered manganite, La$_{0.5}$Sr$_{1.5}$MnO$_{4}$ at X22C and X21 at the NSLS and at 6ID at the APS. The termination of crystal by a surface gives rise to rods of scattered intensity normal to that surface; so-called crystal truncation rods. The intensity distribution along such rods is sensitive to details of the surface order. In principle, similar rods should arise from the surface termination of orbital order. We have successfully observed such orbital truncation rods for the first time. We find that the intensity distribution of the orbital rod has a slightly steeper $L$ dependence than that for the allowed Bragg rods, indicating that the surface roughness of the orbital ordering is larger than the crystallographic roughness. As the transition temperature is approached, this orbital roughness is seen to increase. [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G20.00005: Surface Modification of CMR Perovskite Manganite Thin Films using Atomic Force Microscopy David Schaefer, Rajeswari Kolagani, Cameron Bolling, John Sunderland, Anthony Davidson III, Tyler Bradley, Bonnie Ludka Nanolithography using the atomic force microscope (AFM) is emerging as a promising tool for nanotechnology . We report our results of AFM- induced nanoscale surface modifications in thin films of the CMR manganite material La$_{0.7}$Ba$_{0.3}$MnO$_{3}$. CMR manganite materials have been demonstrated to be useful for a variety of technological applications including magnetic sensors and bolometric infrared detectors. AFM induced surface modifications would enable the realization of such sensors in nanotechnology applications. We have obtained reproducible patterns on these films in the form of nanoscale dots and lines induced by the AFM tip. We have studied these feature dimensions as a function of tip bias voltage, exposure time and humidity. We find that the feature heights are considerably larger on manganite thin film surfaces than those induced on silicon surface under similar conditions. Additionally, the speed of reproducible writing is also found to be significantly higher on the manganite films. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G20.00006: Time-dependent Surface Electronic Structure of EuB$_6$ J.D. Denlinger, F. Wang, J.W. Allen, Han-Oh Lee, Z. Fisk, B. Delley, R. Monnier, J. Lobo-Checa, T. Okuda, J. Osterwalder The surface electronic structure of UHV-cleaved divalent hexaboride EuB$_6$ is investigated using angle-resolved photoemission and the time-dependence of the Eu 4d-character X-point electron pocket, B-p band structure and Eu 4f states are characterized. Surface-slab LDA calculations allow identification in the data of a distinct surface-related band residing in the bulk-projected band gap along X-M and energy-shifted surface-atom Eu 4f states resulting from an electric dipole at the highly ionic surface. The X-point electron pocket size is observed to initially increase in size and then recede to zero occupation. Similarly the Eu 4f surface component initially increases in binding energy and then reverses direction and loses intensity. This behavior is explained in terms clustering of mobile surface Eu atoms on the freshly cleaved surface, followed by adsorption of residual gases. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G20.00007: Formation and stability of surface oxides and oxide surfaces of the O/Cu system: First-principles investigations Aloysius Soon, Mira Todorova, Bernard Delley, Catherine Stampfl Copper-based catalysts are important for several industrial reactions, e.g., the low-temperature water-gas-shift reaction and for methanol oxidation reactions. Despite this, very little is presently known about the surface structure, about the atomic and molecular processes involved and the associated reaction pathways. As a first step towards a microscopic understanding, we use density-functional theory to investigate chemisorption of oxygen on Cu(111), and the stability of surface oxides and oxide surfaces. Surface oxide structures are found to be energetically favoured over chemisorbed~ oxygen even at coverages as low as $\frac{1}{16}$\,ML. Taking into account the pressure and temperature through the framework of \textit{ab initio} thermodynamics [1,2] however, shows that for the conditions relevant to technical catalysis, bulk oxide structures are the appropriate ones to consider. Our results are compared to the behavior of other O/transition-metal systems. \\ $[1]$ K. Reuter, C. Stampfl and M. Scheffler, in Handbook of Materials Modeling, Volume 1, Fundamental Models and Methods, Sidney Yip (Ed). \\ $[2]$ C. Stampfl, Catal. Today 105, 17 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G20.00008: Thermal transport in strongly correlated multilayered nanostructures James Freericks, Veljko Zlatic The formalism for thermal transport in strongly correlated multilayered nanostructures is developed. We employ inhomogeneous dynamical mean-field theory and the Kubo formula to derive relevant thermal transport coefficients, which take the form of matrices with respect to the planar indices. We show how to define the local versions of the current and heat current operators so that heat-current correlation functions can be easily evaluated via the Jonson-Mahan theorem. Thermal transport in nanostructures is complicated by the fact that the thermal current need not be conserved through the device, and a given experimental set-up determines both how the thermal current can change through the device and how the steady-state temperature profile can be determined. Formulae to analyze classic experiments such as the Peltier and Seebeck effects, the thermal conductivity, and for running a thermoelectric cooler or power generator are also discussed. [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G20.00009: Ab Initio Study of Multiferroicity in La(Al,Fe,Cr)O$_3$ Alison Hatt, Nicola Spaldin We present the results of \em ab initio \rm density functional calculations of perovskite-structure La(Al,Fe,Cr)O$_3$. Our calculations reveal two structurally distorted ground states of opposite polarization. Motivated by the growth of three-layer superlattices with enhanced polarization, we investigate the ferroelectricity and magnetic ordering of the La(Al,Fe,Cr)O$_3$ system with the goal of finding emergent multiferroicity due to interfacial strain and inversion symmetry breaking. Finally, we investigate constrained tetragonal LaAlO$_3$ to determine its role in the ferroelectric properties of the supercell. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G20.00010: Density-Functional Study of the Two-Dimensional Electron Gas at the Perovskite Titanate Interface Ranjit Nanda, Zoran Popovic, Sunita Thulasi, Sashi Satpathy Oxide superlattices and microstructures hold the promise for creating a new class of devices with unprecedented functionalities. Density-functional studies$^{1}$ of the recently fabricated, lattice-matched perovskite titanates$^{2}$ (SrTiO$_{3})_{n}$/(LaTiO$_{3})_{m}$ reveal a classic wedge-shaped potential well for the monolayer structure, originating from the Coulomb potential of a charged La sheet. The potential in turn confines the electrons in the Airy-function-localized states. This resulting two-dimensional electron gas may be described in terms of the simplified jellium model$^{3}$ and it describes reasonably well the observed charge modulation of the Ti atoms near the interface. Concerning magnetism, it is suppressed for the monolayer LaTiO$_{3}$ structure, while in structures with a thicker LaTiO$_{3}$ part, bulk antiferromagnetism is recovered, with a narrow transition region separating the magnetic LaTiO$_{3}$ and the non-magnetic SrTiO$_{3}$. 1. Z. S. Popovic and S. Satpathy, Phys. Rev. Lett. \underline {94}, 176805 (2005) 2. A. Ohtomo et al., Nature \underline {419}, 378 (2002) 3. S. Thulasi and S. Satpathy, Phys. Rev. B (2006) [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G20.00011: Lattice relaxation, electronic screening, and spin and orbital phase diagram of LaTiO$_3$/SrTiO$_3$ superlattices Satoshi Okamoto, Andrew Millis, Nicola Spaldin The effect of lattice relaxation on the electronic charge density, band structure, and phase diagram in oxide heterostructure, LaTiO$_3$/SrTiO$_3$ superlattices, the structure grown and measured by Ohtomo {\it et al}.[1], is investigated using density functional theory LDA and LDA+$U$ (VASP implementation). We observe substantial ferroelectric-like distortion of TiO$_6$ octahedra at the nearest-to-La region which screens the electrostatic force originating from La$^{3+}$ ion reducing the charge density in the La-rich region. We determine the changes in the low energy electronic Hamiltonian caused by the lattice relaxation and investigate the consequences for the ground state phase diagram using the Hartree-Fock approximation. Finally, we present a detailed comparison with previous calculations which neglected the lattice effect. [2] Support from DOE ER 46169. [1]Ohtomo et al., Nature \textbf{419}, 378 (2002). [2]Okamoto and Millis, Nature (London) {\bf 428}, 630 (2004), and Phys. Rev. B {\bf 70}, 195120 (2004). [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G20.00012: Total energy calculations for magnetic La$_{2/3}$S$_{1/3}$rMnO$_{3}$/SrRuO$_{3}$ bilayers Yongbin Lee, Benjamin Caes, Bruce Harmon . Recently positive exchange bias was reported for ferromagnetic La$_{2/3}$Sr$_{1/3}$MnO$_{3}$/SrRuO$_{3}$ (LSMO/SRO) bilayers which indicated anti-ferromagnetic exchange interaction across the interface [1]. To address the physical origin of this anti-ferromagnetic exchange interaction, we have performed first principles calculations with both ferro, and anti-ferromagnetic interface structures. We have found that the anti-ferromagnetic interface is preferred to ferromagnetic case energetically. The calculations indicate the interfacial O 2p orbitals and their interaction with Mn, Ru spin polarized 3d orbitals are critical for this anti-ferromagnetic interface preference. In this talk, we will discuss the total energy calculations with various magnetic structures near the interface. [1] X. Ke \textit{et al}. Appl. Phys. Lett. \textbf{84}, 5458 (2004) [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G20.00013: Electronic and magnetic properties of Mn/Si superlattices C.Y. Fong, Mei Qian, Kai Liu We recently predicted a monolayer of Mn doped in 31 layers of Si, a Mn/Si digital ferromagnetic heterostructure, is a half metal.[1] In order to explore the possibility of realizing large magnetoresistance effect, we extend our studies to superlattices composed of periodically embedded monolayers of Mn in Si. By using a first-principles algorithm, we determine whether a superlattice can exhibit half metallic behavior. We then investigate the dependence of the half metallic properties on the thickness of the Si spacer. The energetics of the ferromagnetic and antiferromagnetic phases will be compared for each half metal. For the minimum Si spacer, the band structure near the Fermi energy of the superlattice showing half metallic properties will be given. We also analyze the character of the states at the Fermi energy as well as the associated Fermi surface. \newline \newline [1]M. C. Qian, et al. to appear in Phys. Rev. Lett.; cond- mat/0508269. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G20.00014: Electrostatic Interface Tuning in Correlated Superconducting Heterostructures Natalia Pavlenko, Thilo Kopp In high-temperature superconducting oxide films, transverse electric fields have been used to switch between superconducting and insulating states by electrostatically tuning the free carrier density. We analyze d-wave pairing within a microscopic model for heterogeneous multilayer systems and show that a coupling to electric dipoles and phonons at the interface of cuprate film and dielectric gate localizes the injected charge and leads to a superconductor-insulator transition [1]. We find that in the correlated oxide films, the strong interface polaron effect is inherently related to the stabilization of interface charge ordering and inhomogeneous states. The formation of such states results in a dramatic modification of the doping dependent phase diagram close to and above the optimal doping which is expected to shed light on recent electric field-effect experiments with HTSC cuprates.\\ \noindent [1] N. Pavlenko and T. Kopp, Phys.Rev. B, in print (2005). [Preview Abstract] |
Tuesday, March 14, 2006 10:48AM - 11:00AM |
G20.00015: Quartet formation at (100)/(110) interfaces of d-wave superconductors Arno Kampf, Armando Aligia, Jochen Mannhart Across a faceted (100)/(110) interface between two $d_{x^2-y^2}$-superconductors the structure of the superconducting order parameter leads to an alternating sign of the local Josephson coupling. Describing the Cooper pair motion along and across the interface by a one-dimensional boson lattice model, we show that a small attractive interaction between the bosons strongly enhances their binding at the interface. As a consequence we propose that electrons tunnel in quartets across an interface with a staggered sequence of 0- and $\pi$-junction contacts. We connect this finding to the recently observed $h/4e$ oscillations in bicrystalline (100)/(110) SQUIDs of cuprate superconductors. [Preview Abstract] |
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