Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V42: Surfaces: Phase Transitions, Magnetic and Superconducting Properties |
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Sponsoring Units: DMP Chair: Venkatraman Gopalan, Pennsylvania State University Room: D138 |
Thursday, March 18, 2010 8:00AM - 8:12AM |
V42.00001: First-Principles Cluster Expansions for Predicting Missing-row Surface Reconstructions Wei Chen, Chris Wolverton, David Schmidt, William Schneider It is well-known that the (110) surfaces of late 5d transition metals (e.g. Au, Pt) undergo a (1$\times $2) missing-row reconstruction, while clean surfaces of 3d and 4d metals (e.g. Cu, Ag) do not. Here, we report the use of a cluster expansion (CE) approach to study this particular class of ``missing-row'' surface reconstructions of some transition metals.~We have used first-principles density functional calculations along with a CE to study the (110) surface of late transition metals by considering the reconstructed surface as a 2D binary system of metal atoms and vacancies. Without any experimental input or intuition-based ``guessing'', the CE results demonstrate the (1$\times $2) missing-row structure is the T=0K ground state for the (110) surface of Au and Pt, but not for Cu and Ag. The finite temperature properties of the missing-row surfaces were also studied by a CE+Monte Carlo approach, and we find a (1x2) to (1x1) transition temperature in good agreement with experimental results. [Preview Abstract] |
Thursday, March 18, 2010 8:12AM - 8:24AM |
V42.00002: Temperature Dependent Phases of Sr on Si (001): First principles theory and experiment Kevin F. Garrity, Myrtle-Rose Padmore, Yaron Segal, James W. Reiner, Fred J. Walker, Charles H. Ahn, Sohrab Ismail-Beigi Understanding and predicting temperature dependent surface phases from first principles can be invaluable in developing and optimizing processes for the growth of epitaxial structures. In particular, an ordered sub-monolayer phase of Sr on Si (001) plays a key role in determining the quality of the interface in the most commonly used method for growing epitaxial oxides on Si. Using density functional theory, we build a first principles model of sub-monolayer Sr on Si, which we use to determine the temperature dependent phase equilibrium between an ordered 1/6 ML structure and a disordered lattice gas. In addition, we experimentally determine this phase diagram using RHEED and find quantitative agreement between theoretical predictions and experiment. [Preview Abstract] |
Thursday, March 18, 2010 8:24AM - 8:36AM |
V42.00003: Interactions of oxygen and hydrogen on Pd(111) surface Denis Demchenko, Sacha Gomez, Miquel Salmeron, Lin-Wang Wang The coadsorption and interactions of oxygen and hydrogen on Pd(111) was studied by scanning tunneling microscopy and density functional theory calculations. In the absence of hydrogen oxygen forms a (2$\times$2) ordered structure. Coadsorption of hydrogen leads to a structural transformation from the (2$\times$2) to a ($\sqrt 3\times\sqrt 3$)R30$^\circ$ structure. In addition, hydrogen enhances the mobility of oxygen. To explain these observations, the interaction of oxygen and hydrogen on Pd(111) was studied within the density functional theory. In agreement with the experiment the calculations find a total energy minimum for the oxygen (2$\times$2) structure. The interaction between H and O atoms was found to be repulsive and short ranged, leading to the compression of the O islands from (2$\times$2) to ($\sqrt3\times\sqrt3$)R30$^\circ$ ordered structure at high H coverage. The computed energy barriers for the oxygen diffusion were found to be reduced due to the coadsorption of hydrogen, in agreement with the experimentally observed enhancement of oxygen mobility. The calculations also support the finding that at low temperatures the water formation reaction does not occur on Pd(111). [Preview Abstract] |
Thursday, March 18, 2010 8:36AM - 8:48AM |
V42.00004: Metastability of Bi Films Mediated by Surface Kinetics Guang Bian, Tom Miller, Tai-Chang Chiang We report on a structural phase transformation of bismuth films deposited on Si(111)-(7X7). Films with thicknesses 20 to $\sim $100 angstroms, upon annealing, first order into a metastable pseudocubic phase and then transform into a stable rhombohedral phase. The two phases have very different electronic band structures as measured by angle-resolved photoemission spectroscopy. The formation of the metastable phase over a wide thickness range can be attributed to a surface nucleation mechanism. [Preview Abstract] |
Thursday, March 18, 2010 8:48AM - 9:00AM |
V42.00005: Investigation of Cooling-Induced Phase Transitions of the Surface Structure of w-GaN(000-1) and (0001) Tianjiao Chen, Kangkang Wang, Yinghao Liu, Kendal Clark, Danda Acharya, Muhammad Haider, Arthur Smith, Saw-Wai Hla It is well-known that the surface structure of wurtzite GaN at room temperature consists of multiple reconstructions which depend on both the surface Ga coverage as well as the wurtzite polarity. In this study we investigate the phase transitions of the surface as it is cooled from room temperature down to as low as 190 K. The experiment consists of growth of the GaN using radio frequency plasma molecular beam epitaxy and monitoring the surface using reflection high energy electron diffraction. For GaN layers grown on sapphire (0001), both 5/12 and 7/12 streaks were observed after cooling the as-grown sample down to 255 K. In order to make a connection between a specific room-temperature reconstruction and these observed low-temperature RHEED patterns, we are carrying out a systematic study by preparing both N-polar and Ga-polar surfaces having specific amounts of Ga coverage as the starting points. Initial results show differences depending on the surface treatment. An additional goal of these studies is to explain a set of novel reconstructions seen in LT-STM images at 77K and 4.6 K. Authors acknowledge support by NSF (grant Nos. 0304314 and 0730257). [Preview Abstract] |
Thursday, March 18, 2010 9:00AM - 9:12AM |
V42.00006: Ferroelectricity in Strain-Free SrTiO$_{3}$ Thin Films Amit Kumar, Sava Denev, Ho Won Jang, Chad Folkman, S.H. Baek, Nina Balke, Peter Maksymovych, Mike Biegalski, Sergei Kalinin, Darrell Schlom, C.T. Nelson, X.Q. Pan, L.Q. Chen, Chang Beom Eom, Venkat Gopalan Biaxial strain is known to induce ferroelectricity in thin films of nominally non-ferroelectric materials such as SrTiO$_{3}$. However, we show that even strain-free SrTiO$_{3}$ films and the paraelectric phase of strained films exhibit bulk frequency-dependent polarization hysteresis loops on the nanoscale at room temperature, and stable switchable domains at 50 K. By a direct comparison of the strained and strain-free SrTiO$_{3}$ films using dielectric, ferroelectric, nonlinear optical and nanoscale piezoelectric property measurements, we conclude that both strain states of SrTiO$_{3}$ films are bulk relaxor ferroelectrics, and the role of strain is to stabilize longer-range correlation of preexisting nanopolar regions. This mechanism could have broader applicability to strain-induced ferroelectricity in other incipient ferroelectric materials. [Preview Abstract] |
Thursday, March 18, 2010 9:12AM - 9:24AM |
V42.00007: Non-linear optical probing of strain-enabled ferroelectricity in CaTiO$_{3}$ thin films Eftihia Vlahos, Charles Brooks, Carl Johan Ecklund, Mike Biegalski, Karin Rabe, Darrell Schlom, Venkatraman Gopalan First principles calculations predict CaTiO$_{3}$, under tensile strain, to become ferroelectric with a spontaneous polarization of up to 0.5 C/m$^{2}$.~ Comparative second harmonic generation (SHG) studies of a series of strained CaTiO$_{3}$ thin films were undertaken in order to determine their transition temperature and point group symmetry. The epitaxial strain ranged from -1.7{\%} to 3.3{\%}. Symmetry analysis of the SHG polar plots confirms that for the samples under tensile strain, the polarization is along the $<$110$>_{p }$directions and the point group of the ferroelectric phase is \textit{mm2}. SHG ``hysteresis'' loops were also obtained; these show clear switching. The experimental results are in excellent agreement with the first principles calculations predictions, and low temperature dielectric measurements that were performed on the same samples. [Preview Abstract] |
Thursday, March 18, 2010 9:24AM - 9:36AM |
V42.00008: Phase Transitions and Domain Structure in Mixed Tetragonal-Rhombohedral BiFeO$_{3}$ thin films using Raman Spectroscopy and Nonlinear Optics E. Vlahos, A. Kumar, S. Denev, A. Melville, C. Adamo, J.F. Ihlefeld, G. Sheng, R.J. Zeches, J.X. Zhang, Q. He, C.H. Yang, R. Erni, M.D. Rossell, A.J. Hatt, Y.-H. Chu, C.H. Wang, C. Ederer, V. Gopalan, L.Q. Chen, D.G. Schlom, N.A. Spaldin, L.W. Martin, R. Ramesh, Dmitri Tenne We have shown that biaxially strained BiFeO$_{3}$ thin films can undergo an isosymmetric phase transition from a rhombohedral-like to a tetragonal-like phase. This talk discusses the evolution of the tetragonal and the mixed phases in BiFeO$_{3}$/YAlO$_{3}$ thin films with varying film thickness using optical second harmonic generation (SHG) and Raman spectroscopy. 25nm, 75nm, and 225 nm thick films were studied; thinner films are dominated by the tetragonal phase, whereas thicker films exhibit both tetragonal and rhombohedral phases. The evolution of these phases as function of film thickness and temperature was experimentally determined. [Preview Abstract] |
Thursday, March 18, 2010 9:36AM - 9:48AM |
V42.00009: Structural defect properties of Co-Pt-type magnetic-storage alloys from an optimized basis-set representation Aftab Alam, Duane Johnson Within a rapidly computed site-centered basis-set determined from the local saddle-points in the overlapping atomic charge densities, we shall describe an optimal respresentation which separate the ``spherical'' density and potential around an atom from the symmetry-induced, ``non-spherical'' interstitial part. These saddle-point radii (SPR) define the inscribed ``muffin-tin'' radii that determine uniquely the convex Voronoi polyhedra (VP) surrounding a site, thus more properly reflect the charge transfer, atomic size effects and hence the Energetics. We implement the idea of this SPR-basis within a first principles calculation to investigate the effect of thermal processing in several CoPt-type magnetic storage alloys. We predict accurately the stability of chemically and magnetically ordered and disordered phases, structural defects energies, as well as thermodynamic transformation temperatures in agreement with those of full-potential calculations and experiments. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:00AM |
V42.00010: Spin polarized scanning tunneling microscopy of an antiferromagnetic surface J\'er\^ome Lagoute, Cyril Chacon, Vincent Repain, Yann Girard, Sylvie Rousset Spin polarized scanning tunneling microscopy is a powerfull tool for the invetigation of the local magnetism of surfaces and model systems. The Cr(001) exhibits large terraces which are antiferromagnetically coupled. Magnetic contrast can be achieved on this surface even with a non negligeable surface contamination as we will show. The spectroscopy of the Cr(001) at the atomic scale show specific signatures which will be discussed in detail. In order to get deeper insight in these features we deposited Cr on Cr(001) and studied its spin polarized spectroscopy. This study is indeed necessary in order to understand the electronic and magnetic structure of model systems for nano-spintronics by adsorbing single objects on Cr(001). [Preview Abstract] |
Thursday, March 18, 2010 10:00AM - 10:12AM |
V42.00011: Interface magnetic and orbital reconstruction in an oxide multiferroic BiFeO$_{3}$/ferromagnet La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ heterostructure Pu Yu, Jun-Sik Lee, Satoshi Okamoto, Marta Rossell, Mark Huijben, Chan-Ho Yang, Qing He, Jin-Xing Zhang, Seung-Yeul Yang, Michelle Lee, Quentin Ramasse, Rolf Erni, Ying-Hao Chu, Dario Arena, Chi-Chang Kao, Lane Martin, Ramamoorthy Ramesh Heteroepitaxial complex oxide heterostructures have attracted much scientific attention because of the novel phenomena and functionalities that have been observed to arise at interfaces as a consequence of the interplay between the spin, charge and orbital degrees of freedom. Here we are exploring a model heterointerface in which all the degrees of freedom, namely spin, charge, lattice, and orbital, are operative. The multiferroic perovskite, BiFeO$_{3}$ (BFO) is chosen as the model system that embodies the interplay between these degrees of freedom. This is counterposed across the interface with a highly correlated magnetic oxide, namely the doped manganite La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO). We have discovered the emergence of a novel magnetic state (1.4 $\mu _{B}$/Fe in the interface compared with only $\sim $0.02 $\mu _{B}$/Fe in bulk) in the BFO interface$_{ }$layer. This surprisingly large magnetization is associated with the emergence of spin and electronic orbital reconstruction at the heterointerface. [Preview Abstract] |
Thursday, March 18, 2010 10:12AM - 10:24AM |
V42.00012: SANS study of the vortex lattice PrOs$_{4}$Sb$_{12}$ Andrea Bianchi, Alexandre Desilets-Benoit, Simon Gerber, Nikola Egetenmeyer, Jorge Gavilano, Michel Kenzelmann, Sebastian M\"uhlbauer, Christian Pfleiderer, Ryan Baumbach, Brian Maple We report on the magnetic field ($H$) dependence of the form factor $|F|^2$ of the vortex lattice (VL) in PrOs$_{4}$Sb$_{12}$ obtained by small angle neutron scattering for $H$ applied along the crystallographic $a$-axis. PrOs$_{4}$Sb$_{12}$ is the fist Pr based heavy fermion superconductor (SC) with a critical temperature $T_c$ of 1.85 K and a $H_{c2}$ of 2.5 T Although PrOs$_{4}$Sb$_{12}$ crystallizes in a filled Skutterudite structure with a cubic lattice, this structure has a tetrahedral point group symmetry. In consequence with the application of a magnetic field along the $c$-direction, the $a$ and $b$-axis are no longer equivalent. Measurements of the angular dependence of the thermal conductivity in PrOs$_{4}$Sb$_{12}$ suggest a change of the superconducting order parameter deep inside the SC phase diagram. The thermal conductivity a low fields is compatible with a SC gap structure with a two fold symmetry, while at higher field it changes to a fourfold symmetry. [Preview Abstract] |
Thursday, March 18, 2010 10:24AM - 10:36AM |
V42.00013: Superconductivity in one-atomic-layer metal films Tong Zhang, Peng Cheng, Wen-Juan Li, Yu-Jie Sun, Guang Wang, Xie-Gang Zhu, Ke He, Lili Wang, Xucun Ma, Xi Chen, Yayu Wang, Ying Liu, Hai-Qing Lin, Jin-Feng Jia, Qi-Kun Xue Two-dimensional (2D) superconducting state is a fragile state of matter susceptible to quantum phase fluctuations. It is still not known whether a single layer of ordered metal atoms, which represents the ultimate 2D limit of a crystalline film, could be superconducting. Here we report scanning tunneling microscopy measurements on single atomic-layer films of Pb and In epitaxially grown on Si(111), and demonstrate that superconductivity does exist at such 2D extreme. The superconducting transition temperature (TC) is found to be 1.83K for Pb film and 3.18K for In film. Occurrence of vortex lattice under magnetic field confirms that they are type II superconductor. To understand the mechanism of superconductivity transition, we performed high-resolution ARPES measurement. The result reveals that the interplay between the Pb-Pb (In-In) metallic and the Pb-Si (In-Si) covalence bondings may play a critical role. [Preview Abstract] |
Thursday, March 18, 2010 10:36AM - 10:48AM |
V42.00014: Possible proximity effect in metal/Mott insulator bilayers Ko Munakata, Theodore Geballe, Malcolm Beasley We have synthesized thin Cu/CuO bilayers and studied their transport properties compared with Cu/MgO bilayers as a means of exploring a possible proximity effect between a metal and a correlated (Mott/charge transfer) insulator. Systematic comparisons of the transport properties showed in several different samples that the Cu/CuO samples have distinct features in their magnetoresistance that are not seen in the Cu/MgO bilayers. The data can be analyzed in the framework of the theory of weak localization/antilocalization. The results will also be discussed in light of recent theoretical studies of the proximity effect on the metal side of a metal/Mott insulator interface associated with antiferromagnetism of the Mott insulator [1].\\[4pt] [1]A. Sherman, N. Voropajeva, arXiv: 0904.4314. [Preview Abstract] |
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