Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S11: Focus Session: Surfaces of Correlated Electron Systems |
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Sponsoring Units: DMP Chair: John Hill, Brookhaven National Laboratory Room: Colorado Convention Center Korbel 1F |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S11.00001: Are the surfaces of CrO$_{2}$ metallic? A photoelectron spectroscopy study of epitaxial CrO$_{2}$(100) and CrO$_{2}$(110) films C. A. Ventrice, Jr., D. R. Borst, H. Geisler, G. X. Miao, A. Gupta Previous photoelectron spectroscopy studies of CrO$_{2}$ have found either no density of states or a very low density of states at the Fermi level, suggesting that CrO$_{2}$ is a semiconductor or a semimetal. This is in contradiction to calculations that predict that CrO$_{2}$ should be a half-metallic ferromagnet. We present photoelectron spectroscopy measurements of epitaxial CrO$_{2}$(110)/TiO$_{2}$(110) and CrO$_{2}$(100)/TiO$_{2}$(100) grown using a CrO$_{3}$ precursor. In addition, measurements of epitaxial Cr$_{2}$O$_{3}$(0001)/Pt(111) films grown by thermal evaporation of Cr in an oxygen atmosphere are presented as a reference for reduced CrO$_{2}$ films. The measurements of the CrO$_{2}$ surfaces show no emission at the Fermi level after sputtering and annealing the surfaces in oxygen, even though our soft core photoemission data and low energy electron diffraction measurements provide evidence that stoichiometric CrO$_{2}$ is present. The consequence of this is that neither surface of CrO$_{2}$ is metallic. This behaviour could result from a metal to semiconductor transition at the (110) and (100) surfaces. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S11.00002: Surface vs. Bulk Characterizations in Electronic Inhomogeneity of a VO2 thin film. Young Jun Chang, J.S. Yang, T.W. Noh, D.-W. Kim, J.-S. Chung, E. Oh, B. Kahng We have examined the validity of the percolation model for a VO2 thin film using both surface- and bulk-sensitive measurement during the metal-insulator transition (MIT). VO2 is one of the most widely investigated strongly correlated transition metal oxides, and it displays the MIT at around 340K. The metallic surface area fraction obtained by scanning tunneling spectroscopy fails to reproduce conductivity change, whereas the metallic volume fraction extracted by bulk-sensitive techniques confirms the percolative nature of the transport data. This discrepancy suggests that the surface-sensitive techniques require special care in investigating the electronic structures of strongly correlated transition metal oxides which have strong electron-phonon coupling. [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S11.00003: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S11.00004: Giant Gap Surface Charge Density Waves in NaMo$_6$O$_{17} $ and KMo$_6$O$_{17} $ Feng Wang, S.-K. Mo, J. W. Allen, G.-H. Gweon, J. Marcus, C. Schlenker, H. H\"ochst Quasi-2-dimensional molybdenum bronzes NaMo$_6$O$_{17} $ and KMo$_6$O$_{17} $ have a phase transition into the charge density wave (CDW) state at temperatures (T's) of 80K and 120K respectively. Our recent angle resolved photoemission spectroscopy (ARPES) data confirm in detail our previous findings [1] of nearly identical electronic structures and well nested Fermi surfaces of these two materials. An important new finding in both NaMo$_6$O$_{17} $ and KMo$_6$O$_{17} $ is the opening of giant energy gaps greater than 0.1 eV {\it at T's well above the bulk CDW transitions, even as high as 300K}. The spectra are very dependent on the sample surface and measurement position, and are strongly time dependent. All the evidence indicates formation of a strongly enhanced {\it surface} CDW, in contrast to the interpretation of a recent publication [2] relating these large ARPES gaps to the bulk CDW. We will discuss possible scenarios, e. g. [3], for such surface effects and the implications for measuring the spectral manifestations of the true bulk CDW's. [1] G.-H. Gweon {\it et al.}, Phys. Rev. B {\bf 55}, 13353 (1997). [2] P.-A. Glans {\it et al.}, Phys. Rev. B {\bf 72}, 035115 (2005). [3] S. E. Brown {\it et al.}, Phys. Rev. B {\bf 71}, 224512 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S11.00005: Strong electron correlation on the Fe$_3$O$_4$(0 0 1) surfaces Henry Pinto, Simon D. Elliott, Adam Foster, R. M. Nieminen Magnetite Fe$_3$O$_4$ is a fascinating material that still is not well understood and presents challenges for the state-of-the-art computational methods. This transition metal oxide undergoes a first-order metal-insulator transition at $T_V$=120 K. The ferrimagnetic properties of Fe$_3$O$_4$ makes it a promising material for spintronic applications. We use a plane wave density functional theory in the generalized gradient approximation adding a Hubbard-$U$ parameter to describe properly the strongly correlated Fe--3$d$ electrons. Based on previous results \footnote{H. Pinto, S. Elliott, J.Phys.: Condens. Matter \textbf{18}, 10427 (2006)}, we compute the surface structure, magnetic properties and electronic structure of several Fe$_3$O$_4$(0 0 1) surfaces with $(\sqrt{2}\times\sqrt{2})R45^{\circ}$ reconstruction. The simulated scanning tunneling microscopy images of these surfaces are compared and discussed in the light of available experimental data. Finally, we analyze the possible existence of charge ordering on the Fe$_3$O$_4$(0 0 1) surface and the effect on the surface electronic structure with changing the value of the Hubbard-$U$ parameter on the superficial Fe sites. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S11.00006: Electronic structure of the (001) surface of half-metallic manganites Miguel Pruneda, Valeria Ferrari, Peter B Littlewood, Emilio Artacho A good understanding of the physical properties of surfaces and interfaces of in colossal magneto-resistant hole-doped manganese oxides is highly desirable for future applications of these promising materials in magnetoresistive devices for spintronics. We present results for a fully relaxed (001) surface in the optimally doped half-metallic manganites La$_{0.7}$Sr$_{0.3}$MnO$_3$ in its distorted orthorhombic phase, based on density functional calculations with explicit doping. The crystal termination gives rise to a splitting in the degenerate $t_{2g}$ and $e_g$ levels, and a surface state with $d_{z^2}+d_{xy}$ character develops. The calculations for the relaxed structure show that the tilting of MnO$_6$ octahedra is reduced near the surface, and there is a layer buckling, with a Mn off-centering in the top octahedral layer similar to what observed for Ti in bulk BaTiO$_3$, which decreases rapidly inside the material. As a result of this distortions, the surface state shifts to lower energies, although it is still very localized in the first layer. The Mn off-centering favours the occupation of the conduction $d_{z^2}$ states, which become more localized and affects the magnetic properties at the surface. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S11.00007: A FP-LAPW Study of Atomic Chemisorption on the (100) Surface of $\delta $-Pu Raymond Atta-Fynn, Asok Ray Fully relativistic full potential density functional calculations have been performed to investigate atomic carbon, nitrogen, and oxygen chemisorption on the (100) surface of $\delta $-Pu using the all-electron augmented plane waves plus local basis code WIEN2k. The surface was modeled by a three-layer periodic slab with two atoms per surface unit cell. The center adsorption site is found to be the most preferred site with chemisorption energies of 7.964 eV, 7.665 eV, and 8.335 eV for the C, N, and O adatoms, respectively. The corresponding optimized distances of the adatoms from the surface are found to be 0.26 {\AA}, 0.35 {\AA}, and 0.48 {\AA}. The work functions and the net magnet moments respectively increased and decreased in all cases compared with the bare $\delta $-Pu (100) surface. Analysis of partial charges inside the atomic spheres, charge density distributions, and the local density of states have been performed to investigate the nature of the interaction between the surface Pu atoms and the adatoms.$^{\ast }$ $^{\ast }$R. Atta-Fynn and A. K. Ray, Physica B, in press. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:30PM |
S11.00008: Scanning tunneling spectroscopy study of charge ordering, stripes and phase separation in manganese perovskite oxides. Invited Speaker: Colossal magnetoresistance (CMR) in perovskite-based transition metal oxides keeps challenging our understanding. Constant progress in scanning tunneling microscopy investigations is enabling increasingly detailed experimental insight into the different electronic and structural phases nucleating in these complex materials. I shall review the latest findings emerging from experiments on perovskite- and bilayer-manganites [1], which in particular, reveal the importance of lattice degrees of freedom (polarons) and their contribution to the macroscopic transport properties. I shall also discuss the observation of an unexpected hexagonal phase, glimpses of a stripe phase and, finally, address the question of phase separation in these systems. \newline \newline [1] Ch.Renner and H.M.R{\o}nnow, ``Scanning tunneling microscopy and spectroscopy of manganites'' in Scanning Probe Microscopy: Electrical and Electromechanical Phenomena at the Nanoscale, Springer Series, (2006). [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S11.00009: Scanning Tunneling Spectroscopy Investigation of La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7 }$(x=0.32, 0.4) at Low Temperatures Junwei Huang, Jeehoon Kim, Alex de Lozanne, J.-S. Zhou, J. B. Goodenough We have investigated the surface electronic properties of the ferromagnetic Ruddlesden-Popper compounds La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (x=0.32, 0.4) with our home-made ultra-high vacuum (UHV) and low-temperature scanning tunneling microscope at 20K and 80K. We found that the atomically flat surfaces of both compounds show nano-sized features that may be related to the formation of Mn(4+)-rich and Mn(3+)-rich clusters. The tunneling spectra on those clusters reveal that the local density of states (LDOS) of the Mn(4+)-rich clusters is quite different from those of the Mn(3+)-rich clusters. We also observed a gap that is temperature dependent and spatially inhomogeneous. These unique electronic properties may be explained by the collective polaron model, where the polaron clusters condense below the ferromagnetic transition temperature. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S11.00010: Spectroscopic Imaging Scanning Tunneling Microscopy Studies of Ruthenates M. Wang, Jinho Lee, A. Schmidt, Y. Kohsaka, S.A. Grigera, R.S. Perry, A.P. Mackenzie, J.C. Davis We report atomic resolution spectroscopic imaging studies of Ruthenates, including Sr$_{3}$Ru$_{2}$O$_{7}$ and Ca$_{3}$Ru$_{2}$O$_{7}$. We will discuss the comparison between ARPES and SI-STM studies of these materials. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S11.00011: STM studies of electronic inhomogenieties in $La_{0.35}Pr_{0.275}Ca_{0.375}MnO_{3}$ thin films A. K. Gupta, Udai Raj Singh, S. Chowdhury, R. C. Budhani We report on temperature dependent STM/S measurements of Laser ablated epitaxial thin films ($\simeq$400nm thick) of $La_{0.35}Pr_{0.275}Ca_{0.375}MnO_{3}$ (LPCMO) on $NdGaO_3$ (NGO) substrate. Four-probe resistivity measurement on this film shows a sharp transition near 145K (T$_{MI}$) and a significant hysteresis with temperature between 90K and 160K. The topographic STM images show a clean terraced surface at all temperatures (77-350K) with mono-atomic steps and terrace width of 300-400nm. Some inhomogenieties are observed in the conductance images on a length scale of $\simeq$20nm and more apparent near the terrace steps. Although spatial variations in spectra are seen at all temperatures but with some common features that evolve with temperature. The spectra become gap-like below 210K (i.e. T$_{CO}$) and with cooling this charge-ordering (CO) gap becomes more pronounced with a magnitude of 0.4-0.5eV. This shows an increase in the strength of the CO order parameter and makes us believe that with cooling the CO fraction in the phase-separation scenario is not decreasing with temperature. In this case the large resistivity change at T$_{MI}$ may have to invoke a third phase (other than metallic and CO phases) that becomes metallic with cooling rather than CO fraction melting into metallic phase. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S11.00012: Evidence of Coulomb blockade behavior in finite, one-dimensional quantum well Vincent Meunier, Minghu Pan, Frederic Moreau, Kenneth Park, Ward Plummer We report on a new type of ``quantum box'' that is grown on an insulating TiO$_{2}$(110) surface and is effectively closed by the presence of two charged structures at both ends. The static end charges are responsible for a long-range potential that governs the behavior of the electrons inside the box. As expected from a system with quantum confinement, we observe oscillatory features that can be attributed to standing waves inside the system. The spatial distribution of the charge density fits remarkably well with the solution of the Schrodinger equation, provided that correlation effects are included. However, the astounding result is that they are all observed at room temperature and furthermore unchanged within the range of STM tip potential (about 0.5 V). Because the substrate is insulating and the electrons are well confined inside the structure, we can use the capacitor approach to evaluate the corresponding charging energy. Our theoretical analysis indicates that the energy needed to put an extra electron into the confining structure of 14-16 nm long amounts to about 1.14-1.30 eV in agreement with a simple classical picture of capacitor charging. [Preview Abstract] |
Wednesday, March 7, 2007 5:18PM - 5:30PM |
S11.00013: Surface segregation in La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$(001). V.B. Nascimento, R.G. Moore, H. Liu, M.H. Pan, E.W. Plummer, J. Rundgren, D. Mazur, J.W. Freeland, J.F. Mitchell The (001) clean surface of La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ with nominal dopings of 0.3 $<$ x $<$ 0.4 presents a non-magnetic surface layer tentatively explained by the existence of a surface reconstruction. A surface structural investigation of this system was performed using quantitative Low Energy Electron Diffraction (LEED-IV). Results obtained discard the occurrence of a surface reconstruction. However, a strong Sr segregation is observed in these surfaces leading to an effective doping on the surface of x = 0.8 as well as a possible anti-ferromagnetic phase (in analogy with bulk). Sr segregation is believed to be induced by the creation of a surface which promotes a shift in the phase diagram. [Preview Abstract] |
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