Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J53: Structure and Properties of Oxide Surfaces |
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Sponsoring Units: DCMP Chair: Daniel Dougherty, North Carolina State University Room: Mile High Ballroom 2C |
Tuesday, March 4, 2014 2:30PM - 2:42PM |
J53.00001: Sub-Monolayer Water Adsorption on Alkaline Earth Metal Oxide Surfaces: A First-Principles Study Xunhua Zhao, Saswata Bhattacharya, Luca M. Ghiringhelli, Sergey V. Levchenko, Matthias Scheffler In the present work, we predict atomic structures of adsorbed complexes that should appear on alkaline earth metal oxide (001) terraces in thermodynamic equilibrium with water and oxygen gases. Density-functional theory with the hybrid exchange-correlation functional HSE06 combined with the self-consistent many-body dispersion approach [1] is used to calculate total energies. The choice of this functional is validated by renormalized second-order perturbation theory [2]. An unbiased search for global minima of H$_x$O$_y$ adsorption is performed using first-principles genetic algorithm for periodic models. $x$ and $y$ as a function of temperature and pressure are determined using {\em ab initio} atomistic thermodynamics. We find a range of H$_2$O chemical potentials where one-dimensional adsorbed water structures are thermodynamically stable on CaO(001). On MgO(001) and SrO(001), such structures are not found. The formation of the one-dimensional structures is explained by the balance between water-water and water-surface interactions.\\[4pt] [1] A. Tkatchenko, R. A. DiStasio, Jr., R. Car and M. Scheffler, Phys. Rev. Lett. \textbf{108}, 236402 (2012);\\[0pt] [2] X. Ren, P. Rinke, G. E. Scuseria, M. Scheffler, Phys. Rev. B \textbf{88}, 035120, (2013). [Preview Abstract] |
Tuesday, March 4, 2014 2:42PM - 2:54PM |
J53.00002: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 2:54PM - 3:06PM |
J53.00003: Atomic and electronic structure of polar oxide-wide band gap semiconductor interfaces: MgO(111)/SiC(0001) Vlado Lazarov, Phil Hasnip, Kate Ziemer, Kohji Nakamura, Michael Weinert Atomically sharp polar oxide/semiconductor heterostructures are characterized by the abrupt change of the electrostatic potential across these junctions. This inherent property provides opportunities to tailor the functional properties of these heterojunctions by engineering their atomic interface structure. In this work we present a combined experimental and theoretical study on model MgO(111)/SiC(0001) polar interface. Thin MgO(111) films with rock salt structure were grown by molecular beam epitaxy on two surface reconstructions of SiC(0001). Atomic imaging of the film/substrate interface(s) reveals that chemically abrupt interfaces determined by either O or Mg can be formed, depending on the substrate surface preparation. The density functional theory calculations show that screening of the interfacial dipole moment is the driving force for atomic stacking at the interface, which in turn determines the electronic properties of the MgO(111)/SiC(0001) interfaces. The electronic structure calculations show that O and Mg terminated interface have valence band offsets of 1.0 eV and 3.5 eV, respectively. These results demonstrate the large electronic tunability of this heterostructure, and indicate potentials for number of other polar oxide/semiconductor interfaces. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J53.00004: Structure-Property relationship for H covered Fe3O4(001) Fangyang Liu, Diogo Reis, Von Nascimento, Phillip Sprunger, Richard Kurtz, Rongying Jin, Jiandi Zhang, Ward Plummer Magnetite(Fe3O4), the oldest permanent magnet, is still attracting intense studies due to its fascinating surface physical properties. Previous LEED and STM experiments have reported reconstruction on Fe3O4(001) clean surface, but the origin of this reconstruction is still under debate. Hydrogen removes this reconstruction. LEED, HREELS, XPS are used to study the hydrogen induced surface properties. Surface geometry change was investigated by LEED-I(V) calculation and the structure refinements are being done now. XPS experiments show evidences of OH species on the hydrogen-covered surface, which is consistent with HREELS data, proving H adatoms are bonded to the surface oxygen sites. This suggested the ratio of Fe2$+$/Fe3$+$ on the surface is increased. This change is observed by Fe 2p peak shift in XPS spectra. We will discuss the role hydrogen played in the surface structure and physical properties change induced by reconstruction. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J53.00005: Comparing measurements of iron oxide nano-particle monolayer structure using GISAXS, Langmuir-Schraefer Method and SEM Jacob Stanley, Leandra Boucheron, Yeling Dai, Binhua Lin, Mati Meron, Oleg Shpyrko Iron oxide nanoparticles coated with an oleic acid ligand readily form self-assembled monolayers when drop cast onto water's surface. For low particle densities, upon drop casting, particles form into hexagonally close-packed islands, which then merge together when laterally compressed to higher densities. Using Grazing Incidence Small Angle X-Ray Scattering (GISAXS) off the liquid surface we were able to measure the first through fifth order diffraction peaks. By analyzing the peaks' position and shape we investigated the in-plane structure of these monolayers. Alternatively, using the Langmuir-Schraefer method this film can be transferred to a solid substrate for imaging using SEM. Subsequently, applying a Fourier Transform analysis to the SEM images we demonstrate a comparable measurement to that made using GISAXS - thereby obtaining similar in-plane structural information. Correcting for instrumental factors, we can account for some of the differing features between the data taken by the two techniques. By way of this technique comparison we also demonstrate that the Langmuir-Schraefer method is capable of preserving the nano-particle film structure during transfer from liquid surface to solid substrate. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J53.00006: Distinct Electronic Structure of the Electrolyte Gate Induced Conducting Phase in VO$_{2}$ Revealed by Photoelectron Spectroscopy Julie Karel, Carlos Viol Barbosa, Janos Kiss, Jaewoo Jeong, Nagaphani Aetukuri, Mahesh Samant, Xenia Kozina, Eiji Ikenaga, Gerhard Fecher, Claudia Felser, Stuart Parkin Vanadium dioxide (VO$_{2})$, a strongly correlated material, exhibits a temperature-driven metal to insulator transition (MIT), which is accompanied by a structural transformation from rutile (high-temperature metallic phase) to monoclinic (low-temperature insulator phase). Recently, it was discovered that a low-temperature conducting state emerges in VO$_{2}$ thin films upon gating with a liquid electrolyte. In this talk, photospectroscopy measurements of the core electronic states and valence band of electrolyte gated VO$_{2}$ thin films will reveal electronic features in the gate-induced conducting phase that are distinct from those of the temperature-induced rutile metallic phase. The electronic characteristics of the gated metallic state can be accounted for by oxygen vacancy formation and a consequent reduction in V-V dimerization without lifting the orbital ordering. An electronic bandstructure taking into account these modifications will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J53.00007: Effect of strain on the dynamics of optically induced metal-insulator transition of VO$_2$ thin films Elizabeth Radue, Matt Simons, Lei Wang, S. Kittiwatanakul, J. Lu, S.A. Wolf, R.A. Lukaszew, Irina Novikova Vanadium Dioxide undergoes a first order metal-insulator transition (MIT) when heated to 340K or when stimulated with a strong optical pulse. There is much interest in Vanadium dioxide due to its convenient transition temperature, and the many potential applications. In addition to the dramatic change in conductivity, the advent of ultrafast lasers has made it possible to induce the MIT in the femtosecond regime. Thin films of VO$_2$ are able to robustly undergo reversible MITs over many cycles. Changes in the microstructure can be used to tune the MIT. We are studying the dynamics of the phase transition, specifically how different amount of strain can affect both the quick transition to the metallic phase and the slower relaxation back to the insulating phase. We have found noticeable differences in the threshold fluence needed to optically induce the MIT in films on different substrates, as well as the longevity of the metallic state. We will be discussing the implications of these differences regarding the mechanisms responsible for the optically induced phase transition. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J53.00008: Optical anisotropy in the metal-to-insulator transition in VO$_2$ thin films Matt T. Simons, Elizabeth Radue, Lei Wang, S. Kittiwatanakul, J. Lu, S.A. Wolf, R.A. Lukaszew, Irina Novikova The metal-to-insulator transition in vanadium dioxide (VO$_2$) is being explored for a variety of uses, ranging from opto-electronic switches to nanoparticle coatings for smart windows. The mechanisms behind this transition as well as methods for altering the transition properties are the focus of continuing studies. In particular, the properties of VO$_2$ thin-films are affected by the structure of the underlying substrate material that can influence the temperature, width, and other characteristics of the phase transition. We investigate the anisotropy of the time-resolved optical measurements in an ultrafast photo-induced MIT transition in VO$_2$ on different substrates, including rutile (TiO$_2$) and sapphire (Al$_2$O$_3$). We observe that the optical anisotropy varies with the fluence of the pump used to induce the phase transition on the TiO$_2$ substrate. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J53.00009: High resolution structural and compositional mapping of the SrTiO$_{3}$/LaFeO$_{3}$ interface using chromatic aberration corrected energy filtered imaging Bernd Kabius, Lothar Houben, Christian Dwyer, Robert Colby, Scott A. Chambers, Rafal Dunin-Borkowski Interfaces between insulating polar perovskites have demonstrated a wealth of electronic and magnetic properties. Understanding and predicting the properties of a specific interface requires atomic level knowledge of interface structure and chemistry. Electron microscopy is capable of this task, and has been frequently applied to oxide interfaces using a combination of high-angle angular dark field scanning transmission electron microscopy (HAADF-STEM) and electron energy-loss spectroscopy (EELS). Energy-filtered TEM (EFTEM) captures a full image for a given energy losses, allowing a larger field of view than typical for STEM-EELS in far less time. However, EFTEM has not, to date, demonstrated the spatial resolution of STEM-EELS due to the limits set by chromatic aberration C$_{c}$. This study of LaFeO$_{3}$/SrTiO$_{3}$ demonstrates that C$_{c}$ correction enhances the resolution of EFTEM for elemental mapping, allowing a unit cell-by-unit cell analysis of the concentration gradients across the SrTiO$_{3}$/LaFeO$_{3}$ interface. The charge distribution at the interface will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J53.00010: A first principles study of interstitial transition metals in the SrTiO3 bulk and at the surface Steven Hepplestone, Peter Sushko Recent advances in oxide thin film deposition and growth have given a new momentum to the field of oxide electronics. For example, it was found that La-doped SrTiO$_3$ shows high charge carrier mobilities and conductivity was observed near the interface with LaAlO$_3$. However, forming low-resistance Ohmic metal/oxide constants has proven to be a challenge. Recently, the interface formed by a Cr film grown on SrTiO$_3$ (001) surface has been shown to have such properties. This raises the question of whether Cr or another metal provides the lowest resistance possible. To this end, we perform ab initio calculations using the VASP package and the PBE and PBE+U functionals and examine the electronic properties of interstitial transition metal atoms (M) in the bulk SrTiO$_3$ and near SrTiO$_3$ (001) surface. We show that for surface doping, the variation in the formation energy linearly depends on the number of d-electrons in the valence shell of the M atoms. This contrasts with the formation energies of M atoms in the bulk SrTiO$_3$, where no such trend is observed and, instead, the formation energy depends on the occupancy of the e$_g$ and t$_{2g}$ sub-bands. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J53.00011: Scale-bridging simulations of surfaces and defects in BaTiO3 Andrea Greco, Paul Tangney, John Freeland, Arash Mostofi BaTiO3 (BTO) is a ferroelectric perovskite oxide that is of particular interest in thin-film form for its technological application in tunable nanoelectronic devices. The dielectric properties of BTO thin films depend on many different factors, among which the presence of oxygen vacancies is believed to be one of the most important. Oxygen vacancies, however, are difficult to characterize directly in experiments and usually even their concentration is unknown. On the one hand, first-principles simulations based on density-functional theory (DFT) are invaluable for providing insight into the role of defects in materials and, in principle, could be used for the study of oxygen vacancies in BTO thin films. On the other hand, the large length and time-scales associated with structures and processes in realistic surfaces are well beyond the scope of DFT calculations. To overcome some of these limitations we use DFT in conjunction with a computationally efficient classical interatomic force field that has been fitted to DFT energies, forces and stresses in bulk BTO. We assess the transferability of this potential to defects and surfaces. We then use it to study the prevalence of oxygen vacancies and their structures, both in bulk BTO and near BTO surfaces. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J53.00012: Density Functional Theory studies of metal supported thin Zirconia films Wernfried Mayr-Schmoelzer, Florian Mittendorfer, Josef Redinger We present the results of DFT calculations of the interface between a single layer thin ZrO$_2$ film and the supporting metals Pt$_{3}$Zr [1] and Pd$_{3}$Zr [2]. Both are very stable and are present when thin ZrO$_2$ films are grown by oxidation. Using the Vienna Ab-initio Simulation Package (VASP) employing standard PBE and van-der-Waals density functionals a thorough investigation of the structural properties was done for both small model cells and the large experimentally found super cells. Both substrates have a similar crystal structure but the binding mechanism differs: the Pt$_{3}$Zr substrate shows a pure Pt surface layer, leading to weak bonding between the film and the substrate with large nonlocal contributions. The Pd$_{3}$Zr substrate is stoichiometric and a Zr-O bond forms between substrate and oxide film, leading to higher adsorption energies. The large buckling found experimentally could be attributed to the closer distance resulting from nonlocal contributions for the Pt$_{3}$Zr substrate and to the Zr-O bond for the Pd$_{3}$Zr substrate. Furthermore we present results on the adsorption of atomic hydrogen and water molecules on the surface generated by the thin Zirconia film.\\[4pt] [1] Antlanger, M. et al, Phys Rev B 86, 035451 (2012).\\[0pt] [2] Choi, J et al, submitted. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J53.00013: Density-functional study of the La$_{2}$Zr$_{2}$O$_{7}$ low-index faces Yves Mantz, Yuhua Duan The (001), (011), and (111) faces of the catalytic host lanthanum zirconate (La$_{2}$Zr$_{2}$O$_{7})$ are studied at the level of density-functional theory (DFT), including all surfaces formed by cleaving a perfect crystal (``bulk-truncated'') and a subset of defective surfaces. Surface free energies are computed, dependent on O chemical potential for (001) and (011) surfaces and O and La or Zr chemical potentials for (111) surfaces, with vibrational contributions obtained from computed atomic frequencies and/or phonon dispersions. Taking into account error in determining the conditions where a given surface is thermodynamically preferred, a relaxed defective (001) or (011) surface is shown to be preferred to any relaxed bulk-truncated (001) or (011) surface over a subset of temperatures and oxygen gas partial pressures. Thus, the existence is proven of (001) and (011) faces that cannot be relaxed bulk-truncated, e.g., are defective or reconstructed. Extending this work, the existence of a (111) face that is not relaxed bulk-truncated is addressed. The main finding, of faces that are not relaxed bulk-truncated, is discussed in the context of comparing predicted versus experimental crystallite thicknesses obtained from a data analysis. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J53.00014: Short-Range Charge Transfer Between Oxide Based Superconductor-Ferromagnetic Metal Interfaces Te-Yu Chien, L.F. Kourkoutis, J. Chakhalian, D. Muller, J.W. Freeland Unlike the conventional superconductor (S) and ferromagnetic metal (F) interface, the understanding of the proximity effect between oxide-based S and F is still unclear. One particular question relates to the charge transfer length scale between S and F layers, which resulted from the lack of an appropriate experimental tool. In this talk, we show that by combining the cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S) along with scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS), the charge transfer length scale at the interfaces between YBa Cu O $_{\mathrm{-\delta }}$(YBCO) and La$_{3}$Ca$_{3}$MnO (LCMO) was revealed to have upper limit of 1 nm.\\[4pt] [1] Teyu Chien, et al., Nature Commun.4, 2336 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J53.00015: The Origin of Giant Electrostriction in Gd-Doped Ceria as Studied by Modulation Excitation X-ray Absorption Spectroscopy Alyssa Lerner, Yuanyuan Li, Anatoly Frenkel, Roman Korobko, Igor Lubomirsky Electromechanical materials, such as piezoelectrics and electrostrictors, are ubiquitous. Recently, an unusually large electrostriction effect, which exceeds that of most common electrostrictors, was found in gadolinium-doped ceria thin films. It is likely to be explained by the dynamic response of oxygen vacancies to external electric field. Verifying this hypothesis is very challenging, as it is required to detect local atomic rearrangement at the 0.01 {\AA} scale. Conventional structural methods have neither elemental specificity nor spatial sensitivity to such structural changes. We applied Quick Extended X-Ray Absorption Fine Structure used in the Modulation Excitation mode to directly observe the dynamic response of the Ce and Gd local environments to the electric field. While using periodic stimulation of the films by electric field in situ, we detected X-ray absorption spectra at the Ce and Gd absorption edges, thus enhancing the sensitivity to electro-active species. Our model of electromechanical activity in this system attributes it to a relatively small population of Ce ions with anomalously short Ce-O bonds formed near the oxygen vacancies. This finding suggests that other oxides with a large concentration of vacancies may exhibit even larger electrostriction. [Preview Abstract] |
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