Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J34: Focus Session: Interfaces in Complex Oxides - Microscopy and Local Structure |
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Sponsoring Units: DMP Chair: Suzanne Stemmer, University of California, Santa Barbara Room: C141 |
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J34.00001: Unexpected ordering at the atomic scale interface of SrRuO$_{3}$ and BaTiO$_{3}$ Invited Speaker: Atomically engineered oxide multilayers and superlattices display unique properties responsive to the electronic and atomic structures of the interfaces. Interfaces can exhibit not only two-dimensional functionality, but have the power to dictate the properties of thin films. A clear example is found in ferroelectric thin films, where the domain size, orientation, and transport properties are controlled by top and buried interfaces. We have explored a prototypical ferroelectric - bottom electrode interface by characterizing BaTiO$_{3}$ grown on SrRuO$_{3}$. Films were grown on SrTiO$_{3}$ substrates by pulsed laser deposition, monitored by high-pressure reflection high-energy diffraction, exhibited high crystalline quality in electron diffraction and cross-sectional transmission electron microscopy (STEM), and were flat according to atomic force microscopy. Despite multiple indicators commonly accepted to confirm a sharp interface, atomically the structure is more complex. When grown in a typical oxygen pressure, at or below 10 mTorr, in situ scanning tunneling microscopy (STM) revealed the SrRuO$_{3}$ surface is littered with oxygen vacancies. For growth or post-annealing above $\sim $100 mtorr, vacancies were removed, but STM and low energy electron diffraction (LEED) disclosed a surface reconstruction consisting of parallel rows with periodicity doubled in one direction. Density function theory (DFT) suggests these rows are chains of Sr and O raised by excess oxygen. Both LEED and cross-sectional STEM revealed that this reconstruction persists at the buried interface and modifies the structure of subsequent BaTiO$_{3}$ layers. By four layers, the BaTiO$_{3}$ surface returns to a bulk-like structure with upward polar distortion. This study emphasizes the importance of atomic scale structural studies of what may otherwise appear as sharp interfaces. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J34.00002: Highly confined 2D electron gas in SrTiO$_{3}$/SrRuO$_{3}$ superlattices Marcos Ver\'Issimo-Alves, Daniel Bilc, Pablo Garc\'Ia-Fern\'andez, Philippe Ghosez, Javier Junquera In recent years, experimental advances in the creation of atomically smooth interfaces between complex oxides have made possible the observation of unusual electronic phenomena such as the formation of 2D electron gases confined to the interface between insulating compounds. In this work, we explore an alternative for creating a 2D electron gas in oxide interfaces by means of a (SrTiO$_3$)$_5$/(SrRuO$_3$)$_1$ superlattice. LSDA+U and hybrid functional B1-WC DFT calculations with full relaxation of atomic geometry show the presence of a 2D electron gas, and the system can be described with $U=4$ eV. Electronic correlations thus become important with quantum confinement: the system is a minority spin half metal with a magnetic moment of $\mu=4 \mu_B$. Degenerate Ru 4d$_{(xz,yz)}$ states are the main responsible for the DOS at the Fermi level, whose shape strongly resembles that of 1D free-electron gases. Our results suggest that the material could find uses in spintronic devices. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J34.00003: Ferroelectric thickness effects on LaSrMnO3/PbZrTiO3 Heterostructures Jinling Zhou, Evan Wolf, Charles Frye, Disheng Chen, Srinivas Polisetty, Mikel Holcomb Magnetoelectric (ME) coupling is the coupling of magnetic and electric properties within a material. It allows the possibility of dual control of the material through the manipulation of either electric or magnetic fields and therefore could potentially revolutionize the current technology. However, little is known about the factors that influence the strength of this magnetoelectric coupling. In the presented research, ferromagnetic LSMO and ferroelectric PZT are constructed as wedged adjacent layers for the purpose of studying the coupling effects and physical properties in each layer and the resulting interface. X-ray absorption spectroscopy (XAS) and photoemission electron microscopy (PEEM) are used as the major techniques to map out magnetism, ferroelectricity, and the interfacial coupling. The XAS spectra illustrate a strong effect on the magnetic properties depending on ferroelectric thickness. PEEM images display the magnetic and ferroelectric domains in each material layer, allowing further insight into why the coupling depends on layer thickness. This research will aid the understanding of coupling in not only magnetoelectric heterostructures, but also in other similar complex oxide systems. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J34.00004: SrTiO$_3$ on silicon: interface chemistry, polarization pinning, and electronic states Sohrab Ismail-Beigi, Alexie Kolpak, Fred Walker, Jim Reiner, Charles Ahn We use SrTiO$_3$/Si as a model system to examine the effects of interface atomic structure and composition on the functional properties of epitaxial oxide films on silicon. Using first-principles computations, we show that intrinsic chemo-mechanical boundary conditions at the interface fix a single polarization direction in the SrTiO3 thin film independent of the interface composition, inhibiting ferroelectric functionality. In contrast, the transport properties of the interface are quite sensitive to the interface composition, which can be used to tune the interface from an insulator to an interfacial 2D electron gas. We describe the origins of both functionalities and discuss their applicability to the general class of epitaxial oxides on semiconductors. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J34.00005: First Principles Study of Resonant Phonon Coupling across the LSMO / STO Interface Kevin F. Garrity, Yaron Segal, Carlos A.F. Vaz, Jason D. Hoffman, Fred J. Walker, Charles A. Ahn, Sohrab Ismail-Beigi Epitaxial interfaces permit dynamical modification of the properties of a thin film via coupling to the substrate. In particular, the coupling of phonons between two materials allows one to manipulate the atomic structure and vibrational modes near an interface. We use first principles density functional theory (DFT) to study the octahedral oxygen rotations at and across an interface between La$_x$Sr$_{(1-x)}$MnO$_3$ (LSMO) and SrTiO$_3$. By performing finite temperature Monte Carlo sampling on a classical potential built to reproduce our DFT energetics, we demonstrate that as the SrTiO$_3$ is driven through the phase transition where its octahedral rotations become frozen in place, phonons from the SrTiO$_3$ couple into the interfacial LSMO. These couplings can then modify the LSMO transport properties, as observed in our experiments. The decay length of the phonon coupling into the LSMO agrees with our experimental determinations on this system. We demonstrate that the observed changes in resistance are not due to static changes in the LSMO structure, confirming the phonon coupling. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J34.00006: Coupling of strain and magnetism in LPCMO films Surendra Singh, M. Fitzsimmons, Hyoungjeen Jeen, Amlan Biswas Complex oxides show extraordinary structural, magnetic and magneto-transport properties and these properties are closely coupled with atomic structure and strain. The temperature- magnetic field phase diagram and transport studies of La$_{0.27}$Pr$_{0.40}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) films suggest the existence of two phases at low temperatures, i.e. ferromagnetic metallic (FMM) phase and charge order insulating (COI) phase. We report the magneto transport properties of LPCMO films on application of external strain using mechanical jig. The study shows the shift in metal to insulator transition (MIT) temperature on application of external strain. To understand the effect of strain as well as kinematics of formation of FMM phase from COI phase of LPCMO films and vice versa, we have performed detailed in situ transport and specular polarized neutron reflectivity measurements across the MIT as functions of temperature, magnetic field and applied strain. The study reveals a variation of magnetic scattering length density along the depth of the film, which may be attributed to chemical inhomogeneity of the film as a function of depth. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J34.00007: Coupling between oxygen octahedron rotations, Jahn-Teller distortion, magnetic ordering and epitaxial strain in LaMnO3 from first principles Jun Hee Lee, Karin M. Rabe, Kris Delaney, Eric Bousquet, Nicola Spaldin LaMnO3 is known to have rich physics due to coupling among orbital ordering, Jahn-Teller distortions and magnetism. However, less attention has been paid to the role of oxygen octahedron rotations and epitaxial strain. In this talk, we show from first principles calculations that oxygen octahedron rotations induce weak ferromagnetism in \textit{Pbnm} LaMnO3 and that octahedral rotations are cooperatively coupled to the Jahn-Teller distortion.. Furthermore, we predict that compressive epitaxial strain drives bulk $A$-type antiferromagnetic Pbnm insulating phase to a ferromagnetic metallic phase. At the phase boundary between the ferromagnetic-metallic and antiferromagnetic-insulating phases, colossal magneto-resistance is expected. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J34.00008: Local Octahedral Distortions and Magnetic Properties Controlled by Substrate Symmetry at Perovskite Oxide Interfaces Jun He, Albina Borisevich, Sergei Kalinin, Stephen Pennycook, Sokrates Pantelides We have investigated the oxygen octahedral distortions and local magnetism at the interfaces of magnetic perovskite oxide heterostructures using first principles calculations. The studied prototype oxide heterostructures include La$_{0.7}$Sr$_{0.3}$MnO$_{3}$, SrRuO$_{3}$, and BiFeO$_{3}$. The results show that the symmetry mismatch at interfaces between two perovskite oxides imposes an interfacial layer with distortion modes that do not exist in either bulk material, creating new interface properties by symmetry alone. The thickness of such interface layer depends on the resistance of the octahedra to deformation. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J34.00009: Atomic Resolution and First Principles Study of the Structure and Bonding at SrTiO$_{3}$/GaAs Hetero-interfaces Qiao Qiao, Robert Klie, Serdar Ogut Ultrathin metal-oxide films on polar semiconductor surfaces have received much attention in recent years due to occurrence of novel functional properties, including ferroelectricity, superconductivity and the presence of an interfacial 2-dimensional electron gas. In this study, we examine the atomic and electronic structures of epitaxial ultrathin SrTiO$_{3}$ (100) films on GaAs (001) using aberration corrected atomic-resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) in combination with first principles calculations to develop a fundamental understanding of the interfacial structure-property relationships. Using atomic-column resolved EELS, we show that Ti diffuses into the first few monolayers of GaAs depending on the thin film growth condition. The effects of Ti diffusion into subsurface GaAs (001) with (4x2)-$\beta $2 surface reconstructions will be investigated via first principles calculations. We will also discuss the results for the formation energies of Ti-related impurity defects in the bulk and surface regions of GaAs to help in the interpretation of electron microscopy experiments. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J34.00010: Accessing hidden isosymmetric phase transitions in perovskite thin films James Rondinelli, Sinisa Coh Isosymmetric phase transitions (IPT), which show no change in occupied Wyckoff positions or crystallographic space group, are exceedingly rare in crystalline matter because most condensed systems respond to external stimuli by undergoing ``conventional'' symmetry-lowering displacive, martensitic or reconstructive transitions. In this work, we use first-principles density functional calculations to identify an elusive IPT in orthorhombic $AB$O$_3$ perovskite oxides with tendency towards rhombohedral symmetry. Using perovskite LaGaO$_3$ as our prototypical system, we show that the latent isosymmetric phase transition, which manifests as an abrupt change in the octahedral rotation axis, is accessible only with an external elastic constraint---bi-axial strain. We show the transition originates from a soft phonon that describes the geometric connectivity and relative phase of the GaO$_6$ polyhedra. By connecting the origin of IPT to a chemical and structural incompatibility between the lattice and the elastic constraints, we describe how subtle changes in bulk orthorhombic and monoclinic symmetries are critical to the complete engineering of structure-correlated electronic properties in thin films. Because bi-axial strain is the critical parameter controlling the IPT, we suggest heteroepitaxial synthesis of IPT materials is a plausible route to realize high-$\kappa$ dielectric actuators with variable band gaps and dielectric anisotropies. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J34.00011: Electronic properties of ultrathin GdTiO3 thin films and GdTiO3/SrTiO3 interfaces Pouya Moetakef, Bharat Jalan, Jack Zhang, S. James Allen, Susanne Stemmer Interfaces between Mott insulators, such as the rare earth titanates, and band insulators, such as SrTiO3, have recently attracted much attention. We report on the transport properties of epitaxial rare earth titanate thin films, GdTiO3, grown by molecular beam epitaxy (MBE) and those of heterostructures with SrTiO3 and GdTiO3. Growth of GdTiO3 was performed by shuttered growth of alternating titanium tetra isopropoxide (TTIP) and Gd fluxes, in the absence of any additional oxygen. We show that to stabilize the GdTiO3 perovskite phase, SrTiO3 buffer layers are needed for growth on perovskite substrates, such as LSAT ((LaAlO3)0.3(Sr2AlTaO6)0.7). The contribution of n-type SrTiO3 buffer layers and that of the SrTiO3/GdTiO3 interfaces to the transport properties are determined by measurements of the electrical resistance and Hall coefficient as a function of temperature and magnetic field. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J34.00012: Atomically-resolved mapping of polarization and electric fields across ferroelectric-oxide interfaces by Z-contrast imaging Albina Borisevich, Hye Jung Chang, Sergei Kalinin, Anna Morozovska, Ying-Hao Chu, Pu Yu, Ramamoorthy Ramesh, Stephen Pennycook Polarization, electric field, charge and potential across ferroelectric-oxide interfaces are obtained from direct atomic position mapping by aberration corrected scanning transmission electron microscopy combined with Ginsburg-Landau-Devonshire theory. We compare two antiparallel polarization orientations, which allows separation of the polarization and intrinsic interface charge contributions. Using the Born effective charges, the complete interface electrostatics is obtained in real space, providing an alternative method to holography. The results provide new microscopic insight into the thermodynamics of polarization distribution at the atomic level. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J34.00013: Inherited (In)stabilities in Transition Metal Superlattices Sven Rudin Many transition metals exhibit a solid phase with a body-centered cubic (bcc) crystal structure. For some elements, e.g., tungsten (W), bcc is the only solid phase; for others, e.g., titanium (Ti), the bcc phase only appears at high temperatures. Titanium's high-temperature bcc phase exhibits soft phonon modes. These reflect the atomic movements upon transformation into the low-temperature phases. One such mode shows atomic displacements that also appear in the top few layers of tungsten's surface reconstruction. Superlattices constructed from alternating nanometer-thick layers of W and Ti would allow the two displacement patterns to interact. The work presented here uses density functional theory calculations to predict how the structure and mechanical response of such superlattices depends on the choice of transition metal elements and the layer thicknesses. [Preview Abstract] |
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