Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T6: Focus Session: Emergent Properties in Bulk Complex Oxides: Titanates and Ruthenates |
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Sponsoring Units: GMAG DMP Chair: Matt Langner, Lawrence Berkeley National Laboratory Room: 108 |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T6.00001: Pressure and Temperature Effects on the Antiferrodistortive Phase Transition and Phonon Softening in SrTiO$_3$ Shih-Chang Weng, Ruqing Xu, Ayman Said, Xinyue Fang, Shih-Lin Chang, Tai-C. Chiang Strontium titanate undergoes an antiferrodistortive transition accompanied by a cubic-to-tetragonal structural distortion. This transition can be induced at ambient pressure by lowering the temperature down to 105 K or at room-temperature by applying hydrostatic pressure up to 9.6 GPa. The hydrostatic pressure leads to the same type of symmetry breaking and phonon softening at the \textit{R} point in the Brillouin zone as the temperature does, but give a much larger volume reduction. Herein we report our results of pressure-induced phonon softening determined by inelastic X-ray scattering using a diamond-anvil cell for the required pressure. The phonon softening behavior follows a power law and is accompanied by a central peak. The comparison between pressure and thermal effects is presented. First-principles calculations were preformed as a function of pressure (or unit cell volume), and the results support that the distorted structure becomes stabilized under hydrostatic pressure. [Preview Abstract] |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T6.00002: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T6.00003: Interband and polaronic excitations in YTiO3 from first principles Burak Himmetoglu, Anderson Janotti, Lars Bjaalie, Chris G. Van de Walle YTiO$_3$, as a prototypical Mott insulator, has been the subject of numerous experimental investigations of its electronic structure. The onset of absorption in optical conductivity measurements has generally been interpreted as due to interband transitions at the fundamental gap. Here we re-examine the electronic structure of YTiO$_3$ using density functional theory with either a Hubbard correction (DFT+$U$) or a hybrid functional. Interband transitions are found to be much higher in energy than the observed onset of optical absorption. However, we show that the holes in the lower Hubbard band tend to become self-trapped in the form of small polarons, localized on individual Ti sites. Exciting electrons from the occupied lower Hubbard band to the small-polaron state then leads to broad infrared absorption, consistent with the onset in the experimental optical conductivity spectra. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T6.00004: Development of Electronic and Topographic Structure of the Vacuum-cleaved SrTiO$_{3}$ (001) Surface as a Function of Annealing Wattaka Sitaputra, Marek Skowronski, Nikhil Sivadas, Di Xiao, Randall Feenstra A progressive disappearance of the conductance stripes along with emergence of new surface electronic states were observed at a vacuum-cleaved SrTiO$_{3}$ (001) surface upon annealing at $150-375^{\circ}C$. This disappearance started with an expansion of the TiO$_{2}$ stripe, as seen in conductance mapping of scanning tunneling microscopy, and progressed as the annealing time increases until the TiO$_{2}$ stripes dominate most of the surface. Such development can be associated with a topographic evolution from the initial alternating TiO$_{2}$/SrO surface termination into a step-terrace structure with mainly TiO$_{2}$ termination, which is more thermodynamically favorable during ultra-high vacuum annealing. The completeness of the TiO$_{2}$ terrace after the annealing was found to depend significantly on the original surface structure and, thus, vary across the surface. This different degree of TiO$_{2}$ coverage resulted in different emerging electronic states, in which several were found within the range of 0.6-1.7 eV above the Fermi level on both TiO$_{2}$ and the remaining SrO terminated surface. Interestingly, it was found that a short annealing at $150^{\circ}C$ could produce a significant change in electronic structure where states can be found within 1 eV above and below the Fermi level. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T6.00005: Elastic, structural and magnetic properties of EuTi$_{\mathrm{1-x}}$A$_{\mathrm{x}}$O$_{3}$ (A$=$Zr, Nb) Ling Li, Zhiling Dun, Jiaqiang Yan, Haidong Zhou, David Mandrus, Veerle Keppens The elastic moduli as a function of temperature (280-380 K) and magnetic field (0-9T) for single crystal EuTiO$_{3}$ have been measured using resonant ultrasound spectroscopy (RUS). All the moduli show a sharp step-like softening upon the cubic-to-tetragonal transition at around 288K. We also present low-temperature XRD, magnetic susceptibility, and RUS results on polycrystalline EuTi$_{\mathrm{1-x}}$Zr$_{\mathrm{x}}$O$_{3}$ and EuTi$_{\mathrm{1-x}}$Nb$_{\mathrm{x}}$O$_{3}$ (x$=$0.015, 0.03 and 0.05). All of the compositions investigated present a cubic-to-tetragonal structural transition as temperature is lowered. Our results indicate that the transition temperature of the structural instability increases to higher temperatures with increasing Zr and Nb concentration in both solid-solutions, accompanied by the decrease of the antiferromagnetic transition temperature T$_{\mathrm{N}}$. While the structural distortion in EuTi$_{\mathrm{1-x}}$Zr$_{\mathrm{x}}$O$_{3}$ is suppressed with increasing Zr doping, the magnitude of the structural distortion in EuTi$_{\mathrm{1-x}}$Nb$_{\mathrm{x}}$O$_{3}$ is not affected by Nb-doping. The differences between Zr and Nb as dopants are discussed. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T6.00006: Persistent Optically Induced Magnetism in Oxygen-Deficient Strontium Titanate W.D. Rice, J.D. Thompson, S.A. Crooker, M. Bombeck, P. Ambwani, C. Leighton Strontium titanate (SrTiO$_3$) is a foundational material in the emerging field of complex oxide electronics. While its electronic, optical, and lattice properties have been studied for decades, SrTiO$_3$ has recently become a renewed focus of materials research owing to the discovery of magnetism and superconductivity at interfaces between SrTiO$_3$ and other oxides. The formation and distribution of oxygen vacancies may play an essential but as-yet-incompletely understood role. Here we observe an \textit{optically induced} and \textit{persistent} magnetization in slightly oxygen-deficient bulk SrTiO$_{3-\delta}$ crystals using magnetic circular dichroism spectroscopy and SQUID magnetometry. The optically induced magnetization appears below $\sim$18 K, persists for hours below 10 K, and is tunable via the polarization and wavelength of sub-bandgap (400-500 nm) light. These effects, which only occur in oxygen-deficient samples, reveal a detailed interplay between defects, magnetism, and light in oxide materials. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T6.00007: Structure and Properties of a Metallic Polar Ruthenate Oxide Danilo Puggioni, james Rondinelli Using first-principles density functional theory calculations, we predict a polar-noncentrosymmetric (pNCS) ruthenate exhibiting robust metallicity. We describe a weak coupling ansatz which accounts for the scarcity of noncentrosymmetric metal (NCS-M). We show in this artificial ruthenate that the apparent incompatibility between acentricity and metallicity is circumvented because the polar distortion is largely decoupled from the electronic structure at the Fermi level. Moreover, we discuss the thermopower response showing that this material owns an anomalously anisotropy at 300~K which is comparable to that of YBa$_2$Cu$_3$O$_{7-\delta}$, owing to the polar axis. Our work suggests it is possible to design thermopower anisotropy in noncentroysymmetric conductors for ultrafast-thermoelectric devices. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T6.00008: Ferromagnetism in ruthenate perovskites Hung T. Dang, Jernej Mravlje, Andrew J. Millis In apparent contrast to the usual rule that stronger correlations favor magnetism and other forms of order, while weaker correlations lead to Fermi liquid metals, it has been experimentally established that CaRuO$_3$, a more correlated material, is a paramagnetic metal with a Fermi liquid ground state while SrRuO$_3$, which is less strongly correlated, is ferromagnetic below a Curie temperature of 160K. We present density functional plus dynamical mean field theory calculations which resolve this conundrum. We show that in these materials ferromagnetism occurs naturally for cubic perovskite systems at moderate correlations but is suppressed both by proximity to the Mott insulating phase and by increasing the amplitude of a GdFeO$_3$ distortion. These factors are strongly related to the differences between Ca and Sr ruthenates and are used as the keys to solve the problem. Placement of the ruthenate materials on the metal-insulator phase diagram and comparison to previous works on the Ruddlesden-Popper materials are also discussed. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T6.00009: Anisotropy, Magnetism and Bulk Spin Valve Effect in Fe-doped Bilayer Ruthenate Ca$_{3}$Ru$_{2}$O$_{7}$ T.F. Qi, J. Terzic, G. Cao The bilayered Ruthenate Ca$_{3}$Ru$_{2}$O$_{7}$ displays a wide variety of physical properties derived from the competitions among the orbital degrees of freedom of the Ru-ions, spin-orbit interactions and lattice distortions. We report our recent results of structural and physical properties of single-crystal Ca$_{3}$(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})_{2}$O$_{7}$ (0 \textless x \textless 0.2) as a function of temperature and magnetic field. The central finding of this study is that (1) Ca$_{3}$(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})_{2}$O$_{7}$ display highly anisotropic and antiferromagnetic state that is clearly manifested in the magnetization, electrical resistivity and specific heat; (2) Bulk spin valve effect (SVE) is observed in bulk single crystals of Ca$_{3}$(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})_{2}$O$_{7}$. This study along with our previous work on SVE suggests that the bulk SVE may be commonplace in 3d-element doped Ca$_{3}$Ru$_{2}$O$_{7}$. The results will be presented and discussed along with comparison drawn with other 3d-element doped Ca$_{3}$Ru$_{2}$O$_{7}$ single crystals. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T6.00010: Resistivity for Ru oxides on the basis of a conserving approximation Naoya Arakawa In order to analyze the origin of the non-Fermi liquid behavior in resistivity for Ru oxides, I analyzed the temperature dependence of resistivity, formulated on the basis of a conserving approximation, for the Ru $t_{2g}$ orbital Hubbard model on a 2D square lattice. In this analysis, I focus on the cases of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ with $x=$ 2 and 0.5, and take account of effects of electron correlation by the fluctuation-exchange approximation. In this presentation, I present the results about the effects of not only the self-energy of electrons and but also the Maki-Thompson-type and the Aslamasov-Larkin-type vertex corrections on the temperature dependence of resistivity. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T6.00011: Li$_2$RuO$_3$, a valence bond liquid on the honeycomb lattice Harald Jeschke, Simon A.J. Kimber, I.I. Mazin, Juan Shen, Dimitri N. Argyriou, Sergey V. Streltsov, Roser Valenti, Daniel I. Khomskii Li$_2$RuO$_3$ has been known to form Ru-Ru dimers at low temperature, but was believed to be homogenous above the transition. We provide new experimental evidence for the melting of the low temperature dimer ordering by comparing x-ray diffraction probing the average crystal structure and the pair distribution function, which provides information about local order. We show that strong dimerization survives well above the ordering temperature $T_s\approx 450$ K and that the high temperature structure is a bond liquid of dynamically disordered dimers with the same 1:2 ratio of short and long bonds. Theoretically, we search for low energy structures of Li$_2$RuO$_3$ and find different long range orders of Ru dimer patterns. We can explain not only the low $T$ structure but also the ordering temperature: Due to strong covalency, dimerization leads to a large energy gain but the additional gain through dimer ordering is much smaller and compatible with the experimentally found $T_s$. Moreover, of the two holes present in Li$_2$RuO$_3$ one participates in a strongly covalent bond, which survives at all temperatures, and the other in a weak bond that breaks in the bond-liquid state. This explains why below $T_s$ the effective spin of Ru is 0, and above $1/2$, but never 1. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T6.00012: Optical spectroscopic analysis of Sr$_{2}$RhO$_{4}$ and comparison with Sr$_{2}$RuO$_{4}$ Kim Mortimer, Nathan Armstrong, Jesse Hall, Thomas Timusk Sr$_{2}$RhO$_{4}$ is a strongly correlated electron oxide, similar to the unconventional superconductor Sr$_{2}$RuO$_{4}$. Changing Ru to Rh destroys superconductivity and skews the crystal structure without eliminating T$^{2}$ resistivity [1,2]. Using optical floating zone furnace techniques we grew single crystals of Sr$_{2}$RhO$_{4}$. We then performed spectroscopic measurements of the material at frequencies ranging from 4 meV to 1.2 eV and temperatures from 12 K to 300 K. We compare these results with those from single crystals of Sr$_{2}$RuO$_{4}$, grown by the author at Kyoto University and displaying \textless 10\% 3K phase, which were measured concurrently with Sr$_{2}$RhO$_{4}$ on the same apparatus. The optical resistivity $\rho(\omega,T)$ of both of these materials are then compared to the predictions of Landau-Fermi liquid theory -- in particular, the ratio between the temperature dependence and frequency dependence of resistivity, which yields insight into electron scattering mechanisms. [3] 1. Hase \& Nishihara, doi: 10.1143/JPSJ.65.3957 2. Nagai et al, doi: 10.1143/JPSJ.79.114719 3. Maslov \& Chubukov, doi: 10.1103/PhysRevB.86.155137 [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T6.00013: Angular dependent investigation of the metamagnetism in the itinerant ferromagnet Sr$_{4}$Ru$_{3}$O$_{10}$ by magnetization measurements F. Weickert, L. Civale, M. Jaime, E. Carleschi, B.P. Doyle, A. Strydom, R. Fittipaldi, V. Granata, A. Vecchione, M.B. Salamon We report a detailed study of the magnetization as a function of temperature, field and crystallographic angle of Sr$_{4}$Ru$_{3}$O$_{10}$. The n=3 member of the Sr$_{n+1}$Ru$_{n}$O$_{3n+1}$ Ruddlesden-Popper serie exhibits ferromagnetism below 105$\,$K with magnetic moments aligned along the crystallographic $c$-direction in the tetragonal crystal structure. Metamagnetism is observed at about 2$\,$T below 50$\,$K when a magnetic field is applied in the $ab$-plane. Our study reveals that the metamagnetic transition splits into two distinct anomalies at very low temperatures. Surprisingly, these anomalies are accompanied by a reduction of the total magnetic moment and large hysteresis. Furthermore, the measurements indicate a shift of both metamagnetic signatures to higher fields by rotating the field from $H \parallel ab$ to $H \parallel c$. This observation is in contrast to previously published data, where a single metamagnetic anomaly splits into two that move simultaneously to higher and lower critical magnetic fields with increasing angle. In the presentation we will discuss the application of different spin reorientation models to our experimental findings. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T6.00014: Magnetic order and negative thermal expansion in Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$ Songxue Chi, Feng Ye, Huibo Cao, Tongfei Qi, Gang Cao The recent discovery of colossal negative thermal expansion (NTE) in Ca$_2$Ru$_{1-x}$M$_x$O$_4$ (M=Cr, Mn, Fe and Cu) has highlighted a novel paradigm for NTE functional materials, where the onset of NTE traces the metal insulator (MI) transition temperature while a NTE anomaly is coupled to the magnetic order. This is in contrast to the conventional NTE behavior where electronic and magnetic properties play no roles. The nuclear and magnetic structures of Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$ (x=0.02, 0.05, 0.08 and 0.12) have been studied using neutron scattering. The effect of Fe-doping on two coexisting magnetic modes and its role in the abnormal thermal response of the lattice will be discussed. [Preview Abstract] |
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