Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V17: Focus Session: Bulk Properties of Complex Oxides - 5d Oxides |
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Sponsoring Units: DMP GMAG Chair: Daniel Dessau, University of Colorado at Boulder Room: D174 |
Thursday, March 24, 2011 8:00AM - 8:36AM |
V17.00001: Spin-Orbit Interaction Rediscovered in Transition Metal Oxides Invited Speaker: The 5$d$-transition metal oxides are a class of novel materials that exhibit nearly every collective state known for solids. It is commonly expected that iridium oxides should be more metallic and less magnetic than their 3$d $and 4$f$ counterparts due to the extended nature of the $5d $orbitals. In marked contrast, many iridates are magnetic insulators that exhibit a large array of phenomena seldom or never seen in other materials. We review the anomalous physical properties of several iridates and address potential underlying mechanisms, which include strong orbital magnetism, the J$_{eff}$ = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ insulating state, and spin-orbit coupling; the latter strongly competes with other interactions to create an unusual balance between relevant degrees of freedom in this class of materials. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V17.00002: Resonant inelastic x-ray scattering study of charge ordering in CuIr2S4 Hlynur Gretarsson, Jung-Ho Kim, Diego Casa, Thomas Gog, Sang-Wook Cheong, Young-June Kim We present Ir L$_{3}$-edge resonant inelastic x-ray scattering (RIXS) spectra and resonant x-ray emission spectra (RXES) on the thiospinel, CuIr$_2$S$_4$, which has been attracting much interest due to intriguing metal-insulator transitions. At room temperature CuIr$_2$S$_4$ is metallic, but goes through a metal insulator transition at T$_{\rm{MI}} \sim$ 226 K due to the formation of charge order (CO) of Ir$^{3+}$ and Ir$^{4+}$ together with spin dimerization between Ir$^{4+}$ ions. By exposing the sample to x-ray below T $ = $ 50 K, the crystal symmetry goes from triclinic to tetragonal, accompanied by reduced resistivity. The RIXS signal was dominated by a broad and strong feature around 3 eV, arising from t$_{2g}$ to e$_{g}$ transition, but we were able to observe a clear signature of opening of the insulating gap across the metal-insulator transition. In addition, we also found that this gap is partially filled in the irradiation-induced phase. The emission spectra reveals the existence of an excited Ir-5d \emph{t$_{2g}$} state, which is hidden in the Ir L$_3$-edge XAS of CuIr$_2$S$_4$. The result indicates that the electronic reconstruction that takes place in the irradiation-induced phase comes from the Ir$^{3+}$ while the Ir$^{4+}$ dimers are unchanged. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V17.00003: Electron-doped Sr$_{2}$IrO$_{4-\delta}$ $(0\leq \delta \leq 0.04)$: Evolution of a disordered J$_{eff}=1/2$ Mott insulator into an exotic metallic state O.B. Korneta, T.F. Qi, S. Chikara, L.E. De Long, G. Cao, S. Parkin, P. Schlottmann Stoichiometric Sr$_{2}$IrO$_{4}$ is a ferromagnetic J$_{eff} = 1/2$ Mott insulator driven by strong spin-orbit coupling. Introduction of very dilute oxygen vacancies into single-crystal Sr$_{2}$IrO$_{4-\delta}$ $(\delta < 0.04)$ leads to significant changes in lattice parameters and drives a number of intriguing phenomena such as insulator-to-metal transition at $T_{MI} \approx 105K$, anomalous non-Ohmic behavior and an abrupt current-induced transition in the resistivity. Highly-anisotropic resistivity of the samples continues to decrease by several orders of magnitude below $T_{MI}$ without saturation to a residual limit at the lowest temperature studied $T=1.8K$. The low-temperature metallic state exhibits two distinct regimes (separated at $T\approx 52K$) of switching in the non-linear $I-V$ characteristics. The novel behavior illustrates an exotic ground state and constitutes a new paradigm for device structures. [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V17.00004: Magnetic ordering in Sr$_{2}$IrO$_{4}$ from first principles Sabina Ruiz-Chavarria, Gregorio Ruiz-Chavarria, Pablo de la Mora, Carlos Cosio-Castaneda, Gustavo Tavizon Sr$_{2}$IrO$_{4}$ is a layered compound with IrO$_{2}$ planes, separated by two SrO planes. Experimentally Sr$_{2}$IrO$_{4}$ shows weak ferromagnetism. This behavior can be assigned either as band magnetism or canted antiferromagnetic ordering. The latter has been confirmed by Arpes. With DFT calculations (using the WIEN2k package and Quantum Expresso) we show that the antiferromagnetic ordering is more stable than the ferromagnetic one, and due to the Dzyaloshinskii-Moriya rules there is a possibility of canted magnetic ordering. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V17.00005: Effect of spin-orbit coupling on the band structure, magnetic ground states and low energy excitations of double perovskites Onur Erten, Anamitra Mukherjee, Mohit Randeria, Nandini Trivedi, Patrick Woodward We investigate a model for double perovskites A$_{2}$BB$^{\prime}$O$_{6}$ that describes the coupling of local moments on the B site to itinerant electrons contributed by the B$^\prime$ sites. To model materials like Sr$_2$CrOsO$_6$ we examine the role of spin-orbit coupling on the the B$^\prime$ site, which cannot be ignored because of the large $Z$ of Os. First, we present $T=0$ results for the net moment in the ferrimagnetic state. We show that direct B$^\prime$-B$^\prime$ hopping plays just as important role as the spin orbit coupling in determining the ordered moment. We use our model Hamiltonian approach to discuss the question of metallic versus insulating ground states, by including the effects of Coulomb $U$ on the spin-orbit split electronic structure. Finally, we investigate the low energy excitations of this model to understand the origin of the experimentally observed nonmonotonic behavior of magnetization as a function of temperature. [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V17.00006: Magnetic and structural properties of Sr2CrReO6 epitaxial films fabricated by ultra-high vacuum sputtering A.J. Hauser, B. Peters, J. Soliz, R.E. Williams, M. Dixit, H.L. Fraser, P.M. Woodward, F.Y. Yang Sr2CrReO6, a double-perovskite half-metallic ferromagnet, has attracted much attention due to its high Tc of 620 K. However, balancing the stoichiometry and ordering of a quaternary oxide is no trivial matter. We have deposited pure-phase Sr2CrReO6 epitaxial films on SrTiO3 substrates by ultrahigh vacuum off-axis magnetron sputtering with precise control of the oxygen partial pressure and in-situ monitoring by high-pressure residual gas analyzer. The films exhibit saturation magnetization at T = 5K approaching 0.9 Bohr magnetons per formula unit and Tc close to 600 K. X-ray diffractometry spectra demonstrate epitaxy and phase purity with a rocking curve FWHM of 0.012 degrees. Laue oscillations give evidence of exceptionally smooth surface and interface as well as precise film thickness determination. Finally, direct observation of the films by HAADF STEM show nearly defect free films with double-perovskite ordering. We will discuss the effects of stoichiometry, growth pressure and oxygen content on sample properties. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V17.00007: Electron doped CrO$_2$: An unsual example of a charge ordered ferromagnet Priya Mahadevan, Abhinav Kumar, Debraj Choudhury, D.D. Sarma Usually metallicity accompanies ferromagnetism. K$_2$Cr$_8$O$_{16}$ is one of the less common examples of magnetic materials, exhibiting ferromagnetism in the insulating state [1]. Analyzing the electronic and magnetic properties within first principle electronic structure calculations, we find [2] that K acts like a donor. The doped electrons associated with the introduction of K in the lattice, induces a charge ordered and insulating ground state and interestingly also introduces a ferromagnetic coupling between the Cr ions. The primary considerations driving the charge ordering are found to be electrostatic ones with the charge being localized on two Cr atoms that minimize the electrostatic energy. The structural distortion that accompanies the ordering, generates a pathway for the electron localized on one site to hop on to the neighboring sites, a process more favorable in the ferromagnetic case, thus, giving rise to a rare example of a charge-order driven ferromagnetic insulator. \\[4pt] [1] Kunihiro Hasegawa {\it et al.}, Phys. Rev. Lett. {\bf 103}, 146403(2009). \\[0pt] [2] Priya Mahadevan, Abhinav Kumar, Debraj Choudhury and D.D. Sarma, Phys. Rev. Lett {\bf 104}, 256401 (2010). [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V17.00008: Ab initio study of the anti-ferromagnetic, non-collinear CuB$_2$O$_4$ Crystal Yiing-Rei Chen, P.-R. Lee, J.-Y. Lin, J.-M. Chen, A.N. Vasiliev The spiky features in the crystal absorption spectrum, and the distinct differences in the directional oxygen K-edge absorption spectroscopy of the non-collinear anti-ferromagnetic, incommensurate CuB$_2$O$_4$, had led us to this LDA+U study of the crystal, although in the commensurate phase, due to the instrumental limitation. The calculated band structure matches the spiky features in the absorption spectrum, while the orbital analyzed DOS data explain the differences in the directional oxygen K-edge absorption spectroscopy. The two groups of dispersion-less bands, immediately above the gap, come from different groups of plaquettes, of Cu(A) and Cu(B), and are responsible for the spiky features observed experimentally. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V17.00009: Doping Rules in A$_{2}$BO$_{4}$ Spinel Oxides Alex Zunger, T. Paudel, V. Stevanovic, S. Lany Many of the physical phenomena surrounding Complex Oxide involve the creation and annihilation of charge carriers by cross --substitution of atoms or by the formation of vacancies and interstitials. We have used the machinery of First-Principles defect calculation, developed and tested over the years on semiconductors (where experimental data needed to test DFT corrections is rather clear), applying it to a large number of oxides, initially from the Spinel family. We calculate defect formation energies as a function of temperature and oxygen partial pressure, as well as the concentration of donors and acceptors and the ensuing free carriers. A number of regularities emerge. (i) Oxygen vacancies are not a viable source of electrons and cation vacancies are (usually) not a viable source of holes. (ii) Instead, cation-anti-sites (A-on-B donor and B-on-A acceptors) tend to form in significant numbers and release carriers. (iii) For the group of A3+ and B2+ spinels we find four ``doping classes'' (a) both donor and acceptor are in the gap (Al$_{2}$MgO$_{4})$ (b) Only acceptor is in the gap (Co$_{2}$ZnO$_{4})$ (c) only donor in the gap and (d) none in the gap. Simple regularities can be used as first-order rules to guess electrical behavior from composition. This work was supported through the Center for Inverse Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V17.00010: Magnetoelectricity and magnetostriction due to the rare-earth moment in TmAl$_{3}$(BO$_{3}$)$_{4}$ Rajit Chaudhury, B. Lorenz, Y.Y. Sun, C.W. Chu, L. N. Bezmaternykh, V.L. Temerov We investigated the magnetic, magnetostriction and magnetoelectric properties of d-electron free rare-earth aluminum borate TmAl$_3$(BO$_3$)$_4$ between room temperature and 2 K. The magnetoelectric polarization along the `a' and `c' directions reaches up to 300 $\mu$ C/m$^2$ at 70 kOe with the field is applied along the `a' axis. `c' axis magnetic field does not show any significant effect, which correlates with the fact that $\chi_a$ changed very rapidly compared to $\chi_c$. We find that the polarization is proportional to the magnetostriction. The result of this investigation prove the existence of a significant coupling between the rare-earth magnetic moment and the lattice in RAl$_3$(BO$_3$)$_4$ compounds (R=rare earth). This compound shows that the rare-earth moment is sufficient enough to generate a large magnetoelectric effect. This is comparatively a simpler system to study and understand the origin of magnetoelectric effect. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V17.00011: On the Pairing Instability in Rutile MO$_2$ L. Andrea Salguero, Craig J. Fennie The metal-dioxide family of compounds such as TiO$_2$, SnO$_2$, VO$_2$, NbO$_2$, and WO$_2$ have been of much recent interest for reasons as diverse as understanding novel correlated electron physics to designing new photo/electro-catalysis. All can be thought of as forming in a prototypical te\-tra\-gonal ``rutile'' structure, yet members with unpaired d$^1$ and d$^2$ electrons undergo a structural phase transition to a monoclinic, ``distorted rutile,'' structure. In some cases this metal pairing transition accompanies a metal to insulator transition, the precise role, however, is still not clear. Here we present a comparative first-principles study of the lattice instabilities in d$^1$ and d$^2$ MO$_2$ rutile, paying particular attention to the d$^2$ system WO$_2$, which remains metallic even in the distorted phase. We calculate the phonon dispersion in the tetragonal prototypical structure. Using unstable high symmetry modes as a guide, we calculate the energy surface around the high-symmetry structure and perform full structural relaxations in the distorted ground states. We elucidate the interplay between the electronic structure and the pairing transition and discuss the possibility of controlling it with strain. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V17.00012: Relativistic effect determines the oxidation states: a study of Rh and Ir oxides by first principles methods Maosheng Miao, Ram Seshadri The relativistic effect becomes significant on determining the structure and properties of $4d$ and $5d$ transition-metal compounds. It is found in some iridates that the relativistic effect, mainly contributed as spin-orbit interactions, can enhance the otherwise weak correlation of $5d$ electrons and cause an unusual Mott transition. Utilizing such effects in creating new phase such as topological insulator has grown into a hot spot in the frontier of functional oxide research. However, the relativistic effects on orbital energies, although important on determining the structure, has not been systematically studied. The general trend of the oxidation states of transition metals in the same group is to decrease with increasing atomic number. However, in contrast to this trend, Ir tends to form IrO$_{2}$ (4+) whereas Rh forms both Rh$_{2}$O$_{3}$ (3+) and RhO$_{2}$. Using relativistic and non-relativistic first principles calculations, we demonstrate that the unusually high oxidation state of Ir and the high stability of IrO$_{2}$ is caused by relativistic effect. Because relativity contracts the $s$ and $p$ orbitals, it repels Ir $5d$ electrons outwards and increases their energies. As a consequence, Ir tends to be oxidized to 4+ state and forms IrO$_{2}$. [Preview Abstract] |
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