Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D13: Focus Session: Electronic Structures of Transition-Metal Oxides |
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Sponsoring Units: DMP GMAG Chair: Silke Biermann, Centre de Physique Theorique, Ecole Polytechnique Room: Colorado Convention Center Korbel 4C |
Monday, March 5, 2007 2:30PM - 3:06PM |
D13.00001: Probing the Electronic Structure of Metal Oxides using Resonant Inelastic Soft X-Ray Scattering and Soft X-ray Emission Spectroscopy. Invited Speaker: While photoemission spectroscopy is often the probe of choice in studying the electronic structure of solids, there are many sample and environmental constraints that must be satisfied before meaningful data can be obtained with this spectroscopy. Specifically, samples generally need to be electrically conducting single crystals, with atomically clean and ordered surfaces. Clearly, complimentary electronic structure probes applicable to non-crystalline samples, insulators, or samples with poorly controlled surfaces are highly desirable. I will discuss the application of two such techniques: synchrotron radiation-excited soft x-ray emission spectroscopy and resonant inelastic x-ray scattering. By virtue of being photon-in/ photon-out probes, these techniques can measure the electronic structure of solids in circumstances where photoemission spectroscopy is inapplicable. Soft x-ray emission spectroscopy provides a direct measure of the element- and site-specific local partial density of states, while resonant inelastic soft x-ray scattering measures element specific low energy excitations such as d-d$* $or charge transfer transitions. Recent examples of the application of these spectroscopies to a variety of metal oxide systems, including correlated and low dimensional systems, will be presented. \textit{Research supported in part by the Department of Energy under DE-FG02-98ER45680 and the AFOSR under FA9550-06-1-0157. Experiments were performed at the NSLS and the ALS, which are supported by the Department of Energy. } [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D13.00002: Hybridization of local Frenkel excitons in strongly interacting NiO Wei Ku, Chi-Cheng Lee, Hung-Chung J. Hsueh Recent experimentally observed tightly bound excitons in NiO are explained by a newly developed linear response theory within LDA+U approximation. A novel picture of local Frenkel excitons naturally emerges from a real space formulation of exciton formation using the energy- resolved Wannier functions. Systematic analysis of microscopic interacting processes reveals that the large 1 eV splitting between the excitons is due to a strong hybridization between the Frenkel excitons via strong local interactions. Our new picture can be viewed as a simplest representation of the charge excitation involving complex multiplet structure in strongly correlated systems. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D13.00003: Unusual magnetic ground state in MnO under pressure. Klaus Koepernik, Deepa Kasinathan, Warren E. Pickett A study of the phase transitions in MnO under pressure is presented. The calculations are based on density functional theory. The onsite correlations in the Mn 3d shell are treated within the framework of LSDA+U. The major result is that the first phase transition (with increasing pressure), which is characterized by an isostructural magnetic moment collapse from spin 5/2 to spin 1/2, results in a low spin solution exhibiting an unexpected intra-atomic spin polarization pattern. An analysis of the influence of the symmetry, the magnetic ordering and the LSDA+U interactions shows that this unusal spin arrangement is the result of inter-orbital exchange terms. The dependence of the results on the parameters U and J will be discussed. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D13.00004: Evolution of MnO under Pressure from Dynamical Mean Field Theory Warren E. Pickett, Kwan-woo Lee, Richard T. Scalettar, Jan Kune\v{s}, A.V. Lukoyanov, V.I. Anisimov Late transition metal oxides qualify as so called charge-transfer insulators whose description requires that the simple Hubbard interaction within the $3d$ orbitals has to be augmented by mixing with the ligand $2p$ states. MnO is a relatively simple realization (at ambient pressure) of such a system. Its pressure evolution at room temperature exhibits structural (B1--B8), magnetic (high spin -- low spin) and electronic (insulator -- metal) transitions, and correlated band theories$^1$ predict a S=5/2 to S=1/2 moment collapse. We report All-Electron + DMFT high-spin to low-spin or insulator to metal transitions, and a study of the paramagnetic fcc phase as volume is reduced, focusing on the behavior of the local magnetic moment and the metal-insulator transition. We also present single-particle excitation spectra that illuminate the character of the evolution.\\ $^1$D. Kasinathan et al., Phys. Rev. B {\bf 74}, 195110 (2006) [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D13.00005: Structural and Electronic Properties of Monoclinic TiO$_2$ (B) Polymorph Michel Posternak, Alfonso Baldereschi, Bernard Delley Three major polymorphs of TiO$_2$ are known: rutile, anatase, and brookite. A further phase, TiO$_2$ (B), which coexists with, and derives from natural anatase has recently been identified\footnote{J.F.Banfield \textit{et al.}, Am. Mineral. \textbf{76}, 343 (1991).}. It is monoclinic with $C^3_{2h}$ space group, and its conventional cell contains 8 TiO$_2$ formula units. Using the DMol$^3$ approach\footnote{B. Delley, J. Chem. Phys. \textbf{113}, 7756 (2000).}, we study the structural and electronic properties of this polymorph in terms of the OTi$_3$ complex, that we have recently shown\footnote{M. Posternak \textit{et al.}, Phys. Rev. B \textbf{74}, 125113 (2006).} to be the relevant building block for describing the electronic properties of the three major polymorphs. At variance with these latter cases, the 16 O atoms in TiO$_2$ (B) are \textit{not} all threefold coordinated: indeed, 12 O atoms belong to anataselike OTi$_3$ structural units, and the remaining 4 O atoms are twofold coordinated. The outcome of structural differences on the electronic properties of the TiO$_2$ phases is analyzed. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D13.00006: Photoinduced charge and spin dynamics in strongly correlated electron systems Hiroaki Matsueda, Sumio Ishihara It is widely recognized that the competition among multiple phases is a key issue to understand electronic properties in strongly correlated electron systems. A tiny amount of external perturbation breaks balance among these phases, and gigantic response appears. Photoirradiation by the femtosecond pulse laser is a powerful tool to induce the response. For understanding the mechanism of the response after the photoirradiation, pump-probe spectroscopy measurements on perovskite manganese oxides have been performed in recent years. The main issue is the photoinduced transition between ferromagnetic metallic and charge-ordered insulating (COI) phases. Motivated by the transition, we examine the effect of the photoirradiation on the COI phase in the extended double-exchange model. We calculate the transient optical absorption spectrum by the density matrix renormalization group method. The COI once goes to a metallic state, and the metal tends to go back to the initial COI. The antiferromagnetic spin correlation of the localized spins in the ground state is greatly suppressed. Possible scenarios for photoinduced ferromagnetism are discussed. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D13.00007: Novel electronic and magnetic properties of a new class of cupper oxides Xiao Hu, Xiangang Wan, Masanori Kohno Cuprates have not been considered seriously as candidate of useful magnetic material since the known ferromagnetic cuprates show quite low \(T_c\). The recently reported cuprate Sr\(_8\) CaRe\(_3\)Cu\(_4\)O\(_{24}\), a Mott insulator with perovskite structure, exhibits surprisingly macroscopic magnetization up to \(T_c=440 K\). Doing LSDA+U calculations, we reveal theoretically [X.-G. Wan, M. Kohno, and X. Hu, Phys. Rev. Lett. vol. 94, 087205 (2005).] that an orbital order appears in Cu atoms which results in a ferrimagnetic ground state, and that the pd\(\sigma\) bonds are responsible to the strong super exchange interactions and thus the high \(T_c\). We propose a spin model and perform quantum Monte Carlo simulations, with which we can reproduce accurately the observed magnetization curve including the critical point \(T_c\). Moreover, a half-metal (HM), which behaves as metal for one spin channel and insulator for the opposite, is predicted when replacing Re with W or Mo [X.-G. Wan, M. Kohno, and X. Hu, Phys. Rev. Lett. vol. 95, 146602 (2005)]. Hole doping into the material will also result in HM, with the net magnetic moment changing from negative to positive upon tuning the doping rate. Therefore, an antiferromagnetic HM, a material which has yet been found so far, may be achieved where net magnetization disappears. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D13.00008: Theory of thermopower in strongly correlated electron systems Wataru Koshibae, Sadamichi Maekawa We have studied the effects of spin and orbital degrees of freedom in the strongly correlated electron systems, and have derived the formula of the high-temperature thermopower: \begin{equation} Q = - (k_B/e)\ln (g_e/g_h) - (k_B/e)\ln [n_h/(1 - n_h)], \end{equation} where $n_h$ is the hole concentration, and $g_e$ ($g_h$) denotes the local degeneracy of the electronic configuration on the transition metal ion without (with) hole carrier. The local degeneracy is determined by the spin and orbital degrees of freedom. It has been established that the formula (1) gives a good estimation of the thermopower in not only the 3$d$ transition metal oxides but also the 4$d$ ones, recently. We have studied the thermopower in the oxides composed of several kinds of transition metal ions. Its high-temperature formula shows a complicated expression, however, it is expressed to be the average of the first term of the equation (1) in the case that $n_h = 0.5$, in the double perovskite system. This is because the thermopower is nothing but the entropy carried by the electric current. We will discuss the thermopower of the oxides with several kinds of transition metal ions in the light of the theory. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D13.00009: Hybrid density functional study of Mott transition in MnO Cristian V. Diaconu, Richard L. Martin, Ionut D. Prodan, Gustavo E. Scuseria The electronic structure, the magnetic moment and volume collapse of MnO under pressure is obtained from hybrid density functional theory using the recently developed screened hybrid exchange-correlation functional of Heyd, Scuseria and Ernzerhof (HSE). We study two crystal structures for MnO: cubic (rock salt) and hexagonal (nickel arsenide). We find two antiferromagnetic states for the NaCl structure: a high-spin state that couples two $S=5/2$ moments, and a low-spin state that couples two $S=1/2$ moments. At ambient pressure the high-spin state lies lowest. The low-spin phase becomes favored at a pressure of about $248\,\mathrm{GPa}$, leading to a first order volume collapse. However, this transition is pre-empted by another first-order volume collapse at $178\,\mathrm{GPa}$ from the NaCl structure to a NiAs structure. This transition is predicted to be insulator to metal and is the realization of the Mott transition. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D13.00010: Disorder driven quantum phase transitions in transition metal oxides Kohjiro Kobayashi, Nandini Trivedi We investigate the effect of disorder on a class of transition metal oxides described by a single orbital Hubbard model at half filling and away from half filling. The phases are characterized by the nature of the electronic and spin excitations. We calculate the local density of states, frequency and temperature-dependent conductivity and spin susceptibility as functions of disorder and interaction. The interplay of disorder and correlations produces ususual behavior in the correlated metal, for example, characteristic suppression of density of states at low energies, persistence of gap like features at finite frequency and the presence of local moments. Some of these puzzles can be understood in terms of an inhomogeneous system composed essentially of two-components. We compare our results with recent local scanning tunneling spectroscopy, and optical conductivity measurements. Reference: D. Heidarian and N. Trivedi, Phys. Rev. Lett. 93, 126401 (2004); K. Kobayashi, B.H. Lee, and N. Trivedi, cond-mat. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D13.00011: ABSTRACT WITHDRAWN |
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