Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T32: Focus Session: Transition Metal Oxides II |
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Sponsoring Units: GMAG DMP Chair: Robert Klie, University of Illinois at Chicago Room: 207B |
Thursday, March 5, 2015 11:15AM - 11:27AM |
T32.00001: Spin-orbital fluctuations in the paramagnetic Mott insulator (V$_{1-x}$Cr$_x$)$_2$O$_3$ Jonathan Leiner, Matthew Stone, Mark Lumsden, Wei Bao, Collin Broholm The phase diagram of rhombohedral V$_2$O$_3$ features several distinct strongly correlated phases as a function of doping, pressure and temperature. When doped with chromium for 180 K$<$T$<$450 K the material undergoes a Mott metal to insulator transition. Using the modern time-of-flight neutron scattering spectrometer SEQUOIA at the SNS, we have probed the excitation spectrum of (V$_{0.96}$Cr$_{0.04}$)$_2$O$_3$ in the PI phase and in the monoclinic commensurate anti-ferromagnet for T$<$180 K. An effective exchange Hamiltonian that accounts for the low T spin waves indicates alternating FM and AFM interactions for nearest neighbor spin pairs that are equivalent in the PI. We argue that the corresponding spin-orbital fluctuations are responsible for the extremely short-range dynamic spin correlations that we document in the PI phase. [Preview Abstract] |
Thursday, March 5, 2015 11:27AM - 12:03PM |
T32.00002: Large phonon entropy drives the metallization of vanadium dioxide (VO2) Invited Speaker: Jiawang Hong Vanadium dioxide (VO$_2$) exhibits a first-order metal-insulator transition (MIT) near room temperature, where conductivity is suppressed and the lattice changes from tetragonal to monoclinic on cooling. This MIT in VO$_2$ has attracted intense interest from both fundamental and technological perspectives. However, most studies performed in the past 50 years have focused on the electronic structure and energetics of the transition, ignoring the role of phonons and their entropic contribution to the phase stability. Much of the reason is that the standard tool of neutron scattering does not yield coherent scattering from V nuclei, and first-principles methods with harmonic approximation cannot capture the stable phonons for the rutile phase. We close this gap by using a combination of ab initio molecular dynamics calculations and neutron/x-ray scattering to establish that the entropy driving the MIT is dominated by soft, anharmonic phonons of the metallic phase.\footnote{J. D. Budai*, J. Hong*, M. E. Manley, E. D. Specht, C. W. Li, J. Z. Tischler, D. L. Abernathy, A. H. Said, B. M. Leu, L. A. Boatner, R. J. McQueeney and O. Delaire, Nature, (2014), doi:10.1038/nature13865} The MIT results from the competition between lower electronic energy in insulating M1 phase due to the Peierls instability, and the higher entropy of the metallic rutile phase resulting from soft anharmonic phonons. This understanding of the role of lattice dynamics and their relationship to electronic structure provides a critical component for developing more complete physical models of phase competition in functional transition metal oxides. [Preview Abstract] |
Thursday, March 5, 2015 12:03PM - 12:15PM |
T32.00003: Photoinduced phase transitions in vanadium dioxide revealed by ultrafast electron diffraction and mid-infrared spectroscopy Kunal Tiwari, Vance Morrison, Robert Chatelain, Ali Hendaoui, Andrew Bruhacs, Mohamed Chaker, Bradley Siwick The complex interplay between strong electron-electron correlations and structural distortions is thought to determine the electronic properties of many oxides, but the respective role of these two contributions is often difficult to determine. We report combined radio-frequency compressed ultrafast electron diffraction (RF-UED) and infrared transmissivity experiments in which we directly monitor and separate the lattice and charge density reorganizations associated with the optically induced semiconductor-to-metal phase transition in vanadium dioxide. These studies have uncovered a previously unreported photoinduced transition to a metastable phase retaining the periodic lattice distortion characteristic of the insulating phase, but differing by a reorganization of charge density along the vanadium dimer chains and a transition to metal-like mid IR optical properties. These results demonstrate that UED is able to follow details of both lattice and electronic structural dynamics on the ultrafast timescale. [Preview Abstract] |
Thursday, March 5, 2015 12:15PM - 12:27PM |
T32.00004: Elucidating the Band Gap of Niobium Dioxide Andrew O'Hara, Derek Vigil-Fowler, Steven G. Louie, Alexander A. Demkov Like VO$_{\mathrm{2}}$, niobium dioxide (NbO$_{\mathrm{2}})$ belongs to the family of transition metal oxides with a temperature-driven metal-to-insulator transition. However, NbO$_{\mathrm{2}}$ has received considerably less attention, and several open questions about the material remain. One such question, of both practical and fundamental importance, is the nature and size of the band gap in the low-temperature, distorted rutile phase with a range reported for the gap of 0.5 eV to 1.2 eV. In this work, we investigate the low-temperature phase, utilizing several methodologies - density functional theory within the standard local density approximation (LDA), LDA$+$U, hybrid functional, and the GW approximation, to better understand the physics of the band gap in NbO$_{\mathrm{2}}$. Comparisons of the calculations are made to recent experimental work on NbO$_{\mathrm{2}}$ utilizing photoemission spectroscopy and ellipsometry. This work is supported by DOE under the SciDAC program, the NSF, and SRC. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T32.00005: Orbital and magnetic states in BaV$_{10}$O$_{15}$ Sachith Dissanayake, Tianran Chen, Jooseop Lee, Matthew Stone, Masaaki Matsuda, Tomomasa Kazita, Takuro Katsufuji, Seunghun Lee BaV$_{10}$O$_{15}$ is a new type of frustrated magnet that exhibits interesting physics due to its charge and orbital ordering which has an average V valance of 2.8$+$. The V ions form a lattice with V ``boats,'' each of which is made of five V atoms and connected along ab plane. The system undergoes a structural phase transition at T$_{s}=$130 K and shows an antiferromagnetic long range ordering below T$_{N}=$43 K. Although there have been several studies about this complex V spin system, nature of the orbital and magnetic correlations is still unclear. In this talk I will discuss about our elastic and inelastic neutron scattering results obtained from a powder sample and a single crystal of BaV$_{10}$O$_{15}$. We investigated the magnetic ground states of BaV$_{10}$O$_{15}$ by neutron diffraction refinements and the observed magnetic excitations below 15 meV were analyzed by performing linear spinwave calculations. Another broad high energy excitation around 33 meV was observed at temperatures below the structural transition T$_{s}=$130 K. Possible orbital and magnetic states for this complex V spin system will be discussed. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T32.00006: Pressure-dependent Raman scattering study of CoV$_2$O$_4$ Taylor Byrum, Samuel Gleason, Haidong Zhou, S. Lance Cooper The $A$V$_2$O$_4$ ($A$ = Cd, Mn, Mg, Zn, Co) spinel family provides a fertile ground to explore the crossover between electron localization and itinerancy, which can be tuned with chemical ($A$-site substitution) and/or physical pressure. CoV$_2$O$_4$ resides closest to the predicted itinerant electron limit for the vanadium spinel family. Recent experiments have shown the existence of a critical V-V separation by inducing a metallic transition in CoV$_2$O$_4$ with a pressure of $\sim$6 GPa.\footnote{A. Kismarahardja, \textit{et al}., Phys. Rev. Lett. \textbf{106}, 056602 (2011).} While the structure of CoV$_2$O$_4$ has been theoretically suggested to remain cubic through the metallic transition,\footnote{R. Kaur, \textit{et al}., J. Phys.: Condens. Matter \textbf{26}, 045505 (2014).} no structural measurements under pressure have been performed. Raman spectroscopy is well suited to address this issue, as it is sensitive to even subtle structural changes. In this presentation, I present a pressure-dependent Raman scattering study of CoV$_2$O$_4$ to elucidate the role the structure plays in the metallic transition. [Preview Abstract] |
Thursday, March 5, 2015 12:51PM - 1:03PM |
T32.00007: Spin-glass insulating ground state in Y$_{2}$Os$_{2}$O$_{7}$ Zhiying Zhao, Stuart Calder, Michael McGuire, Brian Sales, Haidong Zhou, Jiaqiang Yan 4$d$/5$d$ transition-metal oxides can display many exotic physical properties due to the interplay between spin-orbit coupling (SOC), Coulomb interaction, crystal field effect, Hund's coupling, and lattice distortion. The magnetic ground state of systems with $d^{4}$ electronic configuration is under hot debate since the consensus between experiments and theories has not been achieved. A non-magnetic ground state is expected under the $J_{\mathrm{eff}}$ scenario in the presence of SOC. However, various ground states are observed in some $d^{4}$ systems (such as Ir$^{5+}$, Os$^{4+}$, and Ru$^{4+})$. In this talk, I will present our study on Y$_{2}$Os$_{2}$O$_{7}$ with nonmagnetic Y$^{3+}$ at A site which allows us to study the magnetism of Os$^{4+}$ ($d^{4})$ sublattice. Polycrystalline Y$_{2}$Os$_{2}$O$_{7}$ was synthesized by solid state reaction and was studied by measuring electrical resistivity, magnetic susceptibility, specific heat, and neutron powder diffraction. A spin-glass insulating ground state is observed in contrast to the long-range magnetic ordered state in Y$_{2}$Ru$_{2}$O$_{7}$. [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:15PM |
T32.00008: First-principles evidence of Slater-type metal-to-insulator transition in NaOsO3 Bongjae Kim, Zeynep Ergonenc, Cesare Franchini NaOsO$_3$ is thought to be the first example of a three-dimensional Slater insulator where the metal-to-insulator transition (MIT) is driven by the emergence of the (antiferro)magnetism in the system. This picture is fundamentally different from the most widely known Mott-Hubbard (MH) insulators for which the opening of the gap is due to electron correlation. Even though there are experimental evidences for the Slater-type sate in NaOsO$_3$ such as the continuous character of the transition and the simultaneous onset of the magnetic and insulating regime at the same temperature (T$_{MIT}$=T$_N$), no direct and unambiguous indication has been provided by the theory side. Based on \emph{ab initio} band structure methods, we have studied the origin of the MIT in NaOsO$_3$ and found that the MIT is predominantly driven by magnetism, and only marginally dependent on electronic correlation. A direct comparison between NaOsO$_3$ and other typical MH oxides allows us to analyze and interpret the distinct role of magnetism and the electronic correlation in favoring Slater- or MH-type states. Finally, fingerprints of the Slater behaviour are found in the optical response, that we have computed within a model Bethe-Salpeter scheme. [Preview Abstract] |
(Author Not Attending)
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T32.00009: Unconventional electronic state in half-frustrated Ca$_2$Os$_2$O$_7$ Marein Rahn, Roger Johnson, James Vale, Christian Donnerer, Pascal Manuel, Dmitry Khalyavin, Gareth Nisbet, Desmond McMorrow, Andrew Boothroyd In 5d transition metal oxides (5d TMOs), the combination of strong spin-orbit coupling and electronic correlation can lead to unusual ground states. Orthorhombic calcium osmate is a ``half frustrated'' compound with a strange phase transition at 327 K, enhancing its resistivity by an order of magnitude. We have investigated the concomitant magnetic ordering process by neutron powder diffraction and single crystal resonant x-ray diffraction. Our results reveal a complex reordering process, likely induced by magnetic frustration. Similar coupling of magnetic order and electronic transport has been in observed in other families of 5d TMOs. Given the itinerant character of these materials, one cannot explained such behaviour in a Mott-Hubbard scenario. We hope that Ca$_2$Os$_2$O$_7$ will serve as a model system to help understand this peculiar interplay of spin-orbit coupling, correlation and frustration. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T32.00010: Magnetic properties of the double perovskite Sr$_2$FeOsO$_6$: microscopic insights from ab-initio density-functional theory study Sudipta Kanungo, Binghai Yan, Martin Jansen, Claudia Felser Using density-functional theory calculations, we investigated the electronic and magnetic properties of the ordered 3d-5d double perovskite Sr$_2$FeOsO$_6$, which has recently drawn attention for interesting antiferromagnetic (AFM) phase transitions in low temperature observed in experiments. The calculated effective magnetic exchange interactions reveal the importance of long-range super-superexchange interactions in this compound. The competition between the ferromagnetic (FM) Os-O-Fe short-range interaction and AFM Os-O-Fe-O-Os long-range interaction induces strong magnetic frustration along the crystallographic c axis. This strong magnetic frustration is proposed to drive the magnetic phase transition between two AFM phases (AFM1 to AFM2) and related lattice distortion, which were also observed in the experiment. [Ref: Sudipta Kanungo, Binghai Yan, Martin Jansen, and Claudia Felser; Phys. Rev. B 89, 214414 (2014)] [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T32.00011: Structure and physical properties of R2Os2O7 Pyrochlores Ryan Rawl, Stuart Calder, Zhiying Zhao, Haidong Zhou, David Mandrus, Jiaqiang Yan The spin-orbit coupling (SOC) in 4d/5d transition metal oxides is enhanced to such a degree, relative to 3d oxides, that it can alter the electronic structure and have a dramatic effect on materials properties. A good example of this is the SOC assisted metal-insulator transition in Sr2IrO4, which has been extensively studied in the last few years. In a strong SOC scenario, a nonmagnetic ground state is expected for systems with d4 electronic configuration. R2Os2O7 pyrochlores, in which Os4$+$ has d4 electron count, provides a material playground to study the magnetism of d4 and effect of R3$+$-Os4$+$ interplay on the ground states. This series has only undergone limited studies, with the only published data investigating the lattice parameters and electrical resistivity at room temperature. In this talk, I will report our study on the magnetic, transport, thermodynamic, and structural properties of R2Os2O7 pyrochlores. [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T32.00012: Evolution of symmetry-broken states in the pseudogap regime of cuprates - the atomic structure footprints Emil Bozin, R. Zhong, K.R. Knox, B.L. Winn, G.D. Gu, J.P. Hill, J.M. Tranquada, S.J.L. Billinge Revealing the nature of the symmetry broken states in strongly correlated electron systems in general, and in the pseudo-gap (PG) phase of cuprates in particular, is instrumental in understanding the underlying properties. To that effect the knowledge of the local atomic structure may reveal relevant details important for more comprehensive understanding of the character of symmetry broken states in strongly correlated electron systems. Atomic pair distribution function (PDF) is one of the few experimental methods that can speak to this problem. Mounting experimental evidence suggests that the pseudogap phase may represent an electronic state in which the four-fold rotational symmetry of the CuO$_{2}$ planes is broken, pointing to stripe or nematic character. Systematic approach has been taken in charting both long and short range structural orders, on an equal footing, across the (x, T) phase diagrams of cuprates. For example, in La$_{2-x}$Ba$_{x}$CuO$_{4}$, by combining inelastic neutron scattering and neutron PDF approaches, we find evidence consistent with there being a dynamic symmetry breaking well above the charge ordering temperature and within the pseudogap regime. The response has non-monotonic doping dependence that peaks at 1/8 composition. [Preview Abstract] |
Thursday, March 5, 2015 2:03PM - 2:15PM |
T32.00013: Prediction of novel perovskite-type oxyhydride KTi(O$_2$H) with two-dimensional electronic states Nobuya Sato, Shinji Tsuneyuki Perovskite-type oxides $AB$O$_3$ have been widely studied for its dielectricity. The properties of these materials significantly change by substituting $A$ or $B$ cations, while compositions of $A$ and $B$ cations are limited to ($A^{2+}$, $B^{4+}$) and ($A^{3+}$, $B^{3+}$) for charge neutrality. Recently, it has been demonstrated that oxygen anions in BaTiO$_3$ can be partially substituted with hydride ions (H$^-$)\footnote{Y. Kobayashi {\it et. al.}, Nature Mater. {\bf 11}, 507 (2012).}, suggesting that more varieties of compositions might be realized (e.g., ($A^+$, $B^{4+}$)) combined with oxygen substitution. In this work, using first-principles calculations, we investigate an unsynthesized model composition with the oxygen substitution, KTi(O$_2$H). This compound is expected to crystalize into the perovskite-type structure from its tolerance factor. We confirm its stability with the crystal structure optimization and calculation of its formation energy for possible reactions. The material is insulating with large electronic polarization and has two-dimensional low-energy electronic states within Ti--O--H planes. For its unconventional composition and the peculiar two-dimensional property, this material may realize novel electronic properties. [Preview Abstract] |
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