Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session P6: Magnetic Complex Oxides IFocus
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Sponsoring Units: GMAG DMP Chair: Gian Guzman-Verri, University of Costa Rica / Argonne National Lab Room: 302 |
Wednesday, March 16, 2016 2:30PM - 3:06PM |
P6.00001: \textbf{High antiferromagnetic transition temperature for a layered hexagonal compound: SrRu}$_{\mathrm{\mathbf{2}}}$\textbf{O}$_{\mathrm{\mathbf{6}}}$ Invited Speaker: Jiaqiang Yan 4d or 5d transition metal oxides (TMOs) are less correlated and have a larger bandwidth than 3d TMOs. A high magnetic ordering temperature for 4d/5d TMOs is not expected. It was therefore a surprise when a perovskite, SrTcO$_{\mathrm{3}}$, was reported to order magnetically around 1000 K. Unfortunately, the radioactive nature of Tc prevented further investigation of the underlying mechanism for the high magnetic ordering temperature. Here we report antiferromagnetic order of SrRu$_{\mathrm{2}}$O$_{\mathrm{6\thinspace }}$at 565 K. Two features distinguish this compound from SrTcO$_{\mathrm{3}}$: (1) SrRu$_{\mathrm{2}}$O$_{\mathrm{6\thinspace \thinspace }}$is not radioactive, which allows the study of the underlying physics by a large variety of techniques as well as the possible fine tuning of the magnetic ground state; and (2) SrRu$_{\mathrm{2}}$O$_{\mathrm{6}}$ crystallizes into a quasi-two-dimensional structure with layers of edge-sharing RuO$_{\mathrm{6}}$ octahedra separated by nonmagnetic Sr layers. Our density functional calculations and Monte Carlo simulations suggest an origin of the reduced moment size and the high Neel temperature. [Preview Abstract] |
Wednesday, March 16, 2016 3:06PM - 3:18PM |
P6.00002: Molecular orbitals vs. relativistic orbitals in t$_{\mathrm{2g}}$ honeycomb lattices: SrRu$_{\mathrm{2}}$O$_{\mathrm{6}}$ as compared to Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$, RuCl$_{\mathrm{3}}$, and Li$_{\mathrm{2}}$RuO$_{\mathrm{3}}$. Igor Mazin, Sergey Streltsov, Kateryna Foyevtseva $t_{2g} $states on a honeycomb lattice tend to form non-dispersive localized states even if large intersite hopping is present. In the nonrelativistic case, these are molecular orbitals (MO) localized on metal hexagons, if the ligand-assisted nearest and next nearest neighbor hoppings, ${t}'_{1} $ and ${t}'_{2} $, dominate, or dimers (DO), if the direct overlap, $t_{1} $, dominates. In the ultrarelativistic limit $t_{2g} $ form effective relativistic orbitals (RO), $j_{eff} =\raise0.7ex\hbox{$3$} \!\mathord{\left/ {\vphantom {3 2}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{$2$}$, which are atomically localized if ${t}'_{1} $is the dominant hopping. On the first glance, the three regimes are defined by the conditions ${t}'_{1} \gg t_{1} ,\lambda $ or $t_{1} \gg {t}'_{1} ,\lambda $ or $\lambda \gg t_{1} ,{t}'_{1} $. In reality, the latter condition is never fulfilled, especially in ruthenates, yet not only Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$, but also RuCl$_{\mathrm{3}}$ appear to be in a regime dominated by RO, even though the residual effect of MO critically influences magnetic interactions, while Li$_{\mathrm{2}}$RuO$_{\mathrm{3}}$, not far removed from RuCl$_{\mathrm{3}}$ in the parameter space, is firmly in the DO regime. Most surprisingly, SrRu$_{\mathrm{2}}$O$_{\mathrm{6}}$, which is even closer to RuCl$_{\mathrm{3}}$, happens to be fully in the MO regime, with negligible spin-orbit effects. In this talk, we will show that an additional, decisive factor is the doping level per site. The principal difference between Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ or RuCl$_{\mathrm{3}}$, Li$_{\mathrm{2}}$RuO$_{\mathrm{3}}$, and SrRu$_{\mathrm{2}}$O$_{\mathrm{6}}$ is that the first two have one $t_{2g} $hole per site, the second one two holes, and the last three electrons. In particular, the total dominance of MO in the latter compound fully explains its unique and unexpected magnetic properties. [Preview Abstract] |
Wednesday, March 16, 2016 3:18PM - 3:30PM |
P6.00003: Magnetization reversal and negative volume thermal expansion in Fe doped Ca$_{2}$RuO$_{4}$ T. F. Qi, S. J. Yuan, F. Ye, S. Chi, J. Terzic, H. Zhang, Z. Zhao, X. Liu, S. Parkin, W. L. Mao, G. Cao We report structural, magnetic, transport and thermal properties of single-crystal Ca$_{2}$Ru$_{1-x}$Fe$_{x}$O$_{4}$ (0 $\le $ x $\le $ 0.2) as functions of pressure, magnetic field and temperature. The central findings of this work are a pronounced magnetization reversal and a negative thermal expansion that are induced by Fe doping. Our results including neutron diffraction data suggest that the magnetization reversal is primarily a result of different temperature dependences of two antiparallel, competing Ru and Fe sublattices and that the negative thermal expansion is achieved via magnetic and metal-insulator transitions. We will present and discuss our results with comparison drawn with relevant systems. [Preview Abstract] |
Wednesday, March 16, 2016 3:30PM - 3:42PM |
P6.00004: NMR study of new ruthenates with high magnetic ordering P.L. Paulose, Tanmoy Chakrabarty The Ru based compounds, Ca$_{3}$LiRuO$_{6}$ and Ca$_{3}$NaRuO$_{6}$ show unusually high magnetic ordering temperature. Extended super exchange model is invoked to explain the magnetic behavior in the isostructural compound Ca$_{3}$LiOsO$_{6}$. We have carried out NMR investigation on these two Ru-based compounds. Ca$_{3}$LiRuO$_{6}$ is a weak ferromagnet with a magnetic ordering temperature (T$_{C})$ of 115 K which is explored by the temperature dependence of $^{7}$Li NMR line shift, line-width and spin-lattice relaxation rate (1/T$_{1})$. Above T$_{C}$, a broad maximum is observed in the evolution of line-width of the spectra. We speculate that this feature might be attributed to some low-dimensional magnetic behavior. Contrastingly, Ca$_{3}$NaRuO$_{6}$ with similar structure and local geometry of the Ru$^{5+}$ ions is a conventional antiferromagnet with a transition temperature of 90 K. The temperature dependence of $^{23}$Na NMR line shift and 1/T$_{1}$ is studied across magnetic transition in Ca$_{3}$NaRuO$_{6}$. The temperature variation of line-width is found to be different compared to Ca$_{3}$LiRuO$_{6}$. In both these systems, 1/T$_{1}$ decreases significantly below ordering temperature, characteristic of many antiferromagnetic systems. [Preview Abstract] |
Wednesday, March 16, 2016 3:42PM - 4:18PM |
P6.00005: Spin-orbit dimers in double perovskites Invited Speaker: George Jackeli In Mott insulators, unquenched orbital degrees of freedom often frustrate the magnetic interactions and lead to a plethora of interesting phases with unusual spin patterns or non-magnetic states without long-range order. Here, we present a theoretical study of interplay of spin and orbital degrees in double-perovskite compounds with d$^1$ ions occupying the fcc sublattice. We show that the ground state of such a system is non-magnetic dimer pseudo-spin singlet with extensive orientational degeneracy of dimers. We discuss how the pseudo-spin state forming the singlet is altered upon increasing the strength of the relativistic spin-orbit coupling and show that the dimer 'gas' phase remains the ground state throughout. Our theoretical findings support and explain the experimentally observed non-magnetic amorphous valence bond state in Ba$_2$YMoO$_6$ and in related compounds. [Preview Abstract] |
Wednesday, March 16, 2016 4:18PM - 4:30PM |
P6.00006: Doping an antiferromagnetic insulator : A route to an antiferromagnetic metallic phase Priya Mahadevan, Shishir Pandey, D.D. Sarma Usually antiferromagnetism is accompanied by an insulating character of the ground state, while ferromagnetism is accompanied by metallicity. In the limit of half-filling, the Hubbard model yields an antiferromagnetic insulator as the ground state. From the Nagaoka theorem we expect ferromagnetism at any finite electron doping of this half filled state. Numerical studies on the other hand, have however shown, that at low doping concentrations one has a narrow region of an antiferromagnetic metallic phase. The question is whether this is realizable in real materials. Among the 3d transition metal oxides, this antiferromagnetic metallic phase has remained elusive as strong electron-phonon coupling results in a different phase diagram. The 5d transition metal oxides are therefore more suitable. In this work we solve a multiband Hubbard model relevant for a 5d transition metal oxide within a mean-field approach and show that the large bandwidth and the small intra-atomic Hund's exchange associated with this limit gives us a robust AFM-M ground state for 25\% electron doping. The conclusions are supported by ab-initio electronic structure calculations for NaOsO$_3$. [Preview Abstract] |
Wednesday, March 16, 2016 4:30PM - 4:42PM |
P6.00007: Excitations and enhanced coupling at the magnetic metal-insulator transition in NaOsO3 and Cd2Os2O7 S. Calder, J. H. Lee, M. B. Stone, J. G. Vale, C Donnerer, N. A. Bogdanov, J Lang, M Feygenson, X Liu, M. H. Upton, D. Casa, M. D. Lumsden, Z. Zhao, J.-Q Yan, Y.G. Shi, Y.S. Sun, Y. Tsujimoto, K Yamaura, D. Mandrus, S. Nishimoto, J. vd Brink, J. P. Hill, D. F. McMorrow, A. D. Christianson 5d oxides provide new paradigms of cooperative interactions that drive novel emergent behavior. This is exemplified in the osmates NaOsO$_3$ and Cd$_2$Os$_2$O$_7$ that host MITs where magnetic order appears intimately entwined. However, unlike the iridates where spin-orbit coupling (SOC) behavior dominates, in the 5d$^3$ osmates an orbital singlet is expected and reduced effect of SOC. We measure the inelastic spectra with neutrons and RIXS. Our results uncover the 5d-manifold splitting to reveal a suppressed role of SOC in the creation of the electronic ground state but dominant behavior in the creation of the magnetic state. Moreover at the MIT in NaOsO$_3$ we find a giant spin-phonon coupling and in Cd$_2$Os$_2$O$_7$ a magnetic excitation corresponding to a superposition of multiple spin-flips. [Preview Abstract] |
Wednesday, March 16, 2016 4:42PM - 4:54PM |
P6.00008: Terahertz Spectroscopy of Osmate Double Perovskites Matthew T. Warren, R. Morrow, T. T. Mai, J. Xiong, P. M. Woodward, R. Vald\'es Aguilar Double perovskites containing 5d transition metal elements allow study of the interplay of spin-orbit coupling and electronic correlations due to the heavy nuclei and large electronic wavefunctions. Here we have studied polycrystalline Sr$_{\mathrm{2}}$MOsO$_{\mathrm{6\thinspace }}$(M $=$ Mg, Fe, Co; with Os electronic configuration of d$^{\mathrm{2}}$, d$^{\mathrm{3}}$, d$^{\mathrm{2}}$, respectively) with time-domain terahertz spectroscopy. Terahertz electrodynamics seem to be decoupled from observed magnetic and structural phase transitions in M$=$Mg, Co. A strong absorption is measured in M$=$Mg, Co around 1.5 THz, which softens with temperature, as expected for an optical phonon. The effectiveness of the variable-range hopping model and the origin of higher temperature conductivity are examined. Work at OSU supported by the NSF MRSEC Center for Emergent Materials under Grant DMR-1420451. [Preview Abstract] |
Wednesday, March 16, 2016 4:54PM - 5:06PM |
P6.00009: Numerical Implementation of a General Spinwave Model to Simulate Spinwave Excitations Found in Inelastic Neutron Scattering Data D. Casavant, I. Brodsky, G. J. MacDougall Many important details regarding magnetism in a material can be inferred from the magnetic excitation spectrum, and in this context, general calculations of the classical spinwave spectrum are often necessary. Beyond the simplest of lattices, however, it is difficult to numerically determine the full spinwave spectrum, due primarily to the non-linearity of the problem. In this talk, I will present MATLAB code, developed over the last few years at the University of Illinois, that calculates the dispersions of spinwave excitations out of an arbitrarily defined ordered spin system. The calculation assumes a standard Heisenberg exchange Hamiltonian with the incorporation of a single-ion anisotropy term which can be varied site-by-site and can also simulate the application of an applied field. An overview of the calculation method and the structure of the code will be given, with emphasis on its general applicability. Extensions to the code enable the simulation of both single-crystal and powder-averaged neutron scattering intensity patterns. As a specfic example, I will present the calculated neutron scattering spectrum for powders of CoV2O4, where good agreement between the simulated and experimental data suggests a self-consistent picture of the low-temperature magnetism. [Preview Abstract] |
(Author Not Attending)
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P6.00010: Unusual behaviour of thermal conductivity in vanadium dioxide across the metal-insulator transition Kedar Hippalgaonkar, Sangwook Lee, Changhyun Ko, Fan Yang, Joonki Suh, Kai Liu, Kevin Wang, Xiang Zhang, Chris Dames, Junqiao Wu In an electrically conductive solid, the Wiedemann-Franz (WF) law requires the electronic contribution to thermal conductivity to be proportional to the product of electrical conductivity and absolute temperature , where the ratio is the Lorenz number, typically not much different from the Sommerfeld value L$_{\mathrm{0}} \quad =$ 2.44x10$^{\mathrm{-8}}$ W-ohm-K$^{\mathrm{-2}}$ at room temperature. The WF law reflects a basic property of metals where charge and heat are both carried by the same quasiparticles that both experience elastic scattering. At temperatures below the Debye temperature, the WF law has been experimentally shown to be robust in conventional conductors, with violations theoretically predicted or experimentally observed in strongly correlated electron systems or Luttinger liquids. However, the experimentally observed violations are at very low temperatures. Here we report breakdown of the WF law in a strongly correlated metal, in which the electronic thermal conductivity and L nearly vanish at temperatures above room temperature, where the electronic thermal conductivity amounts to only \textless \textasciitilde 5{\%} of the value expected from the WF law. [Preview Abstract] |
Wednesday, March 16, 2016 5:18PM - 5:30PM |
P6.00011: Effects of Paramagnetism and Electron Correlations on the Electronic Structure of MnO: \textit{Ab Initio} Study Sangmoon Yoon, Kyoungsuk Jin, Seoung-Hun Kang, Ki Tae Nam, Miyoung Kim, Young-Kyun Kwon Manganese oxide nanoparticles have attracted a lot of attentions as a promising candidate for next-generation catalyst. Therefore, understanding the electronic structure of manganese oxide in room temperature is highly required for the rational design of catalysts. We study the effects of paramagnetism and electron correlations on the electronic structure of MnO using \textit{ab initio} density functional theory. Spin configurations of paramagnetism are postulated as the ensemble average of various spin disorders. Each initial disordered spin configuration is randomly generated with two constraints on magnetic local moments. We first investigate the influence of magnetic ordering on the elctronic structure of MnO using noncollinear spin calculations and find that the magnetic disorders make valence band maximum more delocalized. Moreover, we examine the role of electron correlations in the electronic structure of paramagnetic MnO using DFT$+$U calculations. Strong electron correlations modify not only the size of band gap but also the magnitude of local moments as in the antiferromagnetic MnO. Besides, the initialized spin disorder remains almost unchanged as electron correlation get stronger. Furthermore, our results obtained by considering both strong electron correlation and paramagnetism confirm experimentally-observed oxygen K edge X-ray emission spectra $^{\mathrm{[1]}}$ reflecting the feature of valence bands. $^{\mathrm{[1]}}$ E. Z. Kurmaev et al., Phys. Rev. B. \textbf{77}, 165127 (2008). [Preview Abstract] |
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