Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W6: Focus Session: Emergent Properties in Bulk Complex Oxides: Strongly Spin-Orbit Coupled Systems |
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Sponsoring Units: DMP Chair: Mark Dean, Brookhaven National Laboratory Room: 108 |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W6.00001: Novel Magnetism from the Spin-Orbit-Coupling Strength Alternation Weiguo Yin, X. Liu, A.M. Tsvelik, M.P.M. Dean, M.H. Upton, Jungho Kim, D. Casa, A. Said, T. Gog, T.F. Qi, G. Cao, J.P. Hill We show that bringing close two magnetic ions with strong and weak spin-orbit coupling, respectively, can yield strong ferromagnetic anisotropy from antiferromagnetic superexchange. We applied this novel exchange anisotropy generating mechanism to explain the unique magnetism of the copper-iridium oxide Sr$_3$CuIrO$_6$ containing chains of alternating Cu(II) and Ir(IV) ions. The calculated large-gap spin excitation spectrum agrees well with the Ir $L_3$ edge resonant inelastic x-ray scattering experiment. Our findings point to novel magnetic behavior to be expected in mixed $3d-5d$ transition-metal systems via exchange pathways that are absent in pure $3d$ or $5d$ systems. Reference: W.-G. Yin et al., PRL 111, 057202 (2013). [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W6.00002: J$_{\mathrm{eff}}=$1/2 Mott spin-orbit insulating state close to the cubic limit in Ca$_{4}$IrO$_{6}$ S. Calder, G.-X. Cao, S. Okamoto, J.W. Kim, V.R. Cooper, Z. Gai, B.C. Sales, M.D. Lumsden, D. Mandrus, A.D. Christianson The J$_{\mathrm{eff}}=$1/2 Mott spin-orbit insulating state is manifested in systems with large cubic crystal field splitting and spin-orbit coupling that are comparable to the on-site Coulomb interaction, U. 5d transition metal oxides host parameters in this regime and recently strong evidence for this state in Sr$_{2}$IrO$_{4}$, and additional iridates, has been presented. All the candidates, however, have distorted octahedra, such as the elongation along the c-axis in Sr$_{2}$IrO$_{4}$, and consequently a non-cubic local crystal field environment. Consequently the materials form a mixed J$_{\mathrm{eff}}=$1/2,3/2 ground state. The lack of a material with an unmixed J$_{\mathrm{eff}}=$1/2 has impacted the development and testing of robust models of this novel insulating and magnetic state. We present neutron diffraction, resonant x-ray scattering and DFT calculations that not only reveal Ca$_{4}$IrO$_{6}$ is a new candidate J$_{\mathrm{eff}}=$1/2 material with long-range magnetic order, but furthermore resides close to the required cubic limit. Both our experimental and theoretical investigation indicate Ca$_{4}$IrO$_{6}$ is uniquely positioned to act as a canonical system to investigate of the J$_{\mathrm{eff}}=$1/2 state. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W6.00003: Quantum Critical Phases in Strong Spin-Orbit coupling Systems: Application to Pyrochlore Iridates Eun Gook Moon, Cenke Xu, Yong Baek Kim, Leon Balents We study quantum critical phases in strong spin-orbit coupling systems which are protected by underlying symmetry and topology. Three semi-metallic stable phases are considered, and the absence of the energy gap and density of states at the Fermi level induces non-trivial screening effect. Thus, the Ground states of the phases receive characteristic corrections from the long range Coulomb interaction. The standard renormalization group method is used to investigate properties of the ground states. Considering symmetry breaking terms, we obtain phase diagrams, and generic features of the quantum critical theories between the phases are discussed. We apply our theory to pyrochlore iridates and implication of the presence of the quantum critical phases is discussed. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W6.00004: Strongly spin-orbit coupled Mott insulators Invited Speaker: Hidenori Takagi |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W6.00005: Enhancement of spin-orbit interaction by electron correlation Hiroki Isobe, Naoto Nagaosa We discuss the interplay between relativistic spin-orbit interaction (SOI) and electron correlation, and report the enhancement of the effective strength of SOI by electron correlation. Here we consider a two-site model of $t_{2g}$ orbitals to show that there are cases where the SOI is effectively enhanced by Hund's coupling. Since the strong SOI usually requires heavy and rare elements, it is desirable to realize a strongly spin-orbit coupled electronic system with lighter and abundant elements, especially from the viewpoint of applications. This finding of the effective enhancement of SOI by electron correlation will pave the way for the realization of such systems. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W6.00006: Anisotropic magnetic interactions in 5d iridium oxides by many-body quantum chemistry calculations Vamshi M. Katukuri, Satoshi Nishimoto, Viktor Yushankhai, Ioannis Rousochatzakis, Liviu Hozoi, Jeroen van den Brink Ir 5d$^5$ oxides are being actively studied due to the realization of novel spin-orbit coupled j$_{eff}\approx$1/2 ground states. One remarkable feature in these compounds is the highly anisotropic magnetic interactions, orders of magnitude stronger than in 3d oxides. We address the nature of the anisotropic exchange in the 2D honeycomb (Na/Li)$_2$IrO$_3$ ((Na/Li)213) and square-lattice (Sr/Ba)$_2$IrO$_4$ ((Sr/Ba)213) iridates, by ab initio multireference configuration-interaction calculations on large embedded clusters. For Na213 we find that the Kitaev term is ferromagnetic and defines the dominant energy scale while the nearest-neighbor Heisenberg contribution is antiferromagnetic. Although Li213 is structurally similar, we predict quite different set of interaction parameters in Li213. We further analyze the magnetic order and the essential differences between these two materials by exact diagonalization and density-matrix renormalization-group calculations that additionally include 2nd and 3rd neighbor couplings. Sizable symmetric anisotropic interactions are also computed for Ba214. From the ab initio data, the relevant in-plane spin model for Ba214 turns out to be a Heisenberg-compass effective model. We finally discuss the Dzyaloshinskii-Moriya exchange in Sr214. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W6.00007: Tunnelling into the twisted Mott insulator Sr2IrO4 with atomic resolution Armin Ansary, John Nichols, Noah Bray-Ali, Gang Cao, Kwok-Wai Ng We studied the single-layered iridate Sr$_2$IrO$_4$ with a scanning tunneling microscope. The finite low temperature conductance enables the electronic structure of this antiferromagnetic Mott insulator to be measured by tunneling spectroscopy. We imaged the topography of freshly cleaved surfaces and measured differential tunneling conductance at cryogenic temperatures. We found the Mott gap in the tunneling density of states to be 2$\Delta$ = 615 meV. Within the Mott gap, additional shoulders are observed which are interpreted as inelastic loss features due to magnons. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W6.00008: Topological and magnetic phases in the honeycomb Iridates Stephan Rachel Iridates are amongst the most interesting complex oxide materials. The non-interacting band structure of the honeycomb Iridates has been claimed to feature the quantum spin Hall effect due to large spin orbit coupling. The true materials exhibit considerable Coulomb interactions leading to different types of magnetic order (e.g., zig-zag or spiral order). Here we show how one can obtain such magnetic phases by combining topological band structure and local Coulomb interactions into a topological Hubbard model which we analyze in detail. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W6.00009: Effective J=1/2 insulating state in Ruddlesden-Popper iridates: An LDA+DMFT study Hongbin Zhang, Kristjan Haule, David Vanderbilt Using {\it ab-initio} methods, we\footnote{H. Zhang, K. Haule and D. Vanderbilt, arXiv:1308.4471 (2013).} investigate the metal-insulator transition across the Ruddlesden-Popper (RP) iridates and explore the robustness of the effective $J\!=\!1/2$ insulating state\footnote{B.J. Kim {\it et al.}, Phys. Rev. Lett. {\bf 101}, 076402 (2008).} against band effects due to itineracy, tetragonal distortion, octahedral rotation and Coulomb interaction. The electronic structures we obtained are in good agreement with recent ARPES measurements.\footnote[3]{B.M. Wojek, {\it et al.}, J. Phys.: Condens. Matter {\bf 24}, 415602 (2012).}$^,$\footnote[4]{Q. Wang, {\it et al.}, Phys. Rev. B {\bf 87}, 245109 (2013).}$^,$\footnote[5]{Y. Nie, P. King, and K. Shen, private communication.} We predict the effects of epitaxial strain on the optical conductivity, magnetic moments and effective $J\!=\!1/2$ ground-state wave functions in the RP series. We demonstrate that the deviation from the ideal effective $J\!=\!1/2$ state is negligible at short time scales for both Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$, while it becomes quite significant for Sr$_3$Ir$_2$O$_7$ at long times and low energy, leading to a reconciliation of previous contradictory experimental results. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W6.00010: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W6.00011: Spin orbit coupled magnetism and transport in double perovskites Arun Paramekanti, Ashley Cook We consider a model of the double perovskite Ba$_2$FeReO$_6$, a room temperature ferrimagnet with correlated and spin-orbit coupled Re t$_{2g}$ electrons moving in the background of Fe moments stabilized by Hund's coupling. We show that for such 3d/5d double perovskites, strong correlations on the 5d-element (Re) are essential in driving a half-metallic ground state. Incorporating both strong spin-orbit coupling and the Hubbard repulsion on Re, we uncover (i) Weyl nodes in the band structure, (ii) a significant anomalous Hall effect with hole doping, and (iii) a large spin polarization at the Fermi level. We also obtain a semi-quantitative understanding of (i) the saturation magnetization of Ba$_2$FeReO$_6$, (ii) X-ray magnetic circular dichroism data indicating a significant orbital magnetization, and (iii) the tetragonal distortion accompanying ferrimagnetic order. Strong correlations also lead to local moment formation on Re, and the calculated dynamic spin structure within a local moment picture is in good agreement with neutron scattering experiments. We will also discuss generalizations to other 3d/5d oxides. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W6.00012: Spin waves in half-metallic double perovskite magnets via Dyson's method Kelly Reidy, Katherine Jones-Smith, Harsh Mathur A new method for treating strongly correlated electronic matter is applied to an idealized model of double exchange magnetism and to the half-metallic double perovskite $Sr_{2}FeMoO_{6}$, a conducting material that is magnetically ordered at room temperature and potentially useful for spintronics. Our method [1] is a supersymmetric generalization of Dyson's analysis of spin waves in ferromagnets wherein quasiparticles are governed by a non-Hermitian Hamiltonian. Using this method, we obtain the spin wave dispersion relation for the idealized model and for the double perovskite material. We also reformulate these problems using a supersymmetric generalization of Schwinger boson mean-field theory. The results will be compared to a semi-classical exact diagonalization Monte-Carlo analysis [2], and broader implications of the method of non-Hermitian quasiparticles will be discussed. [1] Katherine Jones-Smith, Phil. Trans. Roy. Soc. A28, 371 (2012). [2] O. Erten et al., Phys. Rev. Lett. 107, 257201 (2011) [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W6.00013: Noncollinear magnetic order in quadruple perovskite LaMn$_3$V$_4$O$_{12}$ Masayuki Toyoda, Kunihiko Yamauchi, Tamio Oguchi The microscopic mechanism of noncollinear magnetic order in quadruple perovskite LaMn$_3$V$_4$O$_{12}$ has been investigated by first-principles density-functional theory calculations. Unlike other provskite-type manganites, Mn ions are located in the A'-sites that are surrounded by four oxygens with square-planar geometry, whereas the octahedrally coordinated B-sites are occupied by magnetically inactive V ions. In order to understand the magnetic interactions between the Mn spins, magnetic exchange coupling constants are estimated by mapping the numerically calculated energies with constraints on spin orientations onto an effective Heisenberg model. The antiferromagnetic coupling between the 2nd-nearest neighbors is found to satbilize the noncollinaer magnetic order which is relatively stronger in the present compound than in other G-type antiferromagnetic quadruple perovskites such as YMn$_3$Al$_4$O$_{12}$ and LaMn$_3$Cr$_4$O$_{12}$. We will also discuss on possibility of the ferroelectricity in the present compound in analogy with CaMn$_7$O$_{12}$, which is a multiferroic quadruple perovskite with noncollinear magnetic order. [Preview Abstract] |
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