Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session C18: Iridates IFocus
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Sponsoring Units: GMAG DMP Chair: Deepak Singh, Universityof Missouri Room: 317 |
Monday, March 14, 2016 2:30PM - 3:06PM |
C18.00001: Topological and unconventional magnetic states in transition metal oxides Invited Speaker: Gregory Fiete In this talk I describe some recent work on unusual correlated phases that may be found in bulk transition metal oxides with strong spin-orbit coupling. I will focus on model Hamiltonian studies that are motivated by the pyrocholore iridates, though the correlated topological phases described may appear in a much broader class of materials. I will describe a variety of fractionalized topological phases protected by time-reversal and crystalline symmetries: The weak topological Mott insulator (WTMI), the TI* phase, and the topological crystalline Mott insulator (TCMI). If time permits, I will also discuss closely related heterostructures of pyrochlore iridates in a bilayer and trilayer film geometry. These quasi-two dimensional systems may exhibit a number of interesting topological and magnetic phases. [Preview Abstract] |
Monday, March 14, 2016 3:06PM - 3:18PM |
C18.00002: Multipolar effects in Eu2Ir2O7 Yilin Wang, Xi Dai We use the density functional theory plus the rotationally invariant Hartree-Fock mean-field method to study the magnetic properties of the pyrochlore iridate material Eu$_2$Ir$_2$O$_7$ (5d$^5$), where the crystal field splitting $\Delta$, spin-orbit coupling (SOC) $\lambda$ and Coulomb interaction $U$ of Ir atoms are all playing significant roles. We have constructed a $t_{2g}$ Wannier tight-binding Hamiltonian and calculated the $U$-$\lambda$ phase diagram, from which we find a very stable all-in/all-out antiferromagnetic ground state for moderate SOC (0.2-0.5 eV). In this magnetic state, except for the dipole moments, we also find considerable multipolar moments (octupole) and large non-linear magnetic susceptibility. With strong enough SOC, the system reduces to a $j_{\text{eff}}=\frac{1}{2}$ single band Hubbard model, and the ground state changes to another antiferromagnetic configuration without multipolar moments. Our results indicate that the coexisting multipolar order is crucial to stabilize the all-in/all-out state and contributes a lot to the non-linear magnetic susceptibility. [Preview Abstract] |
Monday, March 14, 2016 3:18PM - 3:54PM |
C18.00003: Resonant X-ray scattering studies of magnetic order and excitations in pyrochlore iridates. Invited Speaker: Desmond McMorrow The rare-earth pyrochlore iridates (R$_{\mathrm{2}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7}}$, R$=$rare earth) have been proposed to host a number of exotic electronic states as a consequence of the existence of strong spin-orbit coupling of the Ir$^{\mathrm{4+}}$ ion in the presence of significant electron correlations. Of crucial importance to understanding whether any of these states can be realized in practice is to determine the effective low-energy Hamiltonian describing the system. Here we report a comprehensive series of resonant X-ray experiments, both elastic (REXS) and inelastic (RIXS), which reveal the nature of the magnetic order and excitations in single crystals of Sm$_{\mathrm{2}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7\thinspace }}$and Nd$_{\mathrm{2}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7}}$. [Preview Abstract] |
Monday, March 14, 2016 3:54PM - 4:06PM |
C18.00004: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 4:06PM - 4:18PM |
C18.00005: The interplay of ferromagnetic and antiferromagnetic exchanges in the the 3d-5d transition metal oxides Sr2BIrO6 (B=Ni, Cu, Zn) Katharina Rolfs, Ekaterina Pomjakushina, Sandor Toth, Vladimir Pomjakushin, Kazimierz Conder In the field of strongly correlated electron systems significant attention has been drawn towards the study of compounds based on magnetic 4d and 5d transition metal (TM) oxides. The spin orbit coupling (SOC) within these systems becomes non-negligible compared to the crystal field energies and leads to new exotic ground states, such as the Mott insulating state in Sr2IrO4. In order to understand the influence of the SOC on the electronic ground state the focus also turned to mixed 3d-5d systems, which gives the possibility to disentangle SOC effects from common charge-spin-orbital physics, as it is present in pure 3d TMOs and could also introduce new properties. One group within these candidates is the group of Ir-based double perovskites A2BIrO6 (B=3d TM). While in a large number of insulating 3d TMOs, the superexchange interactions between magnetic ions being nearest neighbour is adequate to determine the magnetic order, the SOC of 5d elements can change the exchange topology. This is possibly the case for Sr2NiIrO6, Sr2CuIrO6 and Sr2ZnIrO6. All compounds are high oxygen pressure compounds, which we successfully synthesised. The influence of the 3d metal on the magnetic properties will be discussed based on bulk magnetisation, transport measurements and neutron diffraction. [Preview Abstract] |
Monday, March 14, 2016 4:18PM - 4:30PM |
C18.00006: Magnetic Orders Proximal to the Kitaev Limit in Frustrated Triangular Systems: Application to Ba$_3$IrTi$_2$O$_9$ Andrei Catuneanu, Jeffrey Rau, Heung-Sik Kim, Hae-Young Kee Frustrated transition metal compounds in which spin-orbit coupling (SOC) and electron correlation work together have attracted much attention recently. In the case of 5$d$ transition metals, where SOC is large, $j_\text{eff}=1/2$ bands near the Fermi level are thought to encompass the essential physics of the material, potentially leading to a concrete realization of exotic magnetic phases such as the Kitaev spin liquid. We derive a spin model on a triangular lattice based on $j_\text{eff} = 1/2$ pseudo-spins that interact via antiferromagnetic Heisenberg ($J$) and Kitaev ($K$) exchanges, and crucially, an anisotropic $(\Gamma)$ exchange. Our classical analysis of the spin model reveals that, in addition to small regions of 120$^\circ$, $\mathbb{Z}_2$ / dual-$\mathbb{Z}_2$ vortex crystal and nematic phases, the stripy and ferromagnetic phases dominate the $J$-$K$-$\Gamma$ phase diagram. We apply our model to the 5$d$ transition metal compound, Ba$_3$IrTi$_2$O$_9$, in which the Ir$^{4+}$ ions form layered two-dimensional triangular lattices. By combining our ab-initio and classical analyses, we predict that Ba$_3$IrTi$_2$O$_9$ has a stripy ordered magnetic ground state. [Preview Abstract] |
Monday, March 14, 2016 4:30PM - 4:42PM |
C18.00007: Spin-Orbit Induced Emergent Magnetic Phases in Iridium Based Oxides Indra Dasgupta We shall present our results on the electronic structure of 6H perovskite type quaternary iridates Ba$_{3}$MIr$_{2}$O$_{9}$, where Ir ions form structural dimers and non magnetic M provides a knob to tailor the valence of Ir. We shall first consider the d$^{4.5}$ insulator Ba$_{3}$YIr$_{2}$O$_{9}$ and explain the origin of the pressure induced magnetic transition to a spin-orbital liquid (SOL) state in this system. As a next example [2], we shall consider a pentavalent (d$^{4}$) 6H perovskite iridate Ba$_{3}$ZnIr$_{2}$O$_{9}$ and argue that the ground state of this system is a realization of novel SOL state. Our results reveal that such a system provides a very close realization of the elusive J=0 state where Ir local moments are generated due to the comparable energy scales of the singlet-triplet splitting driven by spin-orbit coupling (SOC) and the superexchange interaction mediated by strong intra-dimer hopping, however substantial frustrated interdimer exchange interactions induce quantum fluctuations favoring SOL phase at low enough temperature. [1] S.K. Panda, S. Bhowal, Ying Li, S. Ganguly, Roser Valenti, L. Nordstrom, and I. Dasgupta Physical Review B (Rapid Communication), 2015 (Accepted for Publication), [2] A. Nag et. al. arXiv:1506.04312 [cond-mat.str-el] [Preview Abstract] |
Monday, March 14, 2016 4:42PM - 4:54PM |
C18.00008: Orbital-selective singlet dimer formation and suppression of double exchange in 4d and 5d systems Sergey Streltsov, Gang Cao, Daniel Khomskii One of the main mechanisms of ferromagnetic ordering in conducting materials is the double exchange (DE). It is usually supposed in DE model that the Hund’s coupling $J_H$ is much larger than electron hopping $t$; in this case one stabilizes the state with maximum spin per pair of ions, which finally leads to ferromagnetism in bulk systems. We show that in the dimerized $4d/5d$ transition metal oxides for which $J_H$ is reduced and $t$ is in contrast enhanced, another situation is possible, when formation of the spin-singlets on delocalized orbitals is more favorable. This leads to suppression of the DE and to a strong decrease of the total spin. The model calculations using the dynamical mean-field theory show that this effect survives even in the extended systems, not only for dimers. Such a situation is realized, e.g., in Y$_5$Mo$_2$O$_{12}$, CrO$_2$ under pressure and in many other $4d/5d$ based materials. Another mechanism, which may suppress DE and which is also typical for $4d/5d$ compounds is the spin-orbit coupling (SOC). We show on the example of Ba$_5$AlIr$_2$O$_{11}$, that in this system it is the combination of molecular-orbital formation and SOC that strongly decreases magnetic moment on Ir. [Preview Abstract] |
Monday, March 14, 2016 4:54PM - 5:06PM |
C18.00009: Investigation of frustrated antiferromagnet on the honeycomb lattice with an applied field Shenxiu Liu, Hongchen Jiang, Tom Devereaux Quantum spin-1/2 honeycomb XY antiferromagnet, or the equivalent hard-core boson system, with both nearest-neighbor J1 and next-nearest-neighbor J2 interactions is a representative frustrated system possibly hosting new phases of matter. Recent theoretical study suggests that this system may exhibit a series of incompressible states, which host fermionic elementary excitations rather than bosonic excitations, at small fixed filling factors or equivalent magnetic field strength. In this work, we will examine the theoretical prediction by directly studying the frustrated honeycomb J1-J2 XY model, using unbiased grand canonical density-matrix renormalization group technique. By searching for magnetization plateaus with an applied magnetic field, we will ultimately determine the presence of these incompressible states as well as their properties. For a more comprehensive study, different variants of this model, including the honeycomb J1-J2 Heisenberg antiferromagnet, will also be investigated. [Preview Abstract] |
Monday, March 14, 2016 5:06PM - 5:18PM |
C18.00010: Topological magnon bands in pyrochlore iridate thin films Pontus Laurell, Gregory A Fiete Thin films of pyrochlore iridates (A$_2$Ir$_2$O$_7$) have previously been studied using weak-coupling techniques such as DFT and DMFT. Here we approach the systems from the strong coupling limit. Since the pyrochlore iridates most likely reside in the difficult to access intermediate coupling regime, a strong coupling study should offer a complementary viewpoint to existing studies. We carry out a variational mean field calculation of the magnetic ground state configurations. We show that the all-in/all-out state, known as the bulk ground state, is generically present in the triangular-Kagome-triangular trilayers. This state can also be found in bilayer films, in specific parameter regimes. A linear spin-wave analysis of the magnetic excitations is also carried out. It shows that when the magnetic order is in (or close to) the all-in/all-out state, the lowest magnon band acquires a non-zero Chern number, leading to the prediction that pyrochlore iridate thin films can host the magnon Hall effect. [Preview Abstract] |
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