Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B17: Focus Session: Iridate Mott Insulators |
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Sponsoring Units: DMP GMAG Chair: Stephen Wilson, Boston College Room: 319 |
Monday, March 18, 2013 11:15AM - 11:27AM |
B17.00001: Infrared study of the electronic structure of metallic pyrochlore iridate Bi$_2$Ir$_2$O$_7$ Yunsang Lee, S.J. Moon, Scott C. Riggs, M.C. Shapiro, I.R. Fisher, A.F. Kemper, D.N. Basov We investigated the electronic properties of a single crystal of metallic pyrochlore iridate Bi$_2$Ir$_2$O$_7$ by using the infrared spectroscopy. Our optical conductivity data show the splitting of t$_{\mathrm{2g}}$ bands into J$_{\mathrm{eff}}$ ones due to strong spin-orbit coupling. We observed a sizable mid-infrared absorption near 0.2 eV within the J$_{\mathrm{eff,1/2}}$ bands, which indicates that this material may belong to a class of correlated system. Our findings suggest that the electronic structure of Bi$_2$Ir$_2$O$_7$ is governed by the strong spin-orbit coupling and the correlation effect, which is prerequisite for theoretically proposed non-trivial topological phases in pyrochlore iridates. We also discuss possible existence of the very far-infrared region of suppression in the optical conductivity of the compound. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B17.00002: Electronic Structure of Spin-Orbital-Coupling-Driven Insulator Sr$_2$IrO$_4$ from Angle-Resolved Photoemission Spectroscopy Yan Liu, Xiaowen Jia, Daixiang Mou, Lin Zhao, Junfeng He, Guodong Liu, Shaolong He, Yingying Peng, Chaoyu Chen, Xiaoli Dong, Jun Zhang, Zuyan Xu, Chuangtian Chen, Gang Cao, X.J. Zhou Sr$_2$IrO$_4$, as a Mott Insulator, is an ideal system to study spin orbital coupling interaction in transition metal oxides. We report a comprehensive investigation on electronic structure of Sr$_2$IrO$_4$ by high resolution angle-resolved photoemission spectroscopy (ARPES). We measured the Fermi surface and band structures at different photon energies, under different photon polarizations. New features have been revealed that were not observed in previous studies. Moreover, the measurement under different polarizations helps identify different orbital characteristics of bands. The comparison between our experimental observations and theoretical calculation proves the important role of spin-orbital coupling interaction in determining its electron structure. The rich information on the electron structure of Sr$_2$IrO$_4$ will provide key insights in understanding the mechanism of various electron interactions in determining its insulator ground state. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B17.00003: Crystal field splitting and correlation effect on the electronic structure of $A_2{\rm IrO}_3$ Hlynur Gretarsson, J.P. Clancy, X. Liu, J.P. Hill, E. Bozin, Y. Singh, S. Manni, P. Gegenwart, J. Kim, A.H. Said, D. Casa, T. Gog, M.H. Upton, H.S. Kim, J. Yu, V.M. Katukuri, L. Hozoi, J.v.d. Brink, Y.J. Kim The electronic structure of the honeycomb lattice iridates Na$_2$IrO$_3$ and Li$_2$IrO$_3$ has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field split $d$--$d$ excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to non-cubic crystal fields, derived from the splitting of $j_{\rm{eff}}$=3/2 states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of $j_{\rm{eff}}$ physics in A$_2$IrO$_3$. We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both Na$_2$IrO$_3$ and Li$_2$IrO$_3$ can be described as spin-orbit Mott insulators, similar to the square lattice iridate Sr$_2$IrO$_4$. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B17.00004: Quasimolecular electronic structure of Na$_2$IrO$_3$ Igor Mazin, Harald Jeschke, Foyevtseva Kateryna, Roser Valenti, Daniel Khomskii Spin-orbit (SO) coupling can lead to many nontrivial effects such as Rashba effect, topological insulators, or topologically protected states in systems described the Heisenberg-Kitaev model, recently proposed for Na$_2$IrO$_3$. This proposal is based on the fact the SO coupling for iridium is very strong, and cannot be quenched by the small trigonal crystal field. We show, however, that Na$_2$IrO$_3$ represents a highly unusual case, in which the electronic structure is dominated by the formation of quasi-molecular composite orbitals (QMOs). The QMOs consist of six atomic orbitals on an Ir hexagon, and the orbital moment of each QMO is quenched, so that spin-orbit effects only affect the inter-QMO interaction. The concept of such composite orbitals in solids is completely new, and invokes very different physics compared to the models considered previously. For instance, one has to account for Hubbard correlations among the QMOs, and not individual atomic orbitals. Both the insulating behavior and the experimentally observed zigzag antiferromagnetism in Na$_2$IrO$_3$ naturally follow from the QMO model. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B17.00005: Magnetic properties of triangular lattice Ca$_4$IrO$_6$ and Ca$_{2.5}$Sr$_{1.5}$IrO$_6$ K.H. Butrouna, L. Li, T.F. Qi, O.B. Korneta, J. Terzic, E. Akbari, S. Parkin, S.J. Yuan, G. Cao We report a structural, thermodynamic, and transport study of single-crystal Ca$_4$IrO$_6$ and Ca$_{2.5}$Sr$_{1.5}$IrO$_6$. The isostructural Ca$_4$IrO$_6$ and Ca$_{2.5}$Sr$_{1.5}$IrO$_6$ feature a triangular lattice of spin chains running along the \emph{c} axis. The underlying properties of the two systems are characterized by a partial antiferromagnetic order occuring at 12 K and 9 K for Ca$_4$IrO$_6$ and Ca$_{2.5}$Sr$_{1.5}$IrO$_6$, respectively, a small entropy removal associated with the phase transition, and a sizable low-temperature specific heat linearly proportional to temperature. The detailed results will be discussed along with comparisons drawn with other related systems such as Ca$_5$Ir$_3$O$_{12}$. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B17.00006: Electronic and magnetic phase evolution in Sr$_3$(Ir$_{\mathrm{1-x}}$Ru$_{\mathrm{x}})_2$O$_7$ Chetan Dhital, Tom Hogan, Kevin Lukas, Steven Dissler, Cyril Opeil, Stephen Wilson A great deal of recent focus has been given to understanding how the interplay of strong spin orbit coupling effects and onsite coulomb repulsion change the conventional energy hierarchy in correlated 5d electron iridium oxides. Contrary to conventional band theory, perovskite iridate compounds Sr2IrO4 and Sr3Ir2O7 have long been known to be insulators; however many of their fundamental electronic properties and the interactions responsible for generating their antiferromagnetic insulating ground states remain under investigation. Here, we report results from our transport and magnetization study of electronic and magnetic phase of Sr3Ir1-xRuxO7. The evolution of the phase behavior as Sr3Ir2O7 is tuned from an AF insulator to a paramagnetic metal and the potential for a first order metal to insulator transition will be discussed. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B17.00007: Magnetic properties of Mn-doped Sr2IrO4 M.D. Lumsden, S. Calder, G.-X. Cao, J.W. Kim, Z. Gai, B.C. Sales, D. Mandrus, A.D. Christianson In 5d electron transition metal oxides, interplay between spin-orbit coupling and electronic interactions can lead to novel properties. One example is the Mott state in Sr$_2$IrO$_4$ which is believed to be associated with the formation of a J$_{eff}$=1/2 band due to large spin-orbit splitting of the t$_{2g}$ band. We use bulk measurements and resonant magnetic x-ray scattering to explore the effects of substituting Mn for Ir in single crystals of Sr$_2$Ir$_{0.9}$Mn$_{0.1}$O$_4$. These measurements indicate that 10\% Mn doping is sufficient to suppress the magnetic ordering temperature from 240 K to ~155 K. Resonant magnetic x-ray scattering measurements indicate a change in the long-range magnetic order when compared to that of undoped Sr$_2$IrO$_4$. Despite the large change in transition temperature and the altered magnetic structure, we observe a difference in the resonant enhancement between the L$_2$ and L$_3$ edges which is very similar to that seen in the pure material. This suggests that the magnetic structure of Sr$_2$IrO$_4$ can be altered by small perturbations whereas the J$_{eff}$=1/2 state is robust. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B17.00008: Weak magnetic transitions in pyrochlore Bi$_2$Ir$_2$O$_7$ Peter Baker, Francis Pratt, Johannes Moeller, Bil Hayes, Stephen Blundell, Tom Lancaster, Tongfei Qi, Gang Cao The pyrochlore iridate Bi$_2$Ir$_2$O$_7$ is analogous to the rare earth pyrochlores $R$Ir$_2$O$_7$ ($R=$ Y and Pr-Lu) but has no rare earth moments or $f$ electrons to interact with the Ir subsystem. This makes it an ideal system in which to study the Ir magnetism in isolation. Bulk measurements showed that it is metallic down to 2K and no indication of magnetic ordering was found down to 50mK. The magnetic field dependence of the low-temperature specific heat shows large changes in both the linear and cubic contributions and the large Wilson ratio of 53.5 suggests proximity to a quantum critical point [1]. Our muon spin relaxation measurements find a bulk magnetic transition at $1.84(3)$K and the form of the data suggests that the low-temperature state represents ordering of exceptionally small magnetic moments with persistent weak dynamics. The relaxation rate increases further below $0.23(4)$K, coincident with a growth in the specific heat, suggesting another magnetic transition. The magnetic field experienced by muons is $\sim 0.7$T at low-temperature, around two orders of magnitude smaller than that in other pyrochlore iridates, corresponding to moments $\sim 0.01~\mu_{\rm B}$/Ir.\\[4pt] [1] T. F. Qi et al., J. Phys.: Condens. Matter 24, 345601 (2012). [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B17.00009: Theory on Magnetic Excitation Spectra in Pyrochlore Iridates Eric Kin-Ho Lee, Subhro Bhattacharjee, Yong Baek Kim Metal-insulator transitions in pyrochlore iridates (A$_2$Ir$_2$O$_7$) are believed to occur due to subtle interplay of spin-orbit coupling, geometric frustration, and electron interactions. In particular, the nature of magnetic ordering of iridium ions in the insulating phase is crucial for understanding of several exotic phases recently proposed for these materials. We study the spectrum of magnetic excitations in the intermediate-coupling regime for the so-called all-in/all-out magnetic state in pyrochlore iridates with non-magnetic A-site ions (A=Eu,Y), which is found to be preferred in previous theoretical studies. We find that the effect of charge fluctuations on the spin-waves in this regime leads to strong departure from the lowest-order spin-wave calculations based on models obtained in strong-coupling calculations. We discuss the characteristic features of the magnetic excitation spectrum that can lead to conclusive identification of the magnetic order in future resonant inelastic x-ray (or neutron) scattering experiments. Knowledge of the nature of magnetic order and its low-energy features may also provide useful information on the accompanying metal-insulator transition. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:39PM |
B17.00010: Spin-Orbit Coupling in Mott Insulators: Unusual Interactions and Possible Exotic Phases Invited Speaker: George Jackeli Over the last few years, there has been an upsurge of interest in materials in which exotic states may emerge as the result of relativistic spin-orbit interactions. We will discuss insulating iridium oxides from this perspective. We show that the strong spin-orbit coupling, through the entanglement of spin and orbital spaces, leads to a variety of interesting Hamiltonians ranging from the Heisenberg model to the Kitaev or quantum compass models, for different lattice geometries [1]. Based on these effective Hamiltonians, we present a comprehensive theoretical study [1-3] of the rich phase behavior and dynamics observed in layered iridium oxides such as tetragonal Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$ and hexagonal \textit{A}$_2$IrO$_3$ (\textit{A}=Na, Li). We suggest that the hexagonal iridates might be close to the Kitaev spin-liquid state. We also discuss the layered tetragonal vanadate Sr$_2$VO$_4$ and argue that magnetically-hidden octupolar order, driven by spin-orbit coupling, is realized in this compound [4].\\[4pt] [1] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. \textbf{102}, 017205 (2009).\\[0pt] [2] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. \textbf{105}, 027204 (2010).\\[0pt] [3] J. Chaloupka, G. Jackeli, and G. Khaliullin, arXiv:1209.5100.\\[0pt] [4] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. \textbf{103}, 067205 (2009). [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B17.00011: ARPES Studies of Low-energy electronic structure of the strong spin-orbit semimetal SrIrO$_{3}$ Yuefeng Nie, Philip D.C. King, Haofei Wei, Masaki Uchida, John Harter, Eric Monkman, Daniel Shai, Darrell Schlom, Kyle Shen The similar energy scales of spin-orbit coupling and electron-electron correlation strength lead to exotic J$_{eff}=$ 1/2 Mott insulating ground states for layered Ruddlesden-Popper 5d iridates, Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$. A metal-insulator transition occurs upon increasing dimensionality from the two-dimensional layered Sr$_{2}$IrO$_{4}$ to the three-dimensional perovskite SrIrO$_{3}$. However, little is known about the electronic structure and nature of the metallic states in SrIrO$_{3}$. We synthesized epitaxial SrIrO$_{3}$ films on (001) LSAT substrates by molecular beam epitaxy and investigated their electronic structure using angle-resolved photoemission spectroscopy. We find an exotic semi-metallic state comprised of massive hole-like bands, whose extrema are pinned very close to the chemical potential, and rapidly dispersive electron bands which dominate the transport. Intriguingly, the bandwidths of SrIrO$_{3}$ are smaller than in its Mott insulating counterpart Sr$_{2}$IrO$_{4}$, indicating that metal-insulator transitions in Ruddlesden-Popper iridates are not simply driven by band narrowing resulting from reduced dimensionality. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B17.00012: High-magnetic-field-tuned insulating state in single-crystal BaIrO$_{3}$ O.B. Korneta, T.F. Qi, L. Li, K. Butrouna, G. Cao, E.S. Choi, Xiangang Wan BaIrO$_{3}$ is a novel magnetic insulator associated with the spin-orbit interaction. It magnetically orders at T$_{C}=182$~K, with an extremely small saturation moment M$_{S} < 0.03~\mu_{B}/Ir$. Application of high magnetic field up to 35~Tesla results in an exotic behavior characterized by: (1) a drastic rise in electrical resistivity by 250\% at low temperatures and (2) highly anisotropic magnetoresistivity with unusually strong hysteretic behavior. Our first principle calculations suggest a band structure near Fermi surface extremely sensitive to slight changes in lattice parameters, which captures underlying physical properties observed experimentally. The giant positive magnetoresistivity along with the extremely small saturation moment signals a delicate interplay between the structural and the electronic degrees of freedom in this compound. The electrical transport and magnetic properties in high magnetic field will be presented and discussed. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B17.00013: Magnetism in double-perovskite Sr$_2$GdIrO$_6$ and Sr$_2$YIrO$_6$ J. Terzic, T.F. Qi, L. Li, O.B. Korneta, S. Parkin, G. Cao {Sr$_2$GdIrO$_6$} and {Sr$_2$YIrO$_6$} with Ir$^{5+}$(5d$^4$) ions are magnetic insulators with a double-perovskite structure derived from the perovskite SrIrO$_3$, which is a paramagnetic metal. We report results of our study of structural and physical properties of single-crystal {Sr$_2$GdIrO$_6$} and {Sr$_2$YIrO$_6$}. This study reveals that while {Sr$_2$YIrO$_6$} exhibits no long-range order above 1.7 K, {Sr$_2$GdIrO$_6$} displays an anisotropic and antiferromagnetic state at low temperatures that is clearly manifested in the magnetization and specific heat. The results will be presented and discussed along with comparison drawn with other related iridates driven by the strong spin-orbit interaction. [Preview Abstract] |
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