Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session F6: Iridates IIFocus
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Sponsoring Units: GMAG DMP Chair: Ilija Zeljkovic, Boston College Room: 302 |
Tuesday, March 15, 2016 11:15AM - 11:27AM |
F6.00001: The role of correlations in the low energy electronic structure of ligthly electron doped Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$. Alberto De la Torre, Flavio Bruno, Zhiming Wang, Anna Tamai, Christophe Berthod, Didier Jaccard, Alaska Subedi, Antoine Georges, Robin Perry, Felix Baumberger We characterized the emergence of exotic electronic ground states in lightly electron doped (Sr$_{1-x}$La$_x$)$_2$IrO$_4$ and (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$ by ARPES. In the single layer iridate, a large Fermi surface with nodal coherent spectral weight and antinodal pseudogap emerges, concomitantly with the collapse of the Mott gap, upon doping [1]. On the other hand, in Sr$_3$Ir$_2$O$_7$ a small non-gapped Fermi surface with coherent quasiparticles, together with a reduction of the correlated gap throughout the entire Brillouin Zone is observed when doping above the insulator to metal transition [2]. By comparing the electronic structure of these two materials, we provide evidence that the interplay between spin-orbit and electron-electron correlations ($U$) in (Sr$_{1-x}$La$_x$)$_2$IrO$_4$ and (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$ is rather different: while in Sr$_2$IrO$_4$ this interplay results in a pseudospin-1/2 single band Mott insulator with a phenomenology very similar to that of cuprates, in Sr$_3$Ir$_2$O$_7$ $U$ enhances the bilayer splitting gap to originate a ground state resembling that of a correlated semiconductor. [1] A. de la Torre et al, PRL 115, 176402 (2015); [2] A. de la Torre et al, PRL 113, 256402 (2014) [Preview Abstract] |
Tuesday, March 15, 2016 11:27AM - 11:39AM |
F6.00002: Electrically tunable transport in antiferromagnetic Sr $_{\mathrm{\mathbf{3}}}$\textbf{Ir}$_{\mathrm{\mathbf{2}}}$\textbf{O}$_{\mathrm{\mathbf{7}}}$ Heidi Seinige, Cheng Wang, Gang Cao, Jianshi-S. Zhou, John B. Goodenough, Maxim Tsoi Recently we demonstrated experimentally the existence of interconnections between magnetic state and transport currents in antiferromagnetic (AFM) Mott insulator Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$. We found a very large anisotropic magnetoresistance [1] and demonstrated a reversible resistive switching driven by high-density currents/high electric fields [2]. These results support the feasibility of AFM spintronics, where antiferromagnets are used in place of ferromagnets, however a low N\'{e}el temperature of this material (240 K) questions any practical applications. Here we present a comparative electrical transport study of its sister compound Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ which has a higher transition temperature (285 K). Similar to the case of Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$, we find a continuous reduction in the resistivity of Sr$_{\mathrm{3}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7}}$ as a function of increasing electrical bias and abrupt reversible changes above a threshold bias current. We explain these results by a reduction of activation energy associated with a field-driven lattice distortion. [1] C. Wang et al., Phys. Rev. X 4, 041034 (2014); [2] C. Wang et al, PRB 92, 115136 (2015). [Preview Abstract] |
Tuesday, March 15, 2016 11:39AM - 11:51AM |
F6.00003: Magnetic excitations and lattice distortions in highly-doped (Sr$_{1-x}$La$_{x}$)$_{3}$Ir$_{2}$O$_{7}$ Tom Hogan, Mary Upton, Xiaoping Wang, Stephen Wilson (Sr$_{1-x}$La$_{x}$)$_{3}$Ir$_{2}$O$_{7}$ has been shown to undergo a first-order phase transition from a localized antiferromagnetic insulating state to a correlated metal. We discuss the further characterization of these correlations by examining the excitation spectra of a highly-doped sample. These reveal evidence of a dispersive feature associated with an over-damped magnon mode, similar to the behavior of the undoped parent compound, as well as a higher energy excitation. The nature of the lattice distortion brought on by La-doping will also be discussed. [Preview Abstract] |
Tuesday, March 15, 2016 11:51AM - 12:27PM |
F6.00004: Nature of the magnetic correlations in photo-doped and chemically-doped spin-orbit Mott insulator Sr$_2$IrO$_4$ Invited Speaker: Mark P. M. Dean In the iridates, competition between spin-orbit coupling, crystal field, and electronic correlation has lead to the observation of several novel states. Particularly notable is the spin-orbit Mott insulating state in $\mathrm{Sr_2IrO_4}$ which has close analogies to the high temperature superconducting cuprates. This talk will describe the nature of the magnetic correlations in $\mathrm{Sr_2IrO_4}$ and how the magnetic correlations can be modified by two different doping schemes. I will first describe doping via photo-excitation in which we use femtosecond infrared pulses to excite carriers across the Mott gap. After excitation, we probe the resulting magnetic state as a function of time delay using the first implementation of magnetic resonant inelastic X-ray scattering at a free electron laser. We find that the non-equilibrium state 2~ps after the excitation has strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. The magnetism recovers its two-dimensional in-plane N{\'e}el correlations on a timescale of a few ps, while the three-dimensional long range magnetic order is restored over a far longer, fluence-dependent timescale of a few 100~ps. In the second part of the talk I will describe chemical doping via Ir-Ru substitution. In this situation, we find that with increased Ru concentration, the dispersive magnetic excitations in the parent compound become almost momentum-independent, opening a magnetic gap $>150$~meV. We attribute this gap to the combined effects of disorder and Ir-Ru interactions. [Preview Abstract] |
Tuesday, March 15, 2016 12:27PM - 12:39PM |
F6.00005: Jahn-Teller effect in systems with strong on-site spin-orbit coupling Ekaterina Plotnikova, Maria Daghofer, Jeroen van den Brink, Krzysztof Wohlfeld When strong spin-orbit coupling removes orbital degeneracy, it would at the same time appear to render the Jahn-Teller mechanism ineffective. We discuss such a situation, the $t_{2g}$ manifold of iridates, and show that, while the Jahn-Teller effect does indeed not affect the $j=1/2$ antiferromagnetically ordered ground state, it leads to distinctive signatures in the $j=3/2$ spin-orbit exciton. It allows for a hopping of the spin-orbit exciton between the nearest neighbor sites without producing defects in the $j=1/2$ antiferromagnet. This arises because the lattice-driven Jahn-Teller mechanism only couples to the orbital degree of freedom, but is not sensitive to the phase of the wave function that defines isospin $j_z$. This contrasts sharply with purely electronic propagation, which conserves isospin, and presence of Jahn-Teller coupling can explain some of the peculiar features of measured resonant inelastic x-ray scattering spectra of Sr$_2$IrO$_4$. [Preview Abstract] |
(Author Not Attending)
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F6.00006: Unveiling the Origin of the Basal-plane Antiferromagnetism in the $J_{\mathrm{eff}}=$1/2 Mott Insulator Ba$_{\mathrm{2}}$IrO$_{\mathrm{4}}$: A Density Functional and Model Hamiltonian Study Yusheng Hou, Hongjun Xiang, Xingao Gong Based on the density functional theory and our new model Hamiltonian, we have studied the basal-plane antiferromagnetism in the novel $J_{\mathrm{eff}}=$1/2 Mott insulator Ba$_{\mathrm{2}}$IrO$_{\mathrm{4}}$. By comparing the magnetic properties of the bulk Ba$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ with those of the single-layer Ba$_{\mathrm{2}}$IrO$_{\mathrm{4}}$, we demonstrate unambiguously that the basal-plane antiferromagnetism is caused by the intralyer magnetic interactions rather than by the previously proposed interlayer ones. In order to reveal the origin of the basal-plane antiferromagnetism, we propose a new model Hamiltonian by adding the single ion anisotropy and pseudo-quadrupole interactions into the general bilinear pseudo-spin Hamiltonian. The obtained magnetic interaction parameters indicate that the single ion anisotropy and pseudo-quadrupole interactions are unexpectedly strong. Systematical Monte Carlo simulations demonstrate that the basal-plane antiferromagnetism is caused by the isotropic Heisenberg, bond-dependent Kitaev and pseudo-quadrupole interactions. Our results show for the first time that the single ion anisotropy and pseudo-quadrupole interaction can play significant roles in establishing the exotic magnetism in the $J_{\mathrm{eff}}=$1/2 Mott insulator. [Preview Abstract] |
Tuesday, March 15, 2016 12:51PM - 1:03PM |
F6.00007: Frustration in square lattice of Iridium Jeff=1/2 moments at high pressure Daniel Haskel, Gilberto Fabbris, Jong Woo Kim, Jung Ho Kim, Bumjoon Kim, Gang Cao, Viktor Struzhkin We study the evolution of magnetic order in the weakly ferromagnetic, insulator Sr2IrO4 under applied pressure using x-ray resonant magnetic scattering and x-ray magnetic circular dichroism techniques in the diamond anvil cell. The weak inter-layer coupling is readily tunable with pressure giving rise to a change in magnetic structure followed by coexisting and competing magnetic phases with different inter-layer coupling. Application of moderate magnetic fields stabilizes one of the magnetic phases. Higher pressures drive the system into a magnetically disordered state, possibly a quantum paramagnetic state. We discuss the results in the context of a J1-J2 model where the increasing strength of next-nearest-neighbor exchange coupling with pressure leads to frustration of in-plane interactions [Preview Abstract] |
Tuesday, March 15, 2016 1:03PM - 1:15PM |
F6.00008: Effect of longer-range lattice anisotropy on the electronic structure and magnetism of spin-orbit-coupled 5$d$ transition-metal oxides Nikolay Bogdanov, Vamshi Katukuri, Judit Romh\'anyi, Viktor Yushankhai, Vladislav Kataev, Berndt B\"uchner, Jeroen van den Brink, Liviu Hozoi Our detailed quantum chemistry calculations provide magnetic $g$ factors and exchange interactions for the quasi two-dimensional iridates Sr$_2$IrO$_4$ and Ba$_2$IrO$_4$. While canonical ligand-field considerations predict $g_{||}$-factors $<2$ for the positive tetragonal distortions present in Sr$_2$IrO$_4$, we find $g_{||} > 2$. This implies that the $d$ levels in Sr$_2$IrO$_4$ are inverted with respect to the ordering deduced from the local ligand distortions, whereas we find them in Ba$_2$IrO$_4$ to be instead normally ordered. Electron spin resonance measurements confirm the level inversion in Sr$_2$IrO$_4$. This $d$-level switching is driven by the specific ionic charge distribution within adjacent IrO$_2$ and SrO layers. Since polar discontinuities and the associated complications do not arise for such layers, our results highlight the tetravalent $d$-metal 214 oxides as ideal platforms to explore $d$-level reconstruction and engineering in the context of oxide heterostructures. [Preview Abstract] |
Tuesday, March 15, 2016 1:15PM - 1:27PM |
F6.00009: Influence of isovalent doping of Ca2$+$ on the Spin orbit Mott insulator Sr2IrO4 Xiang Chen, stephen Wilson Here we investigate the influence of isoelectronic doping into the spin-orbit Mott materials Sr$_{2}$IrO$_{4}$. Specifically, we explore the influence of isovalent Ca substitution as a perturbation to the J$_{eff}=$1/2 Mott ground state by combined transport, bulk magnetization, and scattering measurements. The evolution of the lattice geometry/structure-type as well as the electronic phase behavior will be presented as Ca$^{2+}$ is substituted on the Sr$^{2+}$ site. [Preview Abstract] |
Tuesday, March 15, 2016 1:27PM - 1:39PM |
F6.00010: Crystal growth and intrinsic magnetic behavior of Sr2IrO4 NAKHEON SUNG, H. Gretarsson, D. Proepper, J. Porras, M. Le Tacon, A. V. Boris, B. Keimer, B. J. Kim We report on the growth of stoichiometric Sr2IrO4 single crystals, which allow us to unveil their intrinsic magnetic properties. The effect of different growth conditions has been investigated for crystals grown by the flux method. We find that the magnetic response depends very sensitively on the details of the growth conditions. We assess the defect concentration based on magnetization, X-ray diffraction, Raman scattering, and optical conductivity measurements. We find that samples with a low concentration of electronically active defects show much reduced in-gap spectral weight in the optical conductivity and a pronounced two-magnon peak in the Raman scattering spectrum. A prolonged exposure at high temperature during the growth leads to higher defect concentration likely due to creation of oxygen vacancies. We further demonstrate a systematic intergrowth of Sr2IrO4 and Sr3Ir2O7 phases by varying the growth temperature. Our results thus emphasize that revealing the intrinsic magnetic properties of Sr2IrO4 and related materials requires a scrupulous control of the crystal growth process. [Preview Abstract] |
Tuesday, March 15, 2016 1:39PM - 1:51PM |
F6.00011: Magnetic Properties comparison of 3D Kitaev candidate materials beta and gamma Li2IrO3 Ramon Ruiz Honeycomb iridates have been the focus of substantial interest due to the strong magnetic frustration that arises from their edge-shared bonding environment, which favors a strongly anisotropic Ising-like exchange between bonds. In materials with edge-shared IrO6 octahedra, spin-anisotropy of the exchange between neighboring effective spin-1/2 states is enhanced by the interference of the two exchange paths across the planar Ir-O-Ir bond. In the honeycomb lattice, such an interaction couples different orthogonal spin components for the three nearest neighbors; no single exchange direction can be simultaneously satisfied, leading to strong frustration which can be described by the Kitaev-model. We have recently synthesized two new structure that retains the same bonding environment as the honeycomb lattice, and extends this physics to three-dimensions. In this work, we compare the magnetic properties of these two novel structures, presenting evidences that their high temperature behavior can be explained by geometric g-factor constrains while the low temperature anisotropy and degeneracy of the ground state suggest the presence of spin anisotropic exchange. [Preview Abstract] |
Tuesday, March 15, 2016 1:51PM - 2:03PM |
F6.00012: The Mott Insulating Nature of Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ : DFT$+$DMFT Study Minjae Kim, Beom Hyun Kim, B. I. Min We have investigated the insulating nature of Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$, employing both the density-functional theory (DFT) and the combination of the DFT and the dynamical mean-field theory (DFT$+$DMFT). We have obtained the paramagnetic (PM) insulating state even above the Neel temperature (T$_{\mathrm{N}})$, which reveals that Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ is a Mott-type insulator. The photoemission spectrum is well described by the density of states from the DFT$+$DMFT in this PM insulating state. However, the analysis of optical conductivity suggests that the non-local correlation effect is also important in Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$. We have also found sizable redistribution of both charge and spin densities upon cooling below T$_{\mathrm{N}}$ which suggests that Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ is not a standard Mott insulator having rigid charge density. Therefore, despite the Mott-type insulating state of Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$, the itineracy and the non-local correlation are important as well in describing its electronic and magnetic properties due to the extended nature of Ir 5$d$ state. [Preview Abstract] |
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