Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E43: Spin Orbit Physics in Oxides IIFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: Alex Thaler, Oak Ridge National Lab Room: 390 |
Tuesday, March 14, 2017 8:00AM - 8:36AM |
E43.00001: Transport properties of correlated metals: A dynamical mean field theory perspective Invited Speaker: Xiaoyu Deng Strongly correlated metals, including many transition metal oxides, are characterized by unconventional transport properties with anomalous temperature dependence. For example, in many systems Fermi liquid behavior holds only below an extremely low temperature while at high temperature these bad metals have large resistivity which exceeds the Mott-Ioffe-Regel (MIR) limit. Material specific calculation of these anomalous transport properties is an outstanding challenge. Recent advances enabled us to study the transport and optical properties of two archetypal correlated oxides, vanadium oxides and ruthenates, using the LDA$+$DMFT method. In V2O3, the prototypical Mott system, our computed resistivity and optical conductivity are in very good agreement with experimental measurements, which clearly demonstrates that the strong correlation dominates the transport of this material [4]. Furthermore by expressing the resistivity in terms of an effective plasma frequency and an effective scattering rate, we uncover the so-called "hidden Fermi liquid" [1, 2, 3] behavior, in both the computed and measured optical response of V2O3. This paradigm explains the optics and transport in other materials such as NdNiO3 film and CaRuO3. In the ruthenates family, we carried out a systematical theoretical study on the transport properties of four metallic members, Sr2RuO4, Sr3Ru2O7, SrRuO3 and CaRuO3, which generally encapsulates the gradually structure evolution from two-dimension to three dimension. With a unified computational scheme, we are able to obtain the electronic structure and transport properties of all these materials [5]. The computed effective mass enhancement, resistivity and optical conductivity are good agreement with experimental measurements, which indicates that electron-electron scattering dominates the transport of ruthenates. We explain why the single layered compound Sr2RuO4 has a relative weak correlation with respect to its siblings, which corroborates its good metallicity. Comparing our results with experimental data, benchmarks the capability as well as the limitations of existing methodologies for describing transport properties of realistic correlated materials. [1] Xiaoyu Deng, Jernej Mravlje, Rok Zitko, Michel Ferrero, Gabi Kotliar, Antoine Georges, Physical Review Letters, 110, 086401 (2013). [2] Christophe Berthod, Jernej Mravlje, Xiaoyu Deng, Rok Zitko, Dirk van der Marel, and Antoine Georges, [3] Phys. Rev. B, 87, 115109 (2013) Wenhu Xu, Kristjan Haule, and Gabriel Kotliar, Physical Review Letters, 111, 036401(2013) [4] Xiaoyu Deng, Aaron Sternbach, Kristjan Haule, D. N. Basov, Gabriel Kotliar, Physical Review Letters, 113, 246404 (2014) [5] Xiaoyu Deng, Kristjan Haule, Gabriel Kotliar, Physical Review Letters, 116, 256401 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 8:48AM |
E43.00002: Magnetic structure and anisotropic spin dynamics in Ba$_{\mathrm{\mathbf{2}}}${CoO}$_{\mathrm{\mathbf{4}}}$ Qiang Zhang, Songxue Chi, Feng Ye, Huibo Cao, Masaaki Matsuda, Guixin Cao, Rongying Jin, Ward Plummer, Jiandi Zhang Ba$_{\mathrm{2}}$CoO$_{\mathrm{4}}$ crystallizes in a monoclinic structure with Co in the tetrahedral environment, in contrast to the octahedral coordination of Co in BaCoO$_{\mathrm{3}}$. Single-crystal neutron diffraction determined an antiferromagnetic ground state with propagation vector (1/2 0 1/2), and Co spins mainly confined in the\textit{ ab} plane below $T_{\mathrm{N}} \quad =$ 25 K. The moment is found to be 3.42(4) u$_{\mathrm{B}}$, pointing to the $a $axis with a canting to $b$ axis, indicating a high-spin Co$^{\mathrm{4+}}$ state in Ba$_{\mathrm{2}}$CoO$_{\mathrm{4}}$. Inelastic neutron scattering studies show an energy gap $\approx $ 2 meV, and approaches zero as temperature increases to $T_{\mathrm{N}}$, which probably results from the single-ion anisotropy of high-spin Co$^{\mathrm{4+}}$. The spin wave spectra show distinct dispersion along the $H$ and $K $directions without dispersion along the$ L$ direction in the reciprocal lattice unit, indicating a strong anisotropic spin dynamic behavior. Our findings imply that Ba$_{\mathrm{2}}$CoO$_{\mathrm{4}}$ exhibits a low-dimensional magnetism although the crystalline structure is three dimensional. [Preview Abstract] |
Tuesday, March 14, 2017 8:48AM - 9:00AM |
E43.00003: A Unified Description of the Electronic Structure and Dynamics in the Doped Mott Insulator $Sr_2IrO_4$ Daniel Dessau, Yue Cao, Haoxiang Li, Xiaoqing Zhou, Rajendra Dhaka, Nicholas Plumb, Qiang Wang, Tongfei Qi, Jasminka Terzic, Xiuwen Zhang, Alex Zunger, Gang Cao An important mission of modern condensed matter physics is to forge the link between the local high energy electronic interactions and the emergent collective excitations at lower energies and longer wave lengths. This can be a daunting task when electrons are strongly correlated, e.g. in the evolution of the electronic structure as a Mott insulator is doped into a Fermi liquid. We show the asymmetrical electronic structure evolution in the hole and electron-doped Mott insulator $Sr_2IrO_4$ using angle-resolved photoemission spectroscopy. There is a ubiquitous momentum transfer $(\pi, \pi)$ connecting the dynamically inequivalent parts of the electronic structure near the Fermi level. By parameterizing first-principles calculations, we associate this momentum transfer with an energy scale $V_\pi$$_\pi$ that smoothly connects the electronic structure between the Mott insulator and Fermi liquid limits, allowing us to understand many novel aspects of the physics of these materials. [Preview Abstract] |
Tuesday, March 14, 2017 9:00AM - 9:12AM |
E43.00004: Terahertz circular birefringence in thin films of Sr$_2$IrO$_4$ Thuc T. Mai, R. K. Smith, E. V. Jasper, M. T. Warren, R. Vald\'es Aguilar, S. S. A. Seo Due to its similarity to La$_2$CuO$_4$, the parent compound for the high Tc superconductors, Sr$_2$IrO$_4$ has been in the spotlight of much recent research effort. Sr$_2$IrO$_4$ is one of the first examples of a spin-orbit-assisted Mott insulator. It undergoes a metal to insulator transition around 240 K, coincident with antiferromagnetic ordering that is accompanied by a weak ferromagnetic moment. Motivated by a recent nonlinear optical experiment that insinuates a magnetoelectric ground state of orbital currents in the unit cell, we investigate the circular birefringence and dichroism of THz radiation on thin films of Sr$_2$IrO$_4$. We have developed a novel technique to generate broadband circular polarized THz pulses, and use it to probe the nature of the ground state of Sr$_2$IrO$_4$. The THz measurements reveal a non-trivial behavior of the circular birefringence around the transition temperature and as a function of magnetic field. We will discuss these results in the context of the magnetic structure models in Sr$_2$IrO$_4$, and of the potential implication on its point group symmetry. [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:24AM |
E43.00005: Tuning $J_{\rm eff}$=1/2 magnetism and anisotropy in Sr$_2$IrO$_4$ through Ru substitution Stuart Calder, J. W. Kim, Guixin Cao, A. E. Taylor, A. F. May, C. Cantoni, M. H. Upton, Y. Choi, D. Haskel, M. D. Lumsden, A. D. Christianson Iridates can hosts a spin-orbit entangled ground state with $J_{\rm eff}$=1/2 magnetic moments. Mapping of these pseudo-spins onto several distinct crystal structures has led to the uncovering of a variety of novel properties, such as Kitaev physics and Weyl semimetals. Sr$_2$IrO$_4$ is a canonical example of a $J_{\rm eff}$=1/2 material and much interest has focused on the predictions of unconventional superconductivity driven by analogies to cuprates. In particular, proximity to a superconducting phase was strongly suggested by the magnetic excitation spectra which revealed striking similarities to La$_2$CuO$_4$ and was able to be described within a pure Heisenberg model consisting of spin-1/2 on a square lattice. We have investigated the series Sr$_2$Ir$_{\rm 1-x}$Ru$\rm _x$O$_4$ with neutron and resonant x-ray scattering and found an evolution of magnetism from ab-plane to c-axis aligned spins [1]. Despite this change the $J_{\rm eff}$=1/2 moments remain robust. By probing the excitations we observed a large spin-gap (40 meV) that indicates an alteration of the magnetic anisotropy associated with the parent. [1] S. Calder et al., PRB 92, 165128 (2015). [2] S. Calder et al., arXiv:1610.03399 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 9:24AM - 9:36AM |
E43.00006: Compass Impurity Model of Tb Substitution in Sr$_{2}$IrO$_{4}$ Long Zhang, Fa Wang, Dung-Hai Lee We show that upon Tb substitution the interaction between the magnetic moments on the impurity Tb$^{4+}$ ion and its surrounding Ir$^{4+}$ ions is described by a ``compass'' model, i.e., Ising-like interaction favoring the magnetic moments across each bond to align along the bond direction. Such interaction nucleates quenched magnetic vortices near the impurities and drives a reentrant transition out of the antiferromagnetic ordered phase at low temperatures hence quickly suppresses the N\'eel temperature consistent with the experiment [Phys. Rev. B \textbf{92}, 214411 (2015)]. As a by-product, we propose that the compass model can be realized in ordered double perovskites composed of the spin-orbital-coupled $d^{5}$ ions and the half-closed-shell $f^{7}$ ions. [Preview Abstract] |
Tuesday, March 14, 2017 9:36AM - 9:48AM |
E43.00007: Electrical and magnetic properties of Na doped Ca$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ Yiyao Chen, Ashutosh Dahal, Deepak Singh The coexistence of strong spin-orbit coupling and electron correlation in layered iridates can lead to the interesting J$_{\mathrm{eff}}=$1/2 Mott state, which has been studied in perovskite-like Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ and Ba$_{\mathrm{2}}$IrO$_{\mathrm{4}}$. Unlike its sister compound, the bulk Ca$_{\mathrm{2}}$IrO$_{\mathrm{4\thinspace }}$exhibits hexagonal structure, while the Ruddlesden-Popper phase tetragonal structure$_{\mathrm{\thinspace }}$is thermodynamically unstable. Bulk Ca$_{\mathrm{2}}$IrO$_{\mathrm{4\thinspace }}$has semiconducting behavior, and the spin 1/2 Ir$^{\mathrm{4+}}$ ions carry magnetic moments. Doping Na creates electron deficiency in Ca$_{\mathrm{2}}$IrO$_{\mathrm{4}}$. In this talk, I will discuss electrical and magnetic measurements on newly synthesized Na doped Ca$_{\mathrm{2}}$IrO$_{\mathrm{4}}$. Experimental results on single crystal Ca$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ will also be discussed. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:00AM |
E43.00008: Ultrafast dynamics of hidden order in Sr$_2$IrO$_4$ probed by time-resolved nonlinear optical anisotropy Alberto De la Torre, John Harter, Liuyan Zhao, Gang Cao, David Hsieh The Mott insulator Sr$_2$IrO$_4$ is proposed to be a spin-orbit coupled analogue of the parent compound of cuprate high temperature superconductors. Moreover, the layered iridate has been shown to exhibit both a pseudogap and possibly d-wave superconducting phase upon doping. Recently, rotational anisotropy optical second harmonic generation (RA-SHG) measurements have revealed the existence of an additional hidden order in this system, which breaks both the rotational and inversion symmetries of the host lattice. Here we demonstrate the ability to drive the hidden order-disorder transition by light using a newly developed ultrafast extension of the RA-SHG technique that enables symmetry group changes to be resolved on ultrafast timescales. We will show that there exists a temperature dependent fluence threshold for this photo-induced phase transition and we will present a realistic model for understanding the ultrafast switching phenomenon and subsequent relaxation dynamics. [Preview Abstract] |
Tuesday, March 14, 2017 10:00AM - 10:12AM |
E43.00009: Non-Fermi surface nesting driven commensurate magnetic ordering in Fe-doped Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$. M. Zhu, K.V. Shanavas, Y. Wang, T. Zou, W.F. Sun, W. Tian, V.O. Garlea, A. Podlesnyak, M. Matsuda, M.B. Stone, D. Keavney, Z.Q. Mao, D.J. Singh, X. Ke Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$, an unconventional superconductor, is known to possess an incommensurate spin density wave instability driven by Fermi surface nesting. Here we report a new static spin density wave ordering with a commensurate propagation vector \textbf{q}$_{\mathrm{c}}$ $=$ (0.25 0.25 0) in Fe-doped Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$, despite that the magnetic fluctuations still persist at the incommensurate wave vectors \textbf{q}$_{\mathrm{ic}} \quad =$ (0.3 0.3 $L)$. First principles calculations show that Fe substitution barely changes the nesting vector of the Fermi surface, but leads to antiferromagnetic polarization in the nearest-neighbor Ru magnetic moments. Our results imply that in addition to the known incommensurate magnetic instability, Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ is also in proximity to a commensurate magnetic tendency that can be stabilized via tuning the local exchange interaction between Ru and dilute magnetic impurities. [Preview Abstract] |
Tuesday, March 14, 2017 10:12AM - 10:24AM |
E43.00010: Suppressed magnetism in Ca$_2$RuO$_4$ under applied electric currents Joel Bertinshaw, Namrata Gurung, Maximilian Krautloher, Anil Jain, Juan Porras, Oscar Fabelo Rosa, BJ Kim, Bernhard Keimer The $4d$-electron system Ca$_2$RuO$_4$ plays host to an exciting interplay between spin-orbit coupling and electronic correlation energies that gives rise to exotic ground states and a high sensitivity to external perturbation. Isovalent Sr-doping, hydrostatic pressure and even applied electric currents can induce dramatic changes in the exhibited electronic and magnetic properties. Here, we use single crystal neutron diffraction with in-situ applied electric currents to show that a previously identified current induced metal-insulator transition is linked to a modified distortion of the RuO$_6$ octahedra and a concomitant suppression of antiferromagnetic order. These results indicate a close correlation among the crystal lattice, electronic, and magnetic structures. [Preview Abstract] |
Tuesday, March 14, 2017 10:24AM - 10:36AM |
E43.00011: Electrically tunable transport and resistive switching in doped Ca$_{\mathrm{\mathbf{2}}}$\textbf{RuO}$_{\mathrm{\mathbf{4}}}$ Shida Shen, Morgan Williamson, Gang Cao, Jianshi Zhou, John Goodenough, Maxim Tsoi We study electronic transport properties of Cr doped (2.5{\%}) Mott insulator Ca$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ where electric fields were previously found [1] to induce an insulator-to-metal switching with potential industrial applications. In our experiments we observe a continuous reduction in the resistivity of Ca$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ as a function of increasing electrical bias followed by an abrupt switching at higher biases. Interestingly, the observed switching is non-destructive and requires opposite bias polarities to switch from high-to-low and low-to-high resistance states. Combination of 2-, 3-, and 4-probe measurements provide a means to shed light on the origin of the switching and distinguish between its bulk and interfacial contributions. This work was supported in part by C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA, by NSF grants DMR-1600057, DMR-1265162, and DMR-1122603, and by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No. OSR-2015-CRG4-2626. [1] F. Nakamura et al., Sci. Rep. 3, 2536 (2013). [Preview Abstract] |
Tuesday, March 14, 2017 10:36AM - 10:48AM |
E43.00012: Orbital and spin signatures in the ultrafast lattice vibrations of a Mott insulator Ca$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ Min-Cheol Lee, Choong H. Kim, Inho Kwak, C. W. Seo, Y. S. Lee, C. H. Sohn, F. Nakamura, C. Sow, Y. Maeno, E.-A. Kim, T. W. Noh, K. W. Kim Only within a short period of a decade, ultrafast spectroscopy has dramatically expanded its field in physics. Its unique ability to investigate time-resolved dynamics offers new insight into an interaction between quantum degrees of freedom, such as charge, lattice, orbital and spin. As one of the most interesting ultrafast features, coherent phonon oscillations provide oscillation-\textit{phase} resolved information revealing a coupling of lattice to various order parameters. However, the oscillation-\textit{phase} itself has been overlooked because it has been believed to be robustly determined by generation mechanism. In this talk, I will discuss a novel response from the coherent phonon oscillation depending on the orbital and spin degrees. Surprisingly, the orbital and spin orders in Ca$_{\mathrm{2}}$RuO$_{\mathrm{4}}$, one of the best known 4$d$ Mott insulator, influence a coherent $A_{\mathrm{g}}$ phonon mode with dramatic changes of its \textit{phase} such that the oscillation even flips across the orbital order. DFT calculations and a careful inspection of the crystal structure indicate that an unusually large antipolar distortion of apical oxygen is an essence of the orbital order and the extraordinary \textit{phase} variations. We suggest that such \textit{phase} observation in the ultrafast lattice vibrations can offer a new opportunity to single out a crucial but veiled aspect of the lattice. [Preview Abstract] |
Tuesday, March 14, 2017 10:48AM - 11:00AM |
E43.00013: Effective magnetic interactions in spin-orbit coupled d4 Mott insulators Nandini Trivedi, Christopher Svoboda, Mohit Randeria Transition metal compounds with the $(t_{2g})^4$ electronic configuration are expected to be nonmagnetic atomic singlets due to spin-orbit coupling for both weak and strong interactions . However, starting with the full multi-orbital electronic Hamiltonian, we show the low energy effective magnetic Hamiltonian contains isotropic superexchange spin interactions but anisotropic orbital interactions. By tuning the ratio of superexchange to spin-orbit coupling $J_\mathrm{SE}/\lambda$, we obtain a phase transition from nonmagnetic atomic singlets to novel magnetic phases depending on the strength of Hund's coupling, the crystal structure and the number of active orbitals. Spin-orbit coupling plays a non-trivial role in generating a triplon condensate of weakly interacting excitations at antiferromagnetic ordering vector $\vec k=\vec \pi$, regardless of whether the local spin interactions are ferromagnetic or antiferromagnetic. In the large $J_\mathrm{SE} / \lambda$ regime, the localized spin and orbital moments produce anisotropic orbital interactions that are frustrated or constrained even in the absence of geometric frustration. Orbital frustration leads to frustration in the spin channel opening up the possibility of spin-orbital liquids with both spin and orbital entanglement. [Preview Abstract] |
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