Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W48: Invited Session: Exotic Phase Transitions in 5d Compounds |
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Sponsoring Units: DCMP GMAG Chair: Sang-Wook Cheong, Rutgers University Room: Mile High Ballroom 1A-1B |
Thursday, March 6, 2014 2:30PM - 3:06PM |
W48.00001: Exotic magnetism of Jeff$=$1/2 iso-spins in complex Ir oxides Invited Speaker: Hide Takagi In 5d Iridium oxides, a large spin-orbit coupling of $\sim$ 0.5 eV, inherent to heavy 5d elements, is not small as compared with other relevant electronic parameters including Coulomb U, transfer t and crystal field splitting D, which gives rise to a variety of exotic magnetic ground states. In the layered perovskite Sr$_{2}$IrO$_{4}$, spin-orbital Mott state with Jeff$=$1/2 is realized due to the novel interplay of those energy scales [1, 2]. Despite the strong entanglement of spin and orbital degrees of freedom, surprisingly isotropic, two-dimensional Heisenberg character of $J_{\mathrm{eff}}=$1/2 iso-spins was observed in Sr$_{2}$IrO$_{4}$ with 180 deg Ir-O-Ir bonds, by the recent resonant magnetic x-ray diffuse scattering and the magnetic susceptibility measurements [3]. Complex Na-Ir oxides with honeycomb and more recently identified \textit{hyper-honeycomb} lattices, where 90 deg Ir-O-Ir bonds are realized, are candidates for Kitaev spin liquid. Such exotic magnetism was recently shown to be tailored using super-lattice structure [4] In this talk, we review these unique magnetic phases in Ir oxides. \\[4pt] [1] B. J. Kim et al., \textit{Phys. Rev. Lett. }\textbf{101}, 076402 (2008).\\[0pt] [2] B. J. Kim, H. Ohsumi, T. Komesu, S. Sakai, T. Morita, H. Takagi, and T. Arima, \textit{Science} 323, 1329 (2009). \\[0pt] [3] S. Fujiyama\textit{ et al.} , \textit{Phys. Rev. Lett.} \textbf{108}, 247212 (2012).\\[0pt] [4] J. Matsuno \textit{et al.}, submitted. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W48.00002: Slater and Mott Insulating States in Os- and Ir-Based Transitional Metal Oxides Invited Speaker: A.D. Christianson The discovery of a novel J$_{\mathrm{eff}}=$1/2 electronic configuration and spin-orbit assisted insulating state in Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ has stimulated a fresh look at metal-insulator transitions where relativistic effects participate on an even footing with other energy scales such as crystal field splitting and electron-electron correlations. There are several view points on the origin of the insulating state in Sr$_{2}$IrO$_{4}$, but the most prominent is that spin-orbit coupling modifies the electronic configuration such that a Mott insulting state emerges despite the relatively modest electron-electron correlations within the 5$d$ orbitals. An alternative viewpoint is that magnetic effects enable the opening of the electronic gap giving rise to the insulating state or a Slater metal-insulator transition. Here we describe realizations of both Mott and Slater insulators in the context of Os- and Ir-based 5$d$ transition metal oxides. NaOsO$_{3}$, exhibits a continuous phase transition at 410 K where antiferromagnetism appears in conjunction with the onset of insulating behavior. A combination of neutron diffraction and magnetic resonant x-ray scattering enables the conclusion that G-type magnetic order occurs at the metal-insulator transition providing microscopic evidence that NaOsO$_{3}$ is the first three dimensional realization of a Slater insulator. On the other hand we have probed the robustness of the J$_{\mathrm{eff}}=$1/2 Mott insulating state though studies of Sr$_{2}$Ir$_{\mathrm{1-x}}$T$_{\mathrm{x}}$O$_{4}$ (T$=$Mn, Ru). For both Mn and Ru doping we find that despite qualitative changes in the magnetic order the J$_{\mathrm{eff}}=$1/2 electronic configuration remains robust. In particular, for Ru-doping the signatures of the J$_{\mathrm{eff}}=$1/2 state are observed for all concentrations where magnetic order is present. Finally, we have investigated Ca$_{4}$IrO$_{6}$ which appears to exhibit a nearly ideal J$_{\mathrm{eff}}=$1/2 state which is unperturbed by deviations from cubic crystal field level splitting. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W48.00003: Novel phase transitions in iridium dichalcogenides Invited Speaker: Yoon Seok Oh 5d transition metal oxides has attracted lots of attention because of exotic electronic phase resulted from entanglement of strong spin-orbit coupling and electron correlation in 5d orbital. In this manner, 5d transition metal chalcogenides is another intriguing 5d compound to have a rich variety of strongly correlated electronic states. In fact, recent studies of IrTe$_{2}$ reported chemical-doping/intercalation (Pd, Pt, Cu, and Rh) induced superconductivity and the unconventional structural modulations below $\sim$260 K. The simple empirical features of IrTe$_{2}$ resemble the conventional charge density waves (CDW) in the 3d/4d layered chalcogenides (e.g. 1T-TaS$_{2}$, and 1T-TiSe$_{2}$, etc.). But, recent corroborative experimental results indicate that instability of covalency of Ir ions induces the structural phase transition associated with soliton lattice of Te-Te covalent bonding. So far, there exist controversy to identify the exotic phase transition of IrTe$_{2}$. In this talk, we introduce recent investigations and discuss the phase transition in IrTe$_{2}$. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W48.00004: Excitations, order, and criticality in quantum pyrochlores Invited Speaker: Lucile Savary I will present recent work on quantum criticality in the (conducting) pyrochlore iridates in the context of the wide range of exotic phenomena that occur on the pyrochlore lattice, such as Coulombic quantum spin liquids and quantum order-by-disorder. The physics of the highly-unusual super-universal quantum critical point between a non-Fermi liquid and a Weyl semimetal (with Ising-like order) uncovered in a model relevant to the pyrochlore iridates will be discussed in detail. There, the fluctuations in the parent non-Fermi liquid phase compete with the fluctuations due to the coupling to the Ising order parameter. Remarkably, the fluctuations of both origins are of the same order of magnitude and the resulting quantum critical regime belongs to a unique, very large, universality class, genuinely different from those obtained by considering the effects of a single phenomenon. Moreover, the perturbative analysis is controlled, yielding better faith in the theory. The scaling laws and some unusual coefficients of many physical quantities will be discussed, and a scheme to observe the quantum critical point in experiment provided. Further experimental connections, in particular to Pr$_2$Ir$_2$O$_7$, where Pr spins are important, will be made. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:30PM |
W48.00005: Quantum fluctuations in spin-ice-like Pr$_{2}$Zr$_{2}$O$_{7}$ Invited Speaker: Kenta Kimura Spin ice is a classical frustrated magnet in which ferromagnetic dipolar interactions stabilize a frozen disordered state with Pauling residual entropy and emergent magnetic monopolar quasi-particles [1]. A new class of spin ice has been recently proposed for Pr and Yb pyrochlores, in which quantum-mechanical exchange interactions predominately provides the ferromagnetic coupling between neighboring spins. The resultant strong quantum fluctuations were found to generate exotic quantum magnetism. For example, it has been demonstrated that the Pr-based metallic pyrochlore Pr$_{2}$Ir$_{2}$O$_{7}$ shows a novel chiral spin liquid phase, which may be explained by quantum melting of spin ice. In the talk, we report our recent experimental results on magnetic properties of the insulating analog Pr$_{2}$Zr$_{2}$O$_{7}$ [2]. Pinch-point features in quasi-elastic diffuse neutron scattering reflects adherence to a divergence free local constraint for disordered spins on long time scales. In sharp contrast to conventional ice, however, more than 90{\%} of the neutron scattering is inelastic and devoid of pinch points furnishing evidence for magnetic monopolar quantum fluctuations. \\[4pt] [1] S. T. Bramwell and M. J. P. Gingras, Science 294, 5546 (2001).\\[0pt] [2] K. Kimura \textit{et al}., Nat. Commun. 4, 1934-1-6 (2013). [Preview Abstract] |
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