Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L33: Kitaev Physics in Honeycomb IridatesFocus

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Sponsoring Units: DMP GMAG Chair: George Jackeli, University of Stuttgart Room: 336 
Wednesday, March 16, 2016 11:15AM  11:27AM 
L33.00001: Evidence for coexisting magnetic order in frustrated threedimensional honeycomb iridates Li$_{2}$IrO$_{3}$ Nicholas Breznay, Alejandro Ruiz, Alex Frano, James Analytis The search for unconventional magnetism has found a fertile hunting ground in 5d iridium oxide (iridate) materials. The competition between coulomb, spinorbit, and crystal field energy scales in honeycomb iridates leads to a quantum magnetic system with localized spin1/2 moments communicating through spinanisotropic Kitaev exchange interactions. Although early and ongoing work has focused on layered twodimensional honeycomb compounds such as Na$_{2}$IrO$_{3}$ and a 4d analog, RuCl$_{3}$, recently discovered polytypes of Li$_{2}$IrO$_{3}$ take on threedimensional honeycomb structures. Bulk thermodynamic studies, as well as recent resonant xray diffraction and absorption spectroscopy experiments, have uncovered a rich phase diagram for these threedimensional honeycomb iridates. Low temperature incommensurate and commensurate magnetic orders can be stabilized by tuning the applied magnetic field, displaying a delicate coexistence that signals highly frustrated magnetism. [Preview Abstract] 
Wednesday, March 16, 2016 11:27AM  11:39AM 
L33.00002: Vacancies in a 3DKitaev model on hyperhoneycomb lattice G J Sreejith, Subhro Bhattacharjee, Roderich Moessner We study the properties of isolated single and pairs of vacancies in an exactly solvable Kitaev model on a three dimensional hyperhoneycomb lattice. We show that each vacancy in the lattice is associated with a low energy spin like degree of freedom, similar to the case of previously studied honeycomb model. We calculate the contribution from these vacancy spinmoments to the low field magnetization response to a $z$directed field. Isolated vacancies in the gapped phase act as free spins. In the gapless phase, these spins interact with the surrounding spinliquid suppressing the lowfield magnetization to $\frac{1}{\sqrt{\ln[1/h_z]}}$. Pair of vacancies have a sublatticedependent, anisotropic, spinliquid mediated interaction with each other. In the gapless phase, interaction between vacancies in the same (opposite) sublattice enhances (suppresses) the lowfield magnetization, indicating a ferromagnetic (antiferromagnetic) nature. We also show that, unlike vacancies in the honeycomb lattice, the vacancies here do not bind a flux at lowenergies. [Preview Abstract] 
Wednesday, March 16, 2016 11:39AM  11:51AM 
L33.00003: SpinPeierls instability of threedimensional Kitaev spin liquids with Majorana Fermi surface Maria Hermanns, Simon Trebst, Achim Rosch The Kitaev honeycomb model is one of the paradigmatic examples of a frustrated spin system exhibiting a quantum spin liquid ground state. The emergent lowenergy degrees of freedom are Majorana fermions that can form various different (semi)metallic states. Threedimensional variants of this model can, in particular, harbor gapless quantum spin liquids with a Majorana Fermi surface. In this talk, we discuss Fermi surface instabilities arising from additional spin exchange terms (such as a Heisenberg coupling), which induce interactions between the emergent Majorana fermion degrees of freedom. We show that independent of the details of the interactions, the Majorana Fermi surface is always unstable. Generically, the system spontaneously dimerizes at exponentially small temperatures and forms a quantum spin liquid with nodal lines. Depending on the microscopic details, further symmetries of the system may be broken at this transition. These spinPeierls instabilities of a 3D spin liquid are closely related to BCS instabilities of fermions\footnote{M. Hermanns, S. Trebst, A. Rosch, PRL 115, 117205 (2015).}. [Preview Abstract] 
Wednesday, March 16, 2016 11:51AM  12:27PM 
L33.00004: Kitaev physics in three dimensional honeycomb iridates Invited Speaker: YongBaek Kim It has been realized that Mott insulators with strong spinorbit coupling may allow strongly bonddependent exchange interactions between local moments. Such interactions may lead to magnetic frustration and possible quantum spin liquid phases. This is in contrast to usual frustrated magnets, where the magnetic frustration comes from the geometry of the underlying lattice structure. Hence, it offers a new avenue to generate exotic phases of matter. Recently, both twodimensional ($\alpha$Li$_2$IrO$_3$) and threedimensional honeycomb iridates ($\beta$Li$_2$IrO$_3$ and $\gamma$Li$_2$IrO$_3$) have been discovered and it has been suggested that the magnetic exchange interactions contain the socalled Kitaev interaction, which depends on bond directions. In particular, the local moments of Ir ions in $\beta$Li$_2$IrO$_3$ and $\gamma$Li$_2$IrO$_3$ reside on the threedimensional hyperhoneycomb and stripyhoneycomb lattices. The Kitaev model is exactly solvable on these lattices as well as the twodimensional honeycomb lattice and the ground state is a quantum spin liquid with gapless excitations. We discuss recent progress in theoretical understanding of magnetic exchange interactions, possible presence of quantum spin liquid phases, and unusual magnetic order in $\beta$Li$_2$IrO$_3$ and $\gamma$Li$_2$IrO$_3$. These theoretical results are used to make connections to recent experimental data. [Preview Abstract] 
Wednesday, March 16, 2016 12:27PM  12:39PM 
L33.00005: Detecting Semimetal Surface Modes in Kitaev Spin Liquids Brent Perreault, Johannes Knolle, Natalia B. Perkins, F. J. Burnell Raman scattering is a useful probe for “Kitaevtype" spin liquids because it couples only to the dispersing (and potentially gapless) fermionic degrees of freedom in these systems. I will discuss Raman scattering in Kitaev spin liquids on the 3D hyperhoneycomb (\mathcal{H}0) lattice, where these fermionic degrees of freedom realize topologically nontrivial band structures with protected gapless surface states. I will describe Raman signatures both of bulk 3D samples, and thinfilm samples of these materials, where the resonant Raman response can detect the protected surface modes. [Preview Abstract] 
Wednesday, March 16, 2016 12:39PM  12:51PM 
L33.00006: Large Band Gap of alphaRuCl$_{\mathrm{3}}$ Probed by Photoemission and Inverse Photoemission Spectroscopy Soobin Sinn, Choong Hyun Kim, Luke Sandilands, Kyungdong Lee, Choongjae Won, Ji Seop Oh, Moonsup Han, Young Jun Chang, Namjung Hur, Hitoshi Sato, ByeongGyu Park, Changyoung Kim, HyeongDo Kim, Tae Won Noh The Kitaev honeycomb lattice model has attracted great attention because of its possibility to stabilize a quantum spin liquid ground state. Recently, it was proposed that alphaRuCl$_{\mathrm{3}}$ is its material realization and the first 4$d$ relativistic Mott insulator from an optical spectrum and LDA$+U+$SO calculations. Here, we present photoemission and inverse photoemission spectra of alphaRuCl$_{\mathrm{3}}$. The observed band gap is about 1.8 eV, which suggests that the previously assigned optical gap of 0.3 eV is misinterpreted, and that the strong peak at about 1.2 eV in the optical spectrum may be associated with an actual optical gap. Assuming a strong excitonic effect of 0.6 eV in the optical spectrum, all the structures except for the peak at 0.3 eV are consistent with our electronic spectra. When compared with LDA$+U+$SO calculations, the value of $U$ should be considerably larger than the previous one, which implies that the spinorbit coupling is not a necessary ingredient for the insulating mechanism of alphaRuCl$_{\mathrm{3}}$. We also present angleresolved photoemission spectra to be compared with LDA$+U+$SO and LDA$+$DMFT calculations. [Preview Abstract] 
Wednesday, March 16, 2016 12:51PM  1:03PM 
L33.00007: Abinitio study on crystal structure of $\alpha$RuCl$_3$ HaeYoung Kee, HeungSik Kim $\alpha$RuCl$_3$ was recently proposed as a candidate system for materialization of Kitaev model, but precise structural information of the compound has remained elusive. For the clarification of the full threedimensional crystal structure of $\alpha$RuCl$_3$, we performed abinitio electronic structure calculations including effects of spinorbit coupling (SOC) and electron correlations. We found that SOC prevents dimerization between Ru atoms, and keeps the system close to honeycomb lattice. The ground state crystal structure has monoclinic $C2/m$type layer stacking, but trigonal $P3_112$and orthorhombic $Cmc2_1$type stacking orders are comparable to the $C2/m$ structure in energy, so that stacking faults can be easily introduced. The electronic structure and the $j_{\rm eff}$=1/2 pseudospin exchange interactions and possible magnetic states in $\alpha$RuCl$_3$ will be presented. [Preview Abstract] 
Wednesday, March 16, 2016 1:03PM  1:15PM 
L33.00008: Nanoscale structural and electronic characterization of $\alpha $RuCl$_{\mathrm{3}}$ layered compound Maxim Ziatdinov, Artem Maksov, Arnab Banerjee, Wu Zhou, Tom Berlijn, Jiaqiang Yan, Stephen Nagler, David Mandrus, Arthur Baddorf, Sergei Kalinin The exceptional interplay of spinorbit effects, Coulomb interaction, and electronlattice coupling is expected to produce an elaborate phase space of $\alpha $RuCl$_{\mathrm{3}}$ layered compound, which to date remains largely unexplored. Here we employ a combination of scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM) for detailed evaluation of the system's microscopic structural and electronic orders with a subnanometer precision. The STM and STEM measurements are further supported by neutron scattering, XRay diffraction, density functional theory (DFT), and multivariate statistical analysis. Our results show a trigonal distortion of Cl octahedral ligand cage along the $C_{\mathrm{3}}$ symmetry axes in each RuCl$_{\mathrm{3}}$ layer. The lattice distortion is limited mainly to the Cl subsystem leaving the Ru honeycomb lattice nearly intact. The STM topographic and spectroscopic characterization reveals an intra unit cell electronic symmetry breaking in a spinorbit coupled Mott insulating phase on the Clterminated surface of $\alpha $RuCl$_{\mathrm{3}}$. The associated longrange charge order (CO) pattern is linked to a surface component of Cl cage distortion. We finally discuss a fine structure of CO and its potential relation to variations of average unit cell geometries found in multivariate analysis of STEM data. [Preview Abstract] 
Wednesday, March 16, 2016 1:15PM  1:27PM 
L33.00009: Probing Spin Excitations Using MagnetoRaman Spectroscopy K. Thirunavukkuarasu, Z. Lu, J. Simpson, A. Walker, J. Sears, Y.J. Kim, K. Burch, D. Smirnov The presence of a 2D quantum spin liquid state was recently suggested for the spinorbit coupled Mott insulator $\alpha$RuCl$_3$ with a honeycomb lattice.[Phys. Rev. 90, 041112 (2014)] Optical spectroscopy, Raman scattering, specific heat as well as magnetic susceptibility measurements on $\alpha$RuCl$_3$ identified elementary excitations due to electronic correlations and spinorbit coupling.[arXiv:1503.07593, Phys. Rev. Letters 114, 147201 (2015), and Phys. Rev. 91, 144420 (2015)] These observations appear to be consistent with theoretical expectations for HeisenbergKitaev model for QSL.[Phys. Rev. 91, 241110 (2015)] The underlying mechanism for the unconventional magnetism in $\alpha$RuCl$_3$ was further investigated by probing the effect of external magnetic field on the Raman spectroscopic signatures. Raman scattering experiments were performed at temperatures down to 5 K and magnetic fields up to 10 T. The intensity of strongest A$_{1g}$ phonon was found to decrease with increasing magnetic field strength suggesting the presence of strong magnetic interactions. The experimental observations and its implications will be presented. [Preview Abstract] 
Wednesday, March 16, 2016 1:27PM  1:39PM 
L33.00010: Phase diagram and quantum order by disorder in the Kitaev $K_1$$K_2$ honeycomb magnet Ioannis Rousochatzakis, Johannes Reuther, Ronny Thomale, Stephan Rachel, Natalia Perkins We show that the topological Kitaev spin liquid on the honeycomb lattice is extremely fragile against the second neighbor Kitaev coupling $K_2$, which has been recently identified as the dominant perturbation away from the nearest neighbor model in iridate Na$_2$IrO$_3$, and may also play a role in $\alpha$RuCl$_3$. This coupling explains naturally the zigzag ordering and the special entanglement between real and spin space observed recently in Na$_2$IrO$_3$. The minimal $K_1$$K_2$ model that we present here holds in addition the unique property that the classical and quantum phase diagrams and their respective orderbydisorder mechanisms are qualitatively different due to their fundamentally different symmetry structure. [Preview Abstract] 
Wednesday, March 16, 2016 1:39PM  1:51PM 
L33.00011: High Pressure Transport and Structural Study on the Honeycomb Lattice Iridates $A_2$IrO$_3$ ($A =$ Na, Li) Yogesh Singh, Samar Layek, Kavita Mehlawat, Eran Greenberg, Gregory Kh. Rozenberg, Moshe P. Pasternak The honeycomb lattice iridates $A_2$IrO$_3$ ($A =$ Na, Li) have been predicted and shown to exhibit novel magnetic properties which suggest that these materials could realize bonddirectional Kitaevlike magnetic exchange interactions [1  5]. We will present high pressure (P) electrical transport ($P \leq 80$~GPa) and powder Xray diffraction ($P \leq 40$~GPa) on these materials. The PXRD data for both materials show a structural transition around $P \sim 5$~GPa. The transport data show a dramatic reduction of the charge gap for Na$_2$IrO$_3$. References: [1] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev.Lett. {\bf 105}, 027204 (2010). [2] Y. Singh and P. Gegenwart, Phys. Rev. B {\bf 82}, 064412 (2010). [3] Y. Singh et al., Phys. Rev. Lett. {\bf 108}, 127203 (2012). [3] F. Ye et al., Phys. Rev. B {\bf 85}, 180403 (2012) [5] S. H. Chun et al. Nature Phys. {\bf 11}, 462 (2015). [Preview Abstract] 
Wednesday, March 16, 2016 1:51PM  2:03PM 
L33.00012: Order by disorder in KitaevHeisenberg models on the honeycomb lattice Natalia Perkins, Yuriy Sizyuk, Samuel Ducatman, Peter Woelfle Recent diffuse magnetic xray scattering data in Na2IrO3 [1] clearly determined the spin orientation in this zigzag state and showed that, unexpectedly, it is along the 44.3 degrees direction with respect to a axis, which is approximately half way in between the cubic x and y axes. This experiment provides an important check of the validity of any model proposed to described the magnetic properties of Na2IrO3 as the model should correctly predict not only the type of the magnetic order but also its orientation in space. We propose that order by disorder mechanism in quantum J1K1J2K2J3 model [2] gives the experimentally observed direction along cubic face diagonals. Our findings are based on both the calculation of the contribution of thermal fluctuations of quantum spins into free energy obtained by HubbardStratonovich transformation and the zeropoint correction to the ground state energy due to quantum spin fluctuations obtained by the spinwave expansion at zero temperature. [1] S. H. Chun et al, Nature Physics10,1038 (2015).? [2] Y. Sizyuk, C. Price, P. Woelfle, and N. B. Perkins,? Phys. Rev. B 90, 155126 (2014). [Preview Abstract] 
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