Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A48: Frustrated Magnetism: Kitaev ModelFocus

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Sponsoring Units: GMAG DMP Chair: Masafumi Udagawa, Gakushuin University Room: 395 
Monday, March 13, 2017 8:00AM  8:12AM 
A48.00001: Magnetic Phase Diagram of $\alpha$RuCl$_3$ Jennifer Sears, YoungJune Kim, Yang Zhao, Jeffrey Lynn The layered honeycomb material $\alpha$RuCl$_3$ is thought to possess unusual magnetic interactions including a strong bonddependent Kitaev term, offering a potential opportunity to study a material near a well understood spin liquid phase. Although this material orders magnetically at low temperatures and is thus not a realization of a Kitaev spin liquid, it does show a broad continuum of magnetic excitations reminiscent of that expected for the spin liquid phase. It has also been proposed that a magnetic field could destabilize the magnetic order in this material and induce a transition into a spin liquid phase. Low temperature magnetization and specific heat measurements in this material have suggested a complex magnetic phase diagram with multiple unidentified magnetic phases present at low temperature. This has provided motivation for our work characterizing the magnetic transitions and phase diagram in $\alpha$RuCl$_3$. I will present detailed bulk measurements combined with magnetic neutron diffraction measurements to map out the phase diagram and identify the various phases present. [Preview Abstract] 
Monday, March 13, 2017 8:12AM  8:24AM 
A48.00002: Breakdown of SpinWaves in Anisotropic Magnets: Spin Dynamics in $\alpha$RuCl$_3$ Stephen Winter, Kira Riedl, Andreas Honecker, Roser Valenti $\alpha$RuCl$_3$ has recently emerged as a promising candidate for realizing the hexagonal Kitaev model in a real material. Similar to the related iridates (e.g. Na$_2$IrO$_3$), complex magnetic interactions arise from a competition between various similar energy scales, including spinorbit coupling (SOC), Hund's coupling, and crystalfield splitting. Due to this complexity, the correct spin Hamiltonians for such systems remain hotly debated. For $\alpha$RuCl$_3$, a combination of abinitio calculations, microscopic considerations, and analysis of the static magnetic response have suggested offdiagonal couplings ($\Gamma, \Gamma^\prime$) and longrange interactions in addition to the expected Kitaev exchange (1,2). However, the effect of such additional terms on the dynamic response remains unclear.\\In this contribution, we discuss the recently measured inelastic neutron scattering response in the context of realistic proposals for the microscopic spin Hamiltonian. We conclude that the observed scattering continuum, which has been taken as a signature of Kitaev spin liquid physics, likely persists over a broad range of parameters.\\(1) S. M. Winter, et al, PRB 93, 214431 (2016).\\(2) R. Yadav, et al, arXiv:1604.04755 (2016).\\(3) A. Banerjee, et al, arXiv:1609.00103 (2016). [Preview Abstract] 
Monday, March 13, 2017 8:24AM  8:36AM 
A48.00003: Raman and electronic transport characterization of few and singlelayerthick $\alpha$RuCl$_3$ Boyi Zhou, Erik Henriksen The layered magnetic semiconductor $\alpha$RuCl$_3$, having a honeycomb lattice of spin1/2 moments, has been identified as a potential candidate material to realize the Kitaev quantum spin liquid. In particular, bulk RuCl$_3$ crystals have been studied and found to be on the cusp of manifesting QSL behavior [1]. As the QSL is primarily a twodimensional phenomenon, and since the layers of RuCl$_3$ are weakly coupled, we propose to create and study a 2D spin1/2 honeycomb system by isolating single sheets. Here we report the exfoliation of RuCl$_3$ down to few and singlelayerthick samples, which we characterize by Raman spectroscopy and atomic force microscopy at room temperature. We will also report our progress on measurements of basic electronic transport properties in the 2D RuCl$_3$ system by controlling the chemical potential via gating in a fieldeffect configuration. [1] A. Banerjee et al, Nature Materials \textbf{15}, 733 (2016). [Preview Abstract] 
Monday, March 13, 2017 8:36AM  9:12AM 
A48.00004: Magnetic Majorana Fermions Invited Speaker: Roderich Moessner Condensed matter systems provide emergent miniuniverses in which quasiparticles may exist which do not correspond to any experimentally detected elementary particle. Topological quantum materials have been particularly productive in this regard, with the present search focussing on Majorana fermions, known theoretically already for decades. Here, we discuss manifestations of magnetic Majorana fermions in the Kitaev model. We place particular emphasis on their fate when perturbations, such as Heisenberg terms, are added to the ideal model system, and address experimental signatures of their vestiges in phases adjacent to the spin liquid. [Preview Abstract] 
Monday, March 13, 2017 9:12AM  9:24AM 
A48.00005: Comprehensive study of the dynamics of a classical Kitaev Spin Liquid Anjana Samarakoon, Arnab Banerjee, Cristian Batista, Yoshitomo Kamiya, Alan Tennant, Stephen Nagler Quantum spin liquids (QSLs) have achieved great interest in both theoretical and experimental condensed matter physics due to their remarkable topological properties. Among many different candidates, the Kitaev model on the honeycomb lattice is a 2D prototypical QSL which can be experimentally studied in materials based on iridium or ruthenium.$_{\mathrm{\thinspace }}$Here we study the spin1/2 Kitaev model using classical MonteCarlo and semiclassical spin dynamics of classical spins on a honeycomb lattice. Both real and reciprocal space pictures highlighting the differences and similarities of the results to the linear spin wave theory will be discussed in terms dispersion relations of the pureKitaev limit and beyond. Interestingly, this technique could capture some of the salient features of the exact quantum solution of the Kitaev model, such as features resembling the Majoranalike mode comparable to the Kitaev energy, which is spectrally narrowed compared to the quantum result, can be explained by magnon excitations on fluctuating onedimensional manifolds (loops). Hence the difference from the classical limit to the quantum limit can be understood by the fractionalization of a magnon to Majorana fermions. The calculations will be directly compared with our neutron scattering data on $\alpha $RuCl$_{\mathrm{3}}$ which is a prime candidate for experimental realization of Kitaev physics. [Preview Abstract] 
Monday, March 13, 2017 9:24AM  9:36AM 
A48.00006: LiquidLiquid Transition in Kitaev Magnets Driven by Spin Fractionalization Joji Nasu, Yasuyuki Kato, Junki Yoshitake, Yoshitomo Kamiya, Yukitoshi Motome While phase transitions between magnetic analogs of three states of matter  a longrange ordered state, paramagnet, and spin liquid  have been extensively studied, the possibility of ``liquidliquid'' transitions, namely, between different spin liquids, remains elusive. By introducing the Ising coupling into the honeycomb Kitaev model with bond asymmetry, we discover that the Kitaev spin liquid becomes a bondnematic quantum paramagnet before magnetically ordered. The phase transition between the two liquidlike states with different topological nature is of first order, driven by delocalization of the Z$_{\mathrm{2}}$ gauge fluxes, and persists to a critical point at finite temperature located inside the regime where quantum spins are fractionalized. It is suggested that similar transitions may occur in other perturbed Kitaev magnets with bond asymmetry. [Preview Abstract] 
(Author Not Attending)

A48.00007: Density wave like transport anomalies in surface doped Na$_{2}$IrO$_3$ Yogesh Singh, Kavita Mehlawat We report that the surface conductivity of Na$_{2}$IrO$_3$ crystal is extremely tunable by high energy Ar plasma etching and can be tuned from insulating to metallic with increasing etching time. Temperature dependent electrical transport for the metallic samples show signatures of first order phase transitions which are consistent with charge or spin density wave like phase transitions recently predicted theoretically. Additionally, grazingincidence smallangle xray scattering (GISAXS) reveal that the room temperature surface structure of Na$_{2}$IrO$_3$ does not change after plasma etching. [Preview Abstract] 
Monday, March 13, 2017 9:48AM  10:00AM 
A48.00008: Dynamical properties of fractional excitations in Kitaev spin liquids Masafumi Udagawa Recently, Kitaev model is drawing a considerable interest as a new platform to study quantum spin liquid, and a number of compounds have been proposed as candidates to realize this novel state of matter. The Kitaev spin liquids host unusual excitations: spins are fractionalized into Majorana fermions and Z2 vortices, and the latter behave as abelian/nonabelian anyons. In addition to their own interests, it is quite useful to investigate the dynamical properties of these fractionalized excitations in light of experimental detection of Kitaev spin liquids. Indeed, so far, dynamical responses have been theoretically studied to make connection with experimental probes, such as inelastic neutron scattering, Raman scattering, and nuclear relaxation rate. However, in the previous studies, dynamics of vortex excitations have been less considered, since they are completely static at the solvable limit. In this contribution, we will focus on the dynamical properties of vortex excitations and report how they affect experimental observables. [Preview Abstract] 
Monday, March 13, 2017 10:00AM  10:12AM 
A48.00009: Fieldinduced orders in 3D MottKitaev Li2IrO3 Alejandro Ruiz, Alex Frano, Nicholas Breznay, Itamar Kimchi, Toni Helm, Iain Oswald, Julia Chan, Robert Birgeneau, Zahir Islam, James Analytis Honeycomb iridates have been the focus of substantial interest due to the strong magnetic frustration that arises from their edgeshared bonding environment, which favors a strongly anisotropic Isinglike exchange between bonds. In materials with edgeshared IrO6 octahedra, spinanisotropy of the exchange between neighboring effective spin1/2 states is enhanced by the interference of the two exchange paths across the planar IrO2Ir 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 Kitaevmodel. Previous RXS experiments on the orthorhombic Li2IrO3 samples revealed an incommensurate, noncoplanar magnetic structure with counterrotating moments, suggesting that Kitaev exchange is the dominant spin interaction in this system. In this work, we present thermodynamic and RXS data to illustrate in a concrete way how magnetic frustration and competing interactions combine to produce nearly degenerate and coexisting broken symmetry states. [Preview Abstract] 
Monday, March 13, 2017 10:12AM  10:24AM 
A48.00010: Twopeak structure in temperature dependence of the specific heat in spinS HeisenbergKitaev models on a honeycomb lattice Takafumi Suzuki An interesting phase called as the Kitaev's spin liquid (KSL) phase exists in the S$=$1/2 HeisenbergKitaev (HK) model on a honeycomb lattice. The lowenergy excitations in the KSL state is characterized by Majorana fermions resulting from fractionalization of quantum spins. In the Kitaev model [PRL 113, 197205 (2014)], the fractionalization is observed as a two peak structure in the temperature dependence of the specific heat $C(T)$. This two peak structure of $C(T)$ \quad survives in the magnetic ordered phase, if the system is located in the vicinity of the KSL phase, and this offers criteria for measuring the closeness to the KSL phase [PRB 93, 174425 (2016)]. Similarly, it has been studied that $C(T)$ of the HK model at the large S limit (classical HK model) shows a two peak structure [PRL 109, 187201 (2012)]. In this study, we have calculated $C(T)$ for spinS HK model and found that the origin of two peaks is different between the quantum and classical cases. The difference is evident in the higher temperature peaks in $C(T)$: For the quantum spin S$=$1/2, the higher temperature peak shrinks and the twopeak structure disappears for 3/2 \textless S. At the classical limit, an additional tiny peak seems to appear, independently of the twopeak structures in the quantum case. [Preview Abstract] 
Monday, March 13, 2017 10:24AM  10:36AM 
A48.00011: Quantum Spin Liquids in Hyperhoneycomb Lattices: Classifications and Applications to Pressurized $\beta$Li$_2$IrO$_3$ Biao Huang, Yong Baek Kim, YuanMing Lu Recent discoveries of frustrated magnets in various honeycombbased lattices with strong spinorbit couplings have drawn much attention, due to their proximity to a quantum spin liquid phase captured by the Kitaev model. Though magnetic orders have been identified in most compounds from this family, recent experiments suggested a symmetric spin liquid ground state in $\beta$Li$_2$IrO$_3$ under pressure, with no signatures of a finite temperature phase transition. Motivated by this experimental discovery, we systematically classify U(1) spin liquids on the hyperhoneycomb lattice, and study their properties using the slave fermion representation. The U(1) spin liquids in the neighborhood of the Kitaev $Z_2$ spin liquid are identified, which bridges the experimental observation and previous theoretical studies. We discuss the key features and energetics for various U(1) and $Z_2$ states, and identify the promising candidates for the spin liquid ground state in pressurized $\beta$Li$_2$IrO$_3$. [Preview Abstract] 
Monday, March 13, 2017 10:36AM  10:48AM 
A48.00012: Kitaev materials beyond iridates: order by quantum disorder and Weyl magnons in rareearth double perovskites FeiYe Li, YaoDong Li, Yue Yu, Gang Chen Motivated by the experiments on the rareearth double perovskites, we propose a generalized KitaevHeisenberg model to describe the generic interaction between the spinorbitentangled Kramers doublets of the rareearth moments. We carry out a systematic analysis of the meanfield phase diagram of this new model. In the phase diagram, there exist large regions with a continuous U(1) or O(3) degeneracy. Since no symmetry of the model protects such a continuous degeneracy, we predict that the quantum fluctuation lifts the continuous degeneracy and favors various magnetic orders in the phase diagram. From this order by quantum disorder mechanism, we further predict that the magnetic excitations of the resulting ordered phases are characterized by nearly gapless pseudoGoldstone modes. We find that there exist Weyl magnon excitations for certain magnetic orders. We expect our prediction to inspire further study of Kitaev physics, the order by quantum disorder phenomenon and topological spin wave modes in the rareearth magnets and the systems alike. [Preview Abstract] 
Monday, March 13, 2017 10:48AM  11:00AM 
A48.00013: Topological spin liquids in the ruby lattice with anisotropic Kitaev interactions Saeed S. Jahromi, Mehdi Kargarian, Abdollah Langari The ruby lattice is a fourvalent lattice interpolating between honeycomb and triangular lattices. In this work we investigate the topological spinliquid phases of a spin Hamiltonian with Kitaev interactions on the ruby lattice using exact diagonalization and perturbative methods. The latter interactions combined with the structure of the lattice yield a model with $Z_2 \times Z_2$ gauge symmetry. We mapped out the phase diagram of the model and found gapped and gapless spinliquid phases. While the lowenergy sector of the gapped phase corresponds to the wellknown topological color code model on a honeycomb lattice, the lowenergy sector of the gapless phases is described by an effective spin model with threebody interactions on a triangular lattice. A gap is opened in the spectrum in small magnetic fields, where we showed that the ground state has a finite topological entanglement entropy. We argue that the gapped phases could be possibly described by exotic excitations, and their corresponding spectrum is richer than the Ising phase of the Kitaev model. [Preview Abstract] 
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