Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F24: Kitaev and Other Spin Orbit Coupled SystemsFocus

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Sponsoring Units: GMAG DMP Chair: Itamar Kimchi, Massachusetts Institute of TechnologyMIT Room: LACC 403A 
Tuesday, March 6, 2018 11:15AM  11:27AM 
F24.00001: Polarized neutron study of alphaRuCl3 Christian Balz, Stephen Nagler, Arnab Banerjee, Paula Kelley The magnetic insulator alphaRuCl3 features a layered honeycomb structure of edgesharing octahedrally coordinated Ru3+ ions. The Ru ion is in the lowspin ground state and exhibits strong spinorbit coupling making alphaRuCl3 an interesting candidate for investigating physics related to the Kitaev model. Even though it orders magnetically at low temperatures, alphaRuCl3 exhibits a spectrum of magnetic excitations which includes a continuum previously identified as a signature of fractionalized excitations characteristic of quantum spin liquids. 
Tuesday, March 6, 2018 11:27AM  11:39AM 
F24.00002: NMR Evidence of Lowenergy Spin Excitations in the Fieldinduced Quanum Spin Liquid Phase of αRuCl_{3} Weiqiang Yu, Jiacheng Zheng, Kejing Ran, Jinsheng Wen, Bruce Normand, ZhengXin Liu αRuCl_{3} is a leading candidate material for the observation of physics related to the Kitaev quantum spin liquid (QSL). By combined susceptibility, specificheat, and ^{35}Cl nuclearmagneticresonance (NMR) measurements, we demonstrate that αRuCl3 undergoes a quantum phase transition to a QSL in a magnetic field of 7.5 T applied in the ab plane. We show further by the spinlattice relaxation rates that this highfield QSL phase has gapless spin excitations over a field range up to 16 T. This highly unconventional result, unknown in either Heisenberg or Kitaev magnets, offers insight essential to establish the physics of αRuCl_{3}. 
Tuesday, March 6, 2018 11:39AM  11:51AM 
F24.00003: Searching for Fractionalized particles in a potential Kiteav Spin Liquid Yiping Wang, Gavin Osterhoudt, Paige Kelley, David Mandrus, Kenneth Burch Relativistic Mott insulators on the honeycomb lattice are predicted to realize a novel topologically protected state known as the Kitaev Quantum Spin Liquid(QSL). Recently attention has focused on αRuCl3 that appears to be close to this limit with preliminary evidence from Neutrons and Raman for Majorana fermions. Here I will discuss our recent high resolution Raman spectroscopy of both Stokes and antiStokes spectra. We find new evidence for the nontrivial nature of these excitations. 
Tuesday, March 6, 2018 11:51AM  12:27PM 
F24.00004: Breakdown of Magnons in αRuCl3 Invited Speaker: Stephen Winter Intensive study of αRuCl_{3} has been motivated recently by signatures of strongly anisotropic and frustrated interactions reminiscent of the Kitaev honeycomb model. Even in the zigzag magnetically ordered state, αRuCl_{3} exhibits a broad continuum of magnetic excitations, which is inconsistent with conventional magnons [1]. Many works have taken the breakdown of magnons as a signature of proximity to the Kitaev spin liquid. By analogy, the suppression of magnetic order at finite magnetic field has been discussed in terms of a fieldinduced spinliquid state [2,3,4]. In order to evaluate these proposals, we have considered the dynamical response and stability of magnons at zero and finite field with respect to the full range of realistic magnetic interactions suggested by recent abinitio calculations. Combining extensive exact diagonalization studies with semiclassical analysis [5,6], we will address the (i) relevant mechanisms for magnon breakdown, (ii) origin and robustness of the continuum, (iii) evolution of the spectra under applied field, and (iv) nature of the fieldinduced state. 
Tuesday, March 6, 2018 12:27PM  12:39PM 
F24.00005: Resolving all magnons in antiferromagnetic $\alpha$RuCl$_3$ by timedomain terahertz spectroscopy Liang Wu, Erik Aldape, Arielle Little, Dylan Rees, Thomas Scaffidi, Paula Kelley, Arnab Banerjee, David Mandrus, Stephen Nagler, Ehud Altman, Joseph Orenstein Spin1/2 moments in the antiferromagnetic Mott insulator $\alpha$RuCl$_3$ are coupled by strongly anisotropic bonddependent exchange interactions on a honeycomb lattice. Intense study of RuCl$_3$ has been driven by the proposal that its low energy excitations may be adiabatically connected to the Majorana quasiparticles. However, the low field magnon behavior and the spin Hamiltonian have not been agreed upon, which is the first step to address whether this material hosts exotic fractionalized excitations. Highresolution timedomain terahertz spectroscopy enables us a systematic study of the field dependence of magnon excitations under an inplane magnetic field. I will discuss how many magnons there are below the critical field and the dramatic spectral weight shift between them in various geometries. By combining the linear spinwave theory, our measurements highly constrain the parameters of the spin Hamiltonian. 
Tuesday, March 6, 2018 12:39PM  12:51PM 
F24.00006: UltralowTemperature Thermal Conductivity of the Kitaev Honeycomb Magnet αRuCl_{3} across the FieldInduced Phase Transition Yunjie Yu, Yang Xu, Kejing Ran, Jiamin Ni, Yeyu Huang, Jinghui Wang, Jinsheng Wen, Shiyan Li Recently, there have been increasingly hot debates on whether there exists a quantum spin liquid in the Kitaev honeycomb magnet αRuCl_{3} in high magnetic field. To investigate this issue, we perform the ultralowtemperature thermal conductivity measurements on the single crystals of αRuCl_{3} down to 80 mK and up to 9 T. Our experiments clearly show a fieldinduced phase transition occurring at μ_{0}H_{c} ≈ 7.5 T, above which the magnetic order is completely suppressed. The minimum of thermal conductivity at 7.5 T is attributed to the strong scattering of phonons by the magnetic fluctuations. Most importantly, above 7.5 T, we do not observe any significant contribution of thermal conductivity from gapless magnetic excitations, which puts a strong constraint on the nature of the highfield phase of αRuCl_{3 }[1]. 
Tuesday, March 6, 2018 12:51PM  1:03PM 
F24.00007: Phase diagram of the KitaevHeisenbergJ_{2}J_{3} model: A DMRG study Yifan Jiang, HongChen Jiang The KitaevHeisenberg model is important in understanding the physics of iridates A_{2}IrO_{3} (A=Na,Li). To understand the spin liquid and vairous novel magnetic ordered phases, we investigate the KitaevHeisenbergJ_{2}J_{3} model on the honeycomb lattice by largescale densitymatrix renormalization group simulation. Without J_{2}, we find that a tiny J_{3}∼0.04 interaction is enough to drive the system from the Kitaev spin liquid to the zigzag magnetic ordered phase. When J_{3}=0, the system undergoes a phase transition to a spiral magnetic order phase for relatively larger J_{2}∼0.15. The full phase diagram of the KitaevHeisenberg model in the presence of both J_{2} and J_{3} is obtained and the connection to iridates is also discussed. 
Tuesday, March 6, 2018 1:03PM  1:15PM 
F24.00008: Neutron Scattering Study of Honeycomb Lithium Iridate KitaevSpin Liquid Candidate Thomas Halloran, Collin Broholm, Kemp Plumb The realization of materials that exhibit a quantum spin liquid state beyond one dimension has been a central focus of condensed matter physics research in recent years. The only exactly theoretically solvable model of a spinliquid is the Kitaev model, which consists of a 2D honeycomb lattice of J_{eff }= ½ moment magnetic ions. Here we examine a candidate for the realization of this state. The structure consists of a honeycomb lattice of Ir4+ ions with strong spinorbit coupling to support the bond dependent exchange interactions required in the Kitaev model. Hydrogen present reduces any interlayer nonKitaev interactions which causes order in similar Lithium Iridates. Using inelastic neutron scattering on the SEQUOIA instrument at ORNL, we probed the material for magnetic excitations. NMR studies show no magnetic ordering, ferromagnetic interactions are observed in CurieWeiss fitting, and the inelastic neutron scattering roughly shows broad lowQ excitations predicted by the Kitaev model. Together these traits encourage the possibility of this material exhibiting a spinliquid state. 
Tuesday, March 6, 2018 1:15PM  1:27PM 
F24.00009: Magnons in a Strongly SpinOrbital Coupled Magnet Stephen Winter, Kira Riedl, Pavel Maksimov, Alexander Chernyshev, Andreas Honecker, Roser Valenti We present the nonlinear spinwave theory calculations of the dynamical response of an extended KitaevHeisenberg model for the parameters applicable to αRuCl_{3}. We argue that large anisotropic terms of spinorbit nature necessarily imply strong anharmonic coupling of magnons. Subsequently, the overlap of one and twomagnon states must lead to broad spectral features in magnon spectrum. These are accompanied by a significant longitudinal component of the structure factor from the twomagnon states that are also broadened. We calculate magnon decay rates due to anharmonic coupling within a selfconsistent approach and find our results for the dynamical structure factor to be in a good agreement with both exact diagonalization results for the same set of parameters and experiment. 
Tuesday, March 6, 2018 1:27PM  1:39PM 
F24.00010: NonKitaev Spin Liquids in Kitaev Materials Yaodong Li, Gang Chen We point out that the Kitaev materials may not necessarily support Kitaev spin liquid. It is wellknown that having a Kitaev term in the spin interaction is not the sufficient condition for the Kitaev spin liquid ground state. Many other quantum spin liquids may be stabilized by the competing spin interactions of the systems. We thus explore the possibilities of nonKitaev spin liquids in the honeycomb Kitaev materials. We predict the spectroscopic properties of these nonKitaev spin liquids 
Tuesday, March 6, 2018 1:39PM  1:51PM 
F24.00011: Realizing quantum spin liquid phases in spinorbit driven correlated materials Andrei Catuneanu, Youhei Yamaji, Gideon Wachtel, HaeYoung Kee, YongBaek Kim The spin liquid phase is one of the prominent strongly interacting topological phases of matter whose unambiguous confirmation is yet to be reached despite intensive experimental efforts on numerous candidate materials. The challenge is derived from the difficulty of formulating realistic theoretical models for these materials and interpreting the corresponding experimental data. Here we study a theoretical model with bonddependent interactions, directly motivated by recent experiments on twodimensional correlated materials with strong spinorbit coupling. We show numerical evidence for the existence of an extended family of quantum spin liquids, which are possibly connected to the Kitaev spin liquid state. These results are used to provide an explanation of the scattering continuum seen in neutron scattering on αRuCl_{3}. 
Tuesday, March 6, 2018 1:51PM  2:03PM 
F24.00012: Dynamical Response of Classical Spin Liquid in SpinOrbit Magnets with Strong OffDiagonal Exchange Preetha Saha, GiaWei Chern Spin liquid state emerging in strongly interacting spin systems has been a highly sought after field of study. These materials retain their magnetic disorder even at zero temperatures. While spin liquid state in conventional cooperative paramagnets emerges due to geometrical frustration, in the case of Kitaevtype models it originates from highly anisotropic exchange interactions [1]. Microscopically, such anisotropic interaction is a direct result of strong spinorbit coupling [2]. Recently, a new type of classical spin liquid has been proposed on honeycomb lattice, that is driven by large nearestneighbor offdiagonal exchange interaction Γ [3]. We employ Monte Carlo simulations and LandauLifshitz dynamics to evaluate dynamical spin structure factor and study the excitations in this new classical liquid phase. Temperature evolution of structure factor and wave vector dependance of the spectral energy mode distribution is analyzed for pure Γmodel for both antiferromagnetic and ferromagnetic couplings. [1] A. Kitaev, Ann. Phys. 321, 2 (2005). [2] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009). [3] I. Rousochatzakis and N. B. Perkins Phys. Rev. Lett. 118, 147204 (2017). 
Tuesday, March 6, 2018 2:03PM  2:15PM 
F24.00013: Antiferromagnetism in the Geometrically Frustrated Orthorhombic Li_{4}MgReO_{6} Jeremy Carlo, Graeme Luke, Shahab Derakhshan The exotic physics revealed in geometrically frustrated materials has led to a search for families of geometrically frustrated systems which may be systematically studied. Familiar frustrated systems include the Kagome, pyrochlore, spinel, stacked trianguler, and facecentered cubic systems. Less wellknown are the socalled "rock salt ordered" A_{5}BO_{6} systems, particularly with highZ magnetic ions exhibiting strong spinorbit coupling on the B site. These systems tend to crystallize in either monoclinic (C2/m) or orthorhombic (Fddd) crystal settings, with somewhat different exchange parameters in each case, and these systems exhibit a wide range of ground states ranging from commensurate antiferromagnetism to spinglass behavior. Li_{4}MgReO_{6} had been previously synthesized in the monoclinic space group, exhibiting AFM order at 12K. Here we report synthesis of Li_{4}MgReO_{6} in the orthorhombic space group, and characterization using magnetic susceptibility and muon spin relaxation, finding AFM order setting in below 2K. 
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