Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B24: Spin Frustration: Kitaev SystemsFocus

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Sponsoring Units: GMAG DMP Chair: Yukitoshi Motome, The University of Tokyo Room: LACC 403A 
Monday, March 5, 2018 11:15AM  11:51AM 
B24.00001: Anomalous Thermal Conductivity and Magnetic Torque Response in the Honeycomb Magnet α−RuCl3 Invited Speaker: Minhyea Lee We report on the unusual behavior of the inplane thermal conductivity κ and torque τ response in the KitaevHeisenberg material αRuCl3. κ shows a striking enhancement with linear growth beyond H ~ 7 T, where magnetic order disappears, while τ for both of the inplane symmetry directions shows an anomaly at the same field. The temperature and field dependence of κ are far more complex than conventional phonon and magnon contributions, and require us to invoke the presence of unconventional spin excitations whose properties are characteristic of a fieldinduced spinliquid phase related to the enigmatic physics of the Kitaev model in an applied magnetic field. 
Monday, March 5, 2018 11:51AM  12:03PM 
B24.00002: Magnetic exchange interactions in alphaRuCl3: ab initio study. Hoshin Gong, Bongjae Kim, Kyoo Kim, Sungdae Ji, Byung Min We study the magnetic exchange interactions in alphaRuCl3, a promising candidate of the Kitaev quantum spin liquid, by density functional theory calculation based on the newly reported rhombohedral crystal structure (R3). By precisely mapping the total energies of constrained noncollinear spin configurations into the KitaevHeisenberg Hamiltonian, we have obtained various isotropic and anisotropic interaction parameters of the zigzag magnetic order. Our calculation reveals the importance of further neighbor anisotropic and interlayer interactions to stabilize the zigzagtype antiferromagnetic order as a ground state of the system. Comparison with previous studies are also made. 
Monday, March 5, 2018 12:03PM  12:15PM 
B24.00003: Magnetism of single crystal αRuCl_{3} investigated by muon spin rotation Franz Lang, Stephen Blundell, Dharmalingam Prabhakaran We have performed muon spin rotation measurements on single crystal αRuCl_{3}, a honeycomb layered ruthenate that has been proposed to be close to a Kitaev quantum spin liquid ground state. Our investigation finds a single transition to long range magnetic order near $8$~K, in contrast to the two distinct transitions we previously observed in muon spin rotation measurements of a αRuCl_{3} powder sample [Lang et al, PRB 94 (2016) 020407(R)]. We interpret this in the light of a structural phase transition that has been recently discovered in high purity single crystals of this material. By employing density functional theory calculations we identify the likely muon stopping sites in this material and combine these with dipolar field calculations to investigate which previously reported zigzag antiferromagnetic structures are consistent with the two experimentally observed muon rotation frequencies. 
Monday, March 5, 2018 12:15PM  12:27PM 
B24.00004: Correlations and Thermal Transport in αRuCl3 at Finite Temperature Stephen Winter, Kira Riedl, David Kaib, Roser Valenti αRuCl_{3} has emerged as a candidate material for realising strongly anisotropic and frustrated interactions beyond the celebrated Kitaev model. Despite falling into an antiferromagnetically ordered state at low temperature, the combination of inelastic neutron scattering (INS) [1], electron spin resonance (ESR) [2] and Raman [3] measurements on αRuCl_{3} have revealed persistent magnetic correlations up to high temperatures. Such experiments also show a strong discrepancy between the lowenergy response for the magnetically ordered phase (T < T_{N}) and paramagnetic phase (T > T_{N}). Here we discuss this phenomenology with reference to recent abinitio derived magnetic models including terms beyond the Kitaev interaction. In particular, we will address [4]: (i) The evolution of the dynamical response (INS, ESR, Raman), (ii) the influence of local excitations on the static susceptibility, and (iii) the magnetic thermal transport in different temperature regimes. 
Monday, March 5, 2018 12:27PM  12:39PM 
B24.00005: Possible Proximity Effect in Graphene/$\alpha$RuCl$_3$ Layered Heterostructure Boyi Zhou, Erik Henriksen Recently, the layered antiferromagnetic Mott insulator $\alpha$RuCl$_3$ has been studied in bulk crystals and found to be proximate to a Kitaev quantum spin liquid. Since the QSL is primarily a twodimensional phenomenon, it may be possible to stabilize the QSL phase in exfoliated single or fewlayer $\alpha$RuCl$_3$. We performed DC transport measurement on exfoliated $\alpha$RuCl$_3$ flakes of various thicknesses and found the resistance to freeze out above 30 K, well above the N\'eel temperature. To access the low temperature regime with transport, we fabricated van der Waals heterostructure consisting of graphene and $\alpha$RuCl$_3$ layers in close proximity. The graphene resistance is found to show an unusually strong temperature dependence compared to graphene on SiO$_2$. More strikingly, a ``kink'' in the resistance centered at 20 K is observed, close to the onset of antiferromagnetic ordering in $\alpha$RuCl$_3$. 
Monday, March 5, 2018 12:39PM  12:51PM 
B24.00006: Magnetism in αRuCl_{3 }Under Pressure Gediminas Simutis, Alun Biffin, Rustem Khasanov, Denis Cheptiakov, Karl Krämer, Anna Isaeva, Thomas Doert, HansHenning Klauss, Christian Rüegg, Alex Amato, Hubertus Luetkens Bonddependent interactions between magnetic moments can lead to strong frustration and nontrivial ground states. In particular, the extended Kitaev model can host a spin liquid state or different frozen states depending on the strength of the additional Heisenberg interaction. In experiments, the relative interaction strengths in materials can be varied by applying pressure. 
Monday, March 5, 2018 12:51PM  1:03PM 
B24.00007: Fieldinduced phases in generalized Kitaev models and materials Roser Valenti, David Kaib, Kira Riedl, Radu Coldea, Stephen Winter By using a combination of exact diagonalization and semiclassical calculations we present a detailed analysis of possible fieldinduced phases obtained by adding to the Kitaev model further interaction terms. In particular we discuss the possibility of the appearance of topologically nontrivial fieldinduced spin liquid phases and the implications of these results in relation to the most studied αRuCl_{3} [1,2]. 
Monday, March 5, 2018 1:03PM  1:15PM 
B24.00008: Effect of external magnetic field on KitaevHeisenberg model GiaWei Chern, Yuriy Sizyuk, Craig Price, Natalia Perkins Recent experiments on αRuCl_{3} have shown signatures of Kitaev quantum spin liquid when an external magnetic field larger than 7 T is applied. At zero field the system is known to order into a zigzag pattern that can be explained by several KitaevHeisenberg models. We explore the effects of the magnetic field on this ground state using classical MonteCarlo simulations and spin wave analysis. We will show that the competition of the magnetic field with thermal fluctuations of the anisotropic Kitaev interaction leads to interesting new phases, including partially and fully incommensurate states. 
Monday, March 5, 2018 1:15PM  1:27PM 
B24.00009: Magnetic anisotropy due to pseudodipolar interactions in KitaevHeisenberg Model Hiroaki Ishizuka Magnets with strong spinorbit interactions often have large anisotropic interactions such as Kitaev (pseudodipolar) interactions. Such magnets show rich physics related to fluctuations such as spin liquid and orderbydisorder. On the other hand, later studies considering generalized effective models relevant to materials found that these models show rich ground state phase diagram due to the competition between different anisotropic interactions. Therefore, the understanding of the relative strength of anisotropic interactions is necessary for understanding the magnets, as well as to design a material with a desired property. However, there is no experimental method to estimate the strength of anisotropic interactions. In this work, we theoretically discuss the possibility of measuring the Kitaev interaction by magnetictorque measurement. Focusing on KitaevHeisenberg model, we find that the pseudodipolar interactions give rise to a magnetic anisotropy with a unique temperature dependence [1]. Furthermore, we find that a linear combination of the magnetictorque curve only depends on the Kitaev interactions. This allows an experimental observation of the pseudodipolar interactions. [1] H. Ishizuka, Phys. Rev. B 95, 184413 (2017). 
Monday, March 5, 2018 1:27PM  1:39PM 
B24.00010: Cu2IrO3: a new honeycomb iridate material with an ideal lattice geometry for the Kitaev model Fazel Fallah Tafti, Mykola Abramchuk, Cigdem OzsoyKeskinbora, Jason Krizan, Kenneth Metz, David Bell We present the first binary copper iridium oxide synthesized by a topotactic reaction from sodium iridate and copper chloride under mild conditions. Chemical analysis and crystallographic refinement reveal the composition Cu2IrO3 in a monoclinic C2/c space group with layered honeycomb structure. Magnetization measurements show a magnetic transition at low temperatures T = 2.75 K. The transition is subtle with weak hysteresis in the magnetization and without a clear signature in the heat capacity. The CurieWeiss analysis reveals that the effective iridium magnetic moment and the exchange coupling strength in Cu2IrO3 are similar to its parent compound Na2IrO3. Resistivity measurements show insulator behavior with variablerange hopping. The frustration index f = 40 in Cu2IrO3 compared to 8 in Na2IrO3 brings copper iridate in closer proximity to a Kitaev spin liquid than its predecessors. 
Monday, March 5, 2018 1:39PM  1:51PM 
B24.00011: Interplay between Kitaev Interaction and Single Ion Anisotropy in Ferromagnetic CrI_{3} and CrGeTe_{3} Monolayers Changsong Xu, Feng Jun Sheng, Hongjun Xiang, laurent bellaiche Firstprinciples calculations are performed and analyzed to develop a rather simple Hamiltonian, in order to investigate the magnetic anisotropy of the recently discovered ferromagnetic monolayers made of CrI3 and CrGeTe3. The anisotropic exchange coupling in these two systems is surprisingly determined to be of Kitaevtype, which broadens the candidate materials that may realize quantum spin liquid states. Moreover, the interplay between this Kitaev interaction and single ion anisotropy (SIA) is found to naturally explain the Ising behavior of CrI3 versus the Heisenberg behavior of CrGeTe3, as well as other observed phenomena in other related 2D systems. Finally, both the Kitaev interaction and SIA are further found to be induced by the spinorbit coupling of the heavy ligands (I of CrI3 or Te of CrGeTe3) rather than the 3d magnetic Cr ion. 
Monday, March 5, 2018 1:51PM  2:03PM 
B24.00012: Investigation of pressure driven transition in hyperhoneycomb iridates Panagiotis Peter Stavropoulos, HaeYoung Kee Strong spinorbit coupling in strongly correlated materials plays an important role in achieving quantum spin liquids. Among possible candidates, hyperhoneycomb iridates are particularly interesting as they may offer a threedimensional spin liquid phase. We study the phase diagram of hyperhoneycomb iridate βLi_{2}IrO_{4} under pressure. Using ab initio densety functional theory results and experimental lattice parameters, we first derive a minimal spin model for low to high pressure regime. Exact diagonalization is used to determine the phase diagram as a function of pressure. Experimental implications, including branching ratio, are also discussed. 
Monday, March 5, 2018 2:03PM  2:15PM 
B24.00013: Magnetic excitations in Z_{2} vortex crystals Mengqun LI, Ioannis Rousochatzakis, Natalia Perkins Z_{2} vortex crystals have been recently shown to be realized in triangular KitaevHeisenberg systems [1,2]. These Z_{2} vortices are stablized by spinorbit coupling introduced via the Kitaev term in the Hamiltonian. Here, we analyze the magnetic excitations of these Z_{2} vortex crystals and investigate the spin wave spectrum, energy gap and dynamical spin structure factor in terms of the interplay between the Kitaev and isotropic Heisenberg interaction [3]. 
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