Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B39: Kitaev Systems IIFocus Live

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Sponsoring Units: GMAG DMP Chair: Natalia Perkins, University of Minnesota 
Monday, March 15, 2021 11:30AM  11:42AM Live 
B39.00001: The KitaevHeisenbergΓ Model on the Honeycomb Lattice ShangShun Zhang, Gabor Halasz, Cristian Batista We obtain the lowenergy excitation spectrum and the dynamical spin structure factor of the KitaevHeisenbergΓ model using a novel variational approach, that becomes exact at the exactly solvable Kitaev points. This approach reveals the physical origin of the asymmetry in the stability range of Kitaev spin liquid phases around the ferromagnetic and antiferromagnetic Kitaev points. The same analysis reveals the emergence of bound states of fractionalized excitations in the proximity of the quantum phase transition between the Kitaev spin liquid and different magnetically ordered states. Moreover, the type of magnetic ordering that is inferred from the dominant magnetic susceptibility coincides with the orderings that have been reported in different numerical works. 
Monday, March 15, 2021 11:42AM  11:54AM 
B39.00002: Sign structure of thermal Hall conductivity for inplane field polarized Kitaev magnets Li Ern Chern, Emily Zhang, YongBaek Kim The halfquantized thermal Hall conductivity measured in αRuCl3 in the presence of inplane magnetic fields has been taken as evidence for Kitaev spin liquid. Apart from the quantization, the sign structure of the thermal Hall conductivity is also consistent with theoretical predictions. When the field is perpendicular to one of the nearest neighbour bonds on the honeycomb lattice, the thermal Hall conductivity changes sign when the field direction is reversed, while it is nearly zero when the field is applied along the bond direction. We demonstrate that this sign structure is a generic property of the polarized state in the presence of inplane magnetic fields. Here, the thermal Hall effect arises from topological magnons with finite Chern numbers, and the sign structure follows from the symmetries of the momentum space Berry curvature. We show that the magnitude of the thermal Hall conductivity can be comparable to that observed in the experiments. Thus, the sign structure alone cannot make a strong case for Kitaev spin liquid. The quantization at very low temperatures, however, will be a decisive test as the magnon contribution vanishes in the zero temperature limit. [arXiv:2008.12788] 
Monday, March 15, 2021 11:54AM  12:06PM Live 
B39.00003: Investigation of Sample Dependence in αRuCl_{3} Crystals Subin Kim, YoungJune Kim αRuCl_{3} is a strong candidate for the realization of Kitaev’s honeycomb model, whose ground state is a quantum spin liquid with Majorana fermions. Many studies have been carried out on αRuCl_{3 }and some results have been explained by the fractionalized particle picture. However, many experimental results are still controversial due to the strong sample dependence of αRuCl_{3}. To investigate the sample dependence in αRuCl3, various αRuCl3 crystals grown using chemical vapour transport method with different conditions were characterized. Two types of crystals were grown: thick crystals and thin crystals. We found that thick crystals exhibit consistent xray diffraction pattern, magnetization response, and specific heat. On the other hand, strong sample dependence was found in the characterization data of the thin crystals. This work addresses the sample dependence in αRuCl_{3} by providing a way to synthesize highquality crystals with consistent physical properties and criteria to distinguish them from lowquality crystals. 
Monday, March 15, 2021 12:06PM  12:18PM Live 
B39.00004: Quantumclassical crossover in the spin1/2 HeisenbergKitaev kagome magnet Yang Yang, Ioannis Rousochatzakis, Natalia Perkins In our recent work [1], we revisit the crossover from the quantum resonating valence bond (RVB) phase to a semiclassical regime brought about by anisotropic Kitaev interactions in the spin1/2 Heisenberg kagome antiferromagnet, and focus on the precise mechanisms underpinning this crossover. We parametrize the classical ground states (GSs) in terms of emergent Isinglike variables, and use this: i) to construct an effective lowenergy description of the orderbydisorder mechanism, and ii) to contrast, exact diagonalization data obtained from the full basis with that obtained from the restricted basis of classical GSs. The results reveal that fluctuation corrections from states outside the restricted basis are strongly quenched in the semiclassical regime, and the RVB phase survives up to a relatively large value of Kitaev anisotropy K. Moreover the pure Kitaev model admits a subextensive number of onedimensional symmetries, which explains the absence of classical and quantum order by disorder. 
Monday, March 15, 2021 12:18PM  12:30PM Live 
B39.00005: NMR Investigation on Honeycomb Iridate Ag_{3}LiIr_{2}O_{6} Jiaming Wang, Faranak Bahrami, Weishi Yuan, Philip M Singer, HungYu Yang, Fazel Tafti, Takashi Imai Ag_{3}LiIr_{2}O_{6 }is a Kitaev spin liquid candidate material synthesised from α−Li_{2}IrO_{3} via topotactic reaction. We use ^{7}Li nuclear magnetic resonance (NMR) to investigate the physical properties of two samples of Ag_{3}LiIr_{2}O_{6} based on Knight shift and spinlattice relaxation rate 1/T_{1. }The first sample is shown via Xray diffraction to contain stacking faults between the honeycomb layers and displays a second NMR peak. The small values of its distinct Knight shift and 1/T_{1} suggest that they become nonmagnetic at low temperatures. The second sample contains only one sharp ^{7}Li peak. 
Monday, March 15, 2021 12:30PM  1:06PM Live 
B39.00006: Thermodynamic perspective on the fieldinduced behavior of \alphaRuCl3 Invited Speaker: Philipp Gegenwart Recent efforts in the search for an experimental realization of the Kitaev quantum spin liquid culminated in the observation of quantized thermal Hall effect in \alphaRuCl3. We inspect thermodynamics of this material in the relevant range of temperatures and magnetic fields. Our data exclude a separate spinliquid phase and rule out Kitaev spin liquid as a ground state of \alphaRuCl3, but indicate the presence of competing proximate phases. We put forward Grueneisen parameters as a sensitive experimental probe of this phase competition that will find applications across the field of frustrated magnetism and beyond. 
Monday, March 15, 2021 1:06PM  1:18PM Live 
B39.00007: Magnetic vortex liquid and anomalous thermal Hall conductivity in frustrated magnets with bonddependent interactions Li Ern Chern, Finn Lasse Buessen, YongBaek Kim Recently, the observation of large thermal Hall conductivities in correlated insulators with no apparent broken symmetry have generated immense interest and debates on the underlying ground states. Here, considering frustrated magnets with bonddependent interactions, which are realized in the socalled Kitaev materials, we theoretically demonstrate that a large thermal Hall conductivity can originate from a classical ground state without any magnetic order. We discover a novel liquid state of magnetic vortices, which are inhomogeneous spin textures embedded in the background of polarized spins, under outofplane magnetic fields. In the classical regime, different configurations of vortices form a degenerate manifold. We study the static and dynamical properties of the magnetic vortex liquid state at zero and finite temperatures. In particular, we show that the spin excitation spectrum resembles a continuum of nearly flat Chern bands, which ultimately leads to a large thermal Hall conductivity. Possible connections to experiments are discussed. [arXiv:2009.03332] 
Monday, March 15, 2021 1:18PM  1:30PM Live 
B39.00008: Analyzing thermodynamic signatures of fieldinduced phase transitions in αRuCl_{3} David Kaib, Sananda Biswas, Kira Riedl, Stephen Winter, Roser Valenti In αRuCl_{3} the observed approximately quantized thermal Hall conductivity at intermediate magnetic field strengths has alluded to a potentially fieldinduced Kitaev spin liquid (KSL). As a topologically nontrivial state, it cannot be adiabatically connected to the trivial highfield polarized phase and must be bounded to that by a phase transition. We therefore theoretically analyze recently measured^{1,2} Grüneisen Parameters (GP), which are sensitive probes of quantum phase transitions. 
Monday, March 15, 2021 1:30PM  1:42PM Live 
B39.00009: Revealing the Phase Diagram of Kitaev Materials by Machine Learning Ke Liu, Nicolas Sadoune, Nihal Rao, Jonas Greitemann, Lode Pollet Kitaev materials are promising materials for hosting quantum spin liquids and investigating the interplay of topological and symmetrybreaking phases. We use an unsupervised and interpretable machinelearning method, the tensorialkernel support vector machine (TKSVM), to study the phase diagram of candidate models for Kitaev materials. Our machine learns the global classical phase diagram for the honeycomb KitaevΓ model in a magnetic field and the associated order parameters, identifying several distinct spin liquids and unconventional orders. We find that the emergence of orders in the KitaevΓ model can be consistently explained by competition and cooperation between two spin liquids. We then apply our TKSVM method to the HeisenbergKitaevΓ model and discuss the effects of the Γ' and the J3 interaction. 
Monday, March 15, 2021 1:42PM  1:54PM Live 
B39.00010: Order by disorder and KosterlitzThouless transitions in classical Gammamodel subject to a magnetic field Zhongzheng Tian, Zhijie Fan, Preetha Saha, GiaWei Chern We present a combined analytical and numerical study of honeycomb Gamma model subject to magnetic field in the highsymmetry direction. Extensive Monte Carlo simulations are used to obtain the field H versus temperature T phase diagram. We show that the novel plaquetteordered spin liquid remains stable at low temperatures up to a field H_{c1} ≈ 0.5Γ, where Γ is the energy scale of anisotropic exchange interaction. We argue that the stability of this plaquette spin liquid comes from the extensive configurational entropy of hexagonal fluxes. As temperature is lowered, a longrange magnetic order with tripled unit cell is favored energetically. This magnetic ordered ground state gradually transforms to the fully polarized state at an upper critical field of H_{c2} = 4Γ. Interestingly, this √3 ×√3 spin order is shown to exhibit an emergent accidental O(2) symmetry, reminiscent of the O(3) rotation symmetry of the zerofield case. We show that a similar thermal orderbydisorder lifts this accidental degeneracy and selects a sixfold degenerate magnetic ground state.For intermediate magnetic field H_{c1 }< H < H_{c2} , the system undergoes two KosterlitzThouless transitions, with an intermediate critical phase, into the √3 ×√3 ground state. 
Monday, March 15, 2021 1:54PM  2:06PM Live 
B39.00011: Weakfield induced nonmagnetic state in a Cobased honeycomb Ruidan Zhong, Tong Gao, Nai Phuan Ong, Robert Cava Layered honeycomb magnets are of interest as potential realizations of the Kitaev quantum spin liquid (KQSL), a quantum state with longrange spin entanglement and an exactly solvable Hamiltonian. Conventional magnetically ordered states are present for all currently known candidate materials, however, because nonKitaev terms in the Hamiltonians obscure the Kitaev physics. Current experimental studies of the KQSL are focused on 4d or 5dtransitionmetalbased honeycombs, in which strong spinorbit coupling can be expected, yielding Kitaev interaction that dominate in an applied magnetic field. In contrast, for 3dbased layered honeycomb magnets, spin orbit coupling is weak and thus Kitaevphysics should be substantially less accessible. Here we report our studies on BaCo_{2}(AsO_{4})_{2}, for which we find that the magnetic order associated with the nonKitaev interactions can be fully suppressed by a relatively low magnetic field, yielding a nonmagnetic material and implying the presence of strong magnetic frustration and weak nonKitaev interactions. 
Monday, March 15, 2021 2:06PM  2:18PM Live 
B39.00012: Strain effects in αRuCl_{3} from first principles Sananda Biswas, David Kaib, Kira Riedl, Stephen Winter, Valentin Leeb, Johannes Knolle, Roser Valenti In recent years, the ‘proximate’ Kitaev spin liquid candidate, αRuCl_{3}, is under active study for exploring ways to tune it out of the zigzag antiferromagnetic order. Hence, different routes to enhance magnetic Kitaev interaction in αRuCl_{3} are pursued. First, we studied the effects of uniaxial strain in bulk αRuCl_{3}, when applied perpendicular to the honeymoon plane. In particular, we have explored the magnetoelastic coupling in αRuCl_{3} and the dependence of magnetic coupling constants on strain effects. We find that Kitaev interaction gets strongly enhanced under compression. Secondly, we have explored another route of straining αRuCl_{3}: by a substrate, graphene (gr). Due to proximity effect and charge transfer, αRuCl_{3} in the heterostructure system becomes lightly electrondoped, exhibiting insulator to metal transition. Quantum oscillation measurements of αRuCl_{3}/gr system show anomalous temperature dependence and we will discuss how to extract ab initio parameters for KitaevKondo lattice modelling which can explain such anomaly. 
Monday, March 15, 2021 2:18PM  2:30PM Live 
B39.00013: Testing Topological Phase Transitions in Kitaev Materials Under InPlane Magnetic Fields Jacob S Gordon, HaeYoung Kee Material realization of the nonAbelian Kitaev spin liquid phase  an example of Ising topological order (ITO)  has been the subject of intense research in recent years. The 4d honeycomb Mott insulator αRuCl_{3} has emerged as a leading candidate as it enters a fieldinduced magnetically disordered state where a halfinteger quantized thermal Hall conductivity κ_{xy} was reported. Further, a recent report of a sign change in the quantized κ_{xy} across a certain crystallographic direction is strong evidence for a topological phase transition between two ITOs with opposite chirality. Although this is fascinating, independent verification remains elusive, and one may ask if there is a thermodynamic quantity sensitive to the transition. Here we propose that the magnetotropic coefficient k would serve such a purpose. We report a singular feature in k that indicates a topological phase transition across the baxis where ITO is prohibited by a C_{2} symmetry. If the transition in αRuCl_{3} is indeed a direct transition between ITOs, or a more broad realization of a topological phase transition, then this feature in k should be observable. 
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