Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B39: Kitaev Systems IIFocus Session 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 Kitaev-Heisenberg-Γ Model on the Honeycomb Lattice Shang-Shun Zhang, Gabor Halasz, Cristian Batista We obtain the low-energy excitation spectrum and the dynamical spin structure factor of the Kitaev-Heisenberg-Γ 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 Live |
B39.00002: Sign structure of thermal Hall conductivity for in-plane field polarized Kitaev magnets Li Ern Chern, Emily Zhang, Yong-Baek Kim The half-quantized thermal Hall conductivity measured in α-RuCl3 in the presence of in-plane 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 in-plane 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 α-RuCl3 Crystals Subin Kim, Young-June Kim α-RuCl3 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 α-RuCl3 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 α-RuCl3. To investigate the sample dependence in α-RuCl3, various α-RuCl3 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 x-ray 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 α-RuCl3 by providing a way to synthesize high-quality crystals with consistent physical properties and criteria to distinguish them from low-quality crystals. |
Monday, March 15, 2021 12:06PM - 12:18PM Live |
B39.00004: Quantum-classical crossover in the spin-1/2 Heisenberg-Kitaev 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 spin-1/2 Heisenberg kagome antiferromagnet, and focus on the precise mechanisms underpinning this crossover. We parametrize the classical ground states (GSs) in terms of emergent Ising-like variables, and use this: i) to construct an effective low-energy description of the order-by-disorder 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 one-dimensional 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 Ag3LiIr2O6 Jiaming Wang, Faranak Bahrami, Weishi Yuan, Philip M Singer, Hung-Yu Yang, Fazel Tafti, Takashi Imai Ag3LiIr2O6 is a Kitaev spin liquid candidate material synthesised from α−Li2IrO3 via topotactic reaction. We use 7Li nuclear magnetic resonance (NMR) to investigate the physical properties of two samples of Ag3LiIr2O6 based on Knight shift and spin-lattice relaxation rate 1/T1. The first sample is shown via X-ray 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/T1 suggest that they become non-magnetic at low temperatures. The second sample contains only one sharp 7Li peak. |
Monday, March 15, 2021 12:30PM - 1:06PM Live |
B39.00006: Thermodynamic perspective on the field-induced behavior of \alpha-RuCl3 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 \alpha-RuCl3. We inspect thermodynamics of this material in the relevant range of temperatures and magnetic fields. Our data exclude a separate spin-liquid phase and rule out Kitaev spin liquid as a ground state of \alpha-RuCl3, 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 bond-dependent interactions Li Ern Chern, Finn Lasse Buessen, Yong-Baek 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 bond-dependent interactions, which are realized in the so-called 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 out-of-plane 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 field-induced phase transitions in α-RuCl3 David Kaib, Sananda Biswas, Kira Riedl, Stephen Winter, Roser Valenti In α-RuCl3 the observed approximately quantized thermal Hall conductivity at intermediate magnetic field strengths has alluded to a potentially field-induced Kitaev spin liquid (KSL). As a topologically nontrivial state, it cannot be adiabatically connected to the trivial high-field polarized phase and must be bounded to that by a phase transition. We therefore theoretically analyze recently measured1,2 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 symmetry-breaking phases. We use an unsupervised and interpretable machine-learning method, the tensorial-kernel support vector machine (TK-SVM), to study the phase diagram of candidate models for Kitaev materials. Our machine learns the global classical phase diagram for the honeycomb Kitaev-Γ 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 Kitaev-Γ model can be consistently explained by competition and cooperation between two spin liquids. We then apply our TK-SVM method to the Heisenberg-Kitaev-Γ 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 Kosterlitz-Thouless transitions in classical Gamma-model subject to a magnetic field Zhongzheng Tian, Zhijie Fan, Preetha Saha, Gia-Wei Chern We present a combined analytical and numerical study of honeycomb Gamma model subject to magnetic field in the high-symmetry direction. Extensive Monte Carlo simulations are used to obtain the field H versus temperature T phase diagram. We show that the novel plaquette-ordered spin liquid remains stable at low temperatures up to a field Hc1 ≈ 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 long-range 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 Hc2 = 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 zero-field case. We show that a similar thermal order-by-disorder lifts this accidental degeneracy and selects a six-fold degenerate magnetic ground state.For intermediate magnetic field Hc1 < H < Hc2 , the system undergoes two Kosterlitz-Thouless transitions, with an intermediate critical phase, into the √3 ×√3 ground state. |
Monday, March 15, 2021 1:54PM - 2:06PM Live |
B39.00011: Weak-field induced nonmagnetic state in a Co-based 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 long-range spin entanglement and an exactly solvable Hamiltonian. Conventional magnetically ordered states are present for all currently known candidate materials, however, because non-Kitaev terms in the Hamiltonians obscure the Kitaev physics. Current experimental studies of the KQSL are focused on 4d- or 5d-transition-metal-based honeycombs, in which strong spin-orbit coupling can be expected, yielding Kitaev interaction that dominate in an applied magnetic field. In contrast, for 3d-based layered honeycomb magnets, spin orbit coupling is weak and thus Kitaev-physics should be substantially less accessible. Here we report our studies on BaCo2(AsO4)2, for which we find that the magnetic order associated with the non-Kitaev interactions can be fully suppressed by a relatively low magnetic field, yielding a non-magnetic material and implying the presence of strong magnetic frustration and weak non-Kitaev interactions. |
Monday, March 15, 2021 2:06PM - 2:18PM Live |
B39.00012: Strain effects in α-RuCl3 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, α-RuCl3, 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 α-RuCl3 are pursued. First, we studied the effects of uniaxial strain in bulk α-RuCl3, when applied perpendicular to the honeymoon plane. In particular, we have explored the magnetoelastic coupling in α-RuCl3 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 α-RuCl3: by a substrate, graphene (gr). Due to proximity effect and charge transfer, α-RuCl3 in the heterostructure system becomes lightly electron-doped, exhibiting insulator to metal transition. Quantum oscillation measurements of α-RuCl3/gr system show anomalous temperature dependence and we will discuss how to extract ab initio parameters for Kitaev-Kondo 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 In-Plane Magnetic Fields Jacob S Gordon, Hae-Young Kee Material realization of the non-Abelian 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 α-RuCl3 has emerged as a leading candidate as it enters a field-induced magnetically disordered state where a half-integer quantized thermal Hall conductivity κxy was reported. Further, a recent report of a sign change in the quantized κxy 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 b-axis where ITO is prohibited by a C2 symmetry. If the transition in α-RuCl3 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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