Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F45: Fractionalized Excitations, Nonlinearities and Phonons in Kitaev systemsInvited Live Streamed

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Sponsoring Units: DCMP GMAG DMP Chair: Natalia Perkins, University of Minnesota Room: McCormick Place W375D 
Tuesday, March 15, 2022 8:00AM  8:36AM 
F45.00001: Oscillations of the thermal conductivity in the proximate Kitaev system αRuCl_{3} Invited Speaker: N. Phuan Ong The honeycomb magnet αRuCl_{3} is proximate to the Kitaev Hamiltonian. In an inplane magnetic field Ba (at low temperatures T) a quantum spin liquid (QSL) is sandwiched between the antiferromagnetic (zigzag) state (0 to 7.3 T) and a spinpolarized state (B>11.6 T). Highresolution measurements of the thermal conductivity martix κ_{ij} (from 0.3 to 6 K, with Bx, ▽Tx and xa) have uncovered oscillations in κ_{xx}. The amplitudes are maximal in the QSL regime. Above 11.6 T, they collapse abruptly to zero, wheres below 7 T, they survive as a weak remnant to 4 T. The oscillations are periodic in 1/H_{a} (H_{a} is the component of B along a) with a breakinslope near 7 T. Across samples, phase and periodicity are closely reproducible, with the largest amplitudes observed in the purest crystals (benchmarked by the plateeau value of κ_{xx }at 12 T and 1 K). The oscillations suggest Landau level quantization despite the rigorous absence of free electrons (the charge gap is 2 eV). An intriguing possibility is that the oscillations arise from a quantized neutral Fermi surface in the QSL state. The planar thermal Hall conductivity κ_{xy} is also of strong interest. When Bb, we confirm that κ_{xy} is zero (to a resolution of 20 μK at 0.4 K) in accord with mirror symmetry. If Ba, however, an unusual Onsagertype planar thermal Hall conductivity emerges above ~3 K and grows very rapidly to ~10 K. We do not observe a quantization plateau at any T. Instead, the steep variation with T implies that the heat carriers are bosonic instead of fermionic (this rules out quantization). We show that κ_{xy }arises from spinexcitation edgemodes driven by a Berry curvature Ω. A fit to bosonic modes yields an energy scale (1 meV) that varies with B in agreement with sharp modes seen in spin resonance experiments. Moreover, the overall magnitude of κ_{xy} yields a Chern number C~1 consistent with the edgemode boson model. 
Tuesday, March 15, 2022 8:36AM  9:12AM 
F45.00002: Anisotropic Nonlinear Magnetic Susceptibility in Quantum Spin Systems Invited Speaker: Bellave S Shivaram The leading order nonlinear susceptibility, χ_{3} , in a paramagnet is negative and diverges as T→0. This divergence is destroyed when spins correlate and the nonlinear response provides unique insights into magnetic order. Dimensionality, exchange interaction, and preponderance of quantum effects all imprint their signatures in the nonlinear magnetic response. We will start by reviewing DC nonlinear susceptibility measurements in insulating and itinerant magnets in general. We will follow this with a focus on recent nonlinear susceptibility studies in the proximate Kitaev magnet RuCl_{3}. The possible simultaneous dual role of the spins, itinerant and localized, will be examined. 
Tuesday, March 15, 2022 9:12AM  9:48AM 
F45.00003: Majoranaphonon coupling in Kitaev quantum spin liquid candidate α RuCl_{3} Invited Speaker: Hu Miao

Tuesday, March 15, 2022 9:48AM  10:24AM 
F45.00004: Phonon dynamics in the Kitaev spin liquid Invited Speaker: Mengxing Ye The search for fractionalization in quantum spin liquids largely relies on their decoupling with the environment. However, the spinlattice interaction is inevitable in a real setting. While the Majorana fermion evades a strong decay due to the gradient form of spinlattice coupling, the study of the phonon dynamics may serve as an indirect probe of fractionalization in a quantum spin liquid. We show that the signatures of fractionalization of the Kitaev spin liquid can be seen in the sound attenuation and the Hall viscosity: the former describes the phonon decay into the fractionalized excitations, and the latter is the leading order timereversal symmetry breaking effect on the acoustic phonon. This talk is based on two papers: the analytical calculations in the lowtemperature zeroflux sector [1], and numerical calculations in the inhomogeneous flux sectors at intermediate and high temperatures [2]. 
Tuesday, March 15, 2022 10:24AM  11:00AM 
F45.00005: Quantized thermal Hall effect and topological phase transition in the Kitaev spin liquid candidate αRuCl_{3} Invited Speaker: Yuichi Kasahara Kitaev quantum spin liquid (QSL) hosts Majorana fermions and nonAbelian anyons in magnetid fields as a result of the fractionalization of quantum spins. The emergence of chiral Majorana edge modes leads to a halfinteger quantized thermal Hall conductance, which has been reported for the Kitaev spin liquid candidate αRuCl_{3} [14]. The remarkable feature is that the halfinteger thermal Hall plateau appeas even in a planar Hall geometry, i.e., in magnetic fields with no outofplane components. This is in marked contrast to the quantum Hall effect in twodimensional electron gas. Moreover, the measured fieldangular variation of the quantized plateau has the same sign structure as the topological Chern number proposed in the original Kitaev model [4]. 
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