Evidence of a Phonon Hall Effect in the Kitaev Spin Liquid Candidate α−RuCl3

• Published in: Physical review. X Vol. 12; no. 2
• Main Authors: Lefrançois, É, Grissonnanche, G, Baglo, J, Lampen-Kelley, P, J.-Q. Yan, Balz, C, Mandrus, D, Nagler, S E, Kim, S, Young-June, Kim, Doiron-Leyraud, N, Taillefer, Louis
• Format: Journal Article
• Language: English
• Published: College Park American Physical Society 01.04.2022
• Subjects: Antiferromagnetism; Conduction heating; Conductive heat transfer; Electromagnetism; Excitation; Fermions; Hall effect; Insulators; Liquid phases; Magnetic fields; Phonons; Ruthenium trichloride; Scattering; Spin liquid; Temperature dependence; Thermal conductivity
• ISSN: 2160-3308
• DOI: 10.1103/PhysRevX.12.021025

The materialα−RuCl3has been the subject of intense scrutiny as a potential Kitaev quantum spin liquid, predicted to display Majorana fermions as low-energy excitations. In practice,α−RuCl3undergoes a transition to a state with antiferromagnetic order below a temperatureTN≈7K, but this order can be suppressed by applying an external in-plane magnetic field ofH∥=7T. Whether a quantum spin liquid phase exists just above that field is still an open question, but the reported observation of a quantized thermal Hall conductivity atH∥>7T by Kasahara and co-workers [Nature (London) 559, 227 (2018)] has been interpreted as evidence of itinerant Majorana fermions in the Kitaev quantum spin liquid state. In this study, we reexamine the origin of the thermal Hall conductivityκxyinα−RuCl3. Our measurements ofκxy(T)on several different crystals yield a temperature dependence very similar to that of the phonon-dominated longitudinal thermal conductivityκxx(T), for which the natural explanation is thatκxyis also mostly carried by phonons. Upon cooling,κxxpeaks atT≃20K, then drops untilTN, whereupon it suddenly increases again. The abrupt increase belowTNis attributed to a sudden reduction in the scattering of phonons by low-energy spin fluctuations as these become partially gapped when the system orders. The fact thatκxyalso increases suddenly belowTNis strong evidence that the thermal Hall effect inα−RuCl3is also carried predominantly by phonons. This implies that any quantized signal from Majorana edge modes would have to come on top of a sizable—and sample-dependent—phonon background.