Evidence of a Phonon Hall Effect in the Kitaev Spin Liquid Candidate α − RuCl 3

• Published in: Physical review. X Vol. 12; no. 2
• Main Authors: Lefrançois, É., Grissonnanche, G., Baglo, J., Lampen-Kelley, P., Yan, J.-Q., Balz, C., Mandrus, D., Nagler, S. E., Kim, S., Kim, Young-June, Doiron-Leyraud, N., Taillefer, Louis
• Format: Journal Article
• Language: English
• Published: United States American Physical Society 29.04.2022
• Subjects: antiferromagnetism; antiferromagnets; honeycomb lattice; insulators; Majorana fermions; MATERIALS SCIENCE; quantum spin liquid; thermal Hall effect; transport techniques
• ISSN: 2160-3308; 2160-3308
• DOI: 10.1103/PhysRevX.12.021025

The material α-RuCl3 has 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-RuCl3 undergoes a transition to a state with antiferromagnetic order below a temperature TN≈7 K , but this order can be suppressed by applying an external in-plane magnetic field of H ∥ =7 T . 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 at H ∥ >7 T 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 conductivity κxy in α-RuCl3 . Our measurements of κxy ( T ) on several different crystals yield a temperature dependence very similar to that of the phonon-dominated longitudinal thermal conductivity κxx ( T ) , for which the natural explanation is that κxy is also mostly carried by phonons. Upon cooling, κxx peaks at T≃20 K , then drops until TN , whereupon it suddenly increases again. The abrupt increase below TN is 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 that κxy also increases suddenly below TN is strong evidence that the thermal Hall effect in α-RuCl3 is 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.