Evidence of a Phonon Hall Effect in the Kitaev Spin Liquid Candidate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>α</mml:mi><mml:mtext>−</mml:mtext><mml:msub><mml:mrow><mml:mi>RuCl</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>
Étienne Lefrançois, G. Grissonnanche, Jordan Baglo, Paula Lampen-Kelley, Jiaqiang Yan, Christian Balz, David Mandrus, S. E. Nagler, Subin Kim, Young‐June Kim, N. Doiron-Leyraud, Louis Taillefer
Abstract
The material -RuCl 3 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, -RuCl 3 undergoes a transition to a state with antiferromagnetic order below a temperature T N 7 K, but this order can be suppressed by applying an external in-plane magnetic field of H k 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 k > 7 T by Kasahara and co-workers [Nature (London) 559, 227 ( In this study, we reexamine the origin of the thermal Hall conductivity xy in -RuCl 3 . Our measurements of xy T on several different crystals yield a temperature dependence very similar to that of the phonondominated longitudinal thermal conductivity xx T, for which the natural explanation is that xy is also mostly carried by phonons. Upon cooling, xx peaks at T 20 K, then drops until T N , whereupon it suddenly increases again. The abrupt increase below T N 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 xy also increases suddenly below T N is strong evidence that the thermal Hall effect in -RuCl 3 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.