Abstract
The material 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, undergoes a transition to a state with antiferromagnetic order below a temperature , but this order can be suppressed by applying an external in-plane magnetic field of . 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 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 in . Our measurements of on several different crystals yield a temperature dependence very similar to that of the phonon-dominated longitudinal thermal conductivity , for which the natural explanation is that is also mostly carried by phonons. Upon cooling, peaks at , then drops until , whereupon it suddenly increases again. The abrupt increase below 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 also increases suddenly below is strong evidence that the thermal Hall effect in 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.
- Received 11 November 2021
- Revised 2 February 2022
- Accepted 22 February 2022
DOI:https://doi.org/10.1103/PhysRevX.12.021025
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Quantum spin liquids are materials that have the potential, in theory, to host novel kinds of particles or excitations, such as spinons or Majorana fermions. The challenge for experimentalists is to conclusively detect these. Some of the predicted excitations are mobile and thus detectable with thermal transport measurements. Using thermal Hall conductivity—a transverse thermal conductivity generated by a magnetic field—we shed new light on mobile carriers of heat in one spin liquid candidate, .
Thermal Hall conductivity has emerged as a probe of choice for detecting exotic low-energy excitations and, more recently, for measuring phonons. We perform such measurements on five different samples of at temperatures from 2 to 100 K under an external magnetic field of 15 T. Contrary to prior reports, we find no evidence of Majorana fermions, one type of exotic excitation predicted for this material. Instead, we attribute heat conduction—both longitudinal and transverse—to phonons scattering off spin excitations.
Along with other studies, our work shows that phonons can respond to magnetic fields and generate a thermal Hall signal. The underlying mechanism—in this and other insulators—remains to be elucidated.
