Abstract
The material a-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, a-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)NATUAS0028-083610.1038/s41586-018-0274-0] 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 ?xy in a-RuCl3. Our measurements of ?xy(T) on several different crystals yield a temperature dependence very similar to that of the phonon-dominated 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 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 ?xy also increases suddenly below TN is strong evidence that the thermal Hall effect in a-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.
| Original language | English |
|---|---|
| Article number | 021025 |
| Journal | Physical Review X |
| Volume | 12 |
| Issue number | 2 |
| DOIs | |
| State | Published - Jun 2022 |
Funding
We thank L. Balents, K. Behnia, B. Flebus, C. Hess, H.-Y. Kee, Y.\u2009B. Kim, S.\u2009A. Kivelson, A.\u2009H. MacDonald, N. Perkins, J.\u2009A. Quilliam, S. Sachdev, L. Savary, and R. Valent\u00ED for stimulating discussions. We thank S. Fortier for his assistance with the experiments and M. Dion for his assistance with sample orientation. L.\u2009T. acknowledges support from the Canadian Institute for Advanced Research (CIFAR) as a CIFAR Fellow and funding from the Institut Quantique, the Natural Sciences and Engineering Research Council of Canada (NSERC; PIN:123817), the Fonds de Recherche du Qu\u00E9bec\u2013Nature et Technologies (FRQNT), the Canada Foundation for Innovation (CFI), and a Canada Research Chair. This research was undertaken thanks in part to funding from the Canada First Research Excellence Fund. C.\u2009B. and S.\u2009E.\u2009N. were supported by the U.S. Department of Energy (DOE), Basic Energy Sciences, Scientific User Facilities Division. S.\u2009E.\u2009N. also acknowledges support from the Quantum Science Center (QSC), a National Quantum Information Science Research Center of the U.S. DOE. Materials synthesis by J.\u2009Q.\u2009Y. was supported by the U.S. DOE, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. D.\u2009M. acknowledges support from the Gordon and Betty Moore Foundation\u2019s EPiQS Initiative, Grant No. GBMF9069. Work at the University of Toronto was supported by NSERC (RGPIN-2019-06449 and RTI-2019-00809), CFI, and Ontario Ministry of Research and Innovation.