Title: Evidence of a Phonon Hall Effect in the Kitaev Spin Liquid Candidate $\alpha \text{−}{\mathrm{RuCl}}_3$

Abstract

The material $\alpha \text{-}{\mathrm{RuCl}}_3$ 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, $\alpha \text{-}{\mathrm{RuCl}}_3$ undergoes a transition to a state with antiferromagnetic order below a temperature $T_N\approx 7\mathrm{K}$, but this order can be suppressed by applying an external in-plane magnetic field of $H_{\parallel }=7\mathrm{T}$. 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 $H_{\parallel }>7\mathrm{T}$ 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 ${\kappa }_{xy}$ in $\alpha \text{-}{\mathrm{RuCl}}_3$. Our measurements of ${\kappa }_{xy}\left(T\right)$ on several different crystals yield a temperature dependence very similar to that of the phonon-dominated longitudinal thermal conductivity ${\kappa }_{xx}\left(T\right)$, for which the natural explanation is that ${\kappa }_{xy}$ is also mostly carried by phonons. Upon cooling, ${\kappa }_{xx}$ peaks at $T\simeq 20\mathrm{K}$, 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 ${\kappa }_{xy}$ also increases suddenly below $T_N$ is strong evidence that the thermal Hall effect in $\alpha \text{-}{\mathrm{RuCl}}_3$ 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.

Authors:

Lefrançois, É. ;

• Search DOE PAGES for author "Lefrançois, É."
• Search DOE PAGES for ORCID "0000-0001-6661-0352"

Grissonnanche, G. ;

• Search DOE PAGES for author "Grissonnanche, G."
• Search DOE PAGES for ORCID "0000-0002-5115-3125"

Baglo, J. ;

• Search DOE PAGES for author "Baglo, J."
• Search DOE PAGES for ORCID "0000-0003-2800-0638"

Lampen-Kelley, P.; Yan, J. -Q. ;

• Search DOE PAGES for author "Yan, J. -Q."
• Search DOE PAGES for ORCID "0000-0001-6625-4706"

Balz, C.; Mandrus, D.; Nagler, S. E. ;

• Search DOE PAGES for author "Nagler, S. E."
• Search DOE PAGES for ORCID "0000-0002-7234-2339"

Kim, S. ;

• Search DOE PAGES for author "Kim, S."
• Search DOE PAGES for ORCID "0000-0002-1191-9640"

Kim, Young-June ;

• Search DOE PAGES for author "Kim, Young-June"
• Search DOE PAGES for ORCID "0000-0002-1172-8895"

Doiron-Leyraud, N.; Taillefer, Louis

• Search DOE PAGES for author "Taillefer, Louis"
• Search DOE PAGES for ORCID "0000-0002-8729-2194"

Publication Date:

Fri Apr 29 00:00:00 EDT 2022

Research Org.:

Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE Office of Science (SC), Basic Energy Sciences (BES). Materials Sciences & Engineering Division; Gordon and Betty Moore Foundation; Natural Sciences and Engineering Research Council of Canada (NSERC)

OSTI Identifier:

1865992

Alternate Identifier(s):

OSTI ID: 1869089

Grant/Contract Number:  

AC05-00OR22725; GBMF9069; RGPIN-2019-06449; RTI-2019-00809

Resource Type:

Published Article

Journal Name:

Physical Review. X

Additional Journal Information:

Journal Name: Physical Review. X Journal Volume: 12 Journal Issue: 2; Journal ID: ISSN 2160-3308

Publisher:

American Physical Society

Country of Publication:

United States

Language:

English

Subject:

36 MATERIALS SCIENCE; antiferromagnetism; Majorana fermions; quantum spin liquid; thermal Hall effect; antiferromagnets; honeycomb lattice; insulators; transport techniques