title: Emergence of nontrivial magnetic excitations in a spin-liquid state of kagomé volborthite.
abstract: When quantum fluctuations destroy underlying long-range ordered states, novel quantum states emerge. Spin-liquid (SL) states of frustrated quantum antiferromagnets, in which highly correlated spins fluctuate down to very low temperatures, are prominent examples of such quantum states. SL states often exhibit exotic physical properties, but the precise nature of the elementary excitations behind such phenomena remains entirely elusive. Here, we use thermal Hall measurements that can capture the unexplored property of the elementary excitations in SL states, and report the observation of anomalous excitations that may unveil the unique features of the SL state. Our principal finding is a negative thermal Hall conductivity [Formula: see text] which the charge-neutral spin excitations in a gapless SL state of the 2D kagomé insulator volborthite Cu3V2O7(OH)2[Formula: see text]2H2O exhibit, in much the same way in which charged electrons show the conventional electric Hall effect. We find that [Formula: see text] is absent in the high-temperature paramagnetic state and develops upon entering the SL state in accordance with the growth of the short-range spin correlations, demonstrating that [Formula: see text] is a key signature of the elementary excitation formed in the SL state. These results suggest the emergence of nontrivial elementary excitations in the gapless SL state which feel the presence of fictitious magnetic flux, whose effective Lorentz force is found to be less than 1/100 of the force experienced by free electrons.
authors.affiliation: Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan; Department of Physics, Kyoto University, Kyoto 606-8502, Japan;
authors.forename: Daiki
authors.initials: D
authors.lastname: Watanabe
authors.affiliation: Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan;
authors.forename: Kaori
authors.initials: K
authors.lastname: Sugii
authors.affiliation: Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan;
authors.forename: Masaaki
authors.initials: M
authors.lastname: Shimozawa
authors.affiliation: Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan;
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Suzuki
authors.affiliation: Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan;
authors.forename: Takeshi
authors.initials: T
authors.lastname: Yajima
authors.affiliation: Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan;
authors.forename: Hajime
authors.initials: H
authors.lastname: Ishikawa
authors.affiliation: Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan;
authors.forename: Zenji
authors.initials: Z
authors.lastname: Hiroi
authors.affiliation: Department of Advanced Materials Science, University of Tokyo, Chiba 277-8561, Japan.
authors.forename: Takasada
authors.initials: T
authors.lastname: Shibauchi
authors.affiliation: Department of Physics, Kyoto University, Kyoto 606-8502, Japan;
authors.forename: Yuji
authors.initials: Y
authors.lastname: Matsuda
authors.affiliation: Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan; my@issp.u-tokyo.ac.jp.
authors.forename: Minoru
authors.initials: M
authors.lastname: Yamashita
keywords: frustrated magnetism; spin liquid; thermal transport