Emergence of nontrivial magnetic excitations in a spin liquid state of kagome 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 keep to 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 utilize thermal Hall measurements that can capture the unexplored property of the elementary excitations in SL states, and report on the observation of anomalous excitations that may unveil the unique features of the SL state. Our principal finding is a negative thermal Hall conductivity (k_xy) which the charge-neutral spin excitations in a gapless SL state of the two-dimensional kagome insulator volborthite Cu_3V_2O_7(OH)_2 \cdot 2H_2O exhibit, in much the same way in which charged electrons give rise to the conventional electric Hall effect. We find that k_xy 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 k_xy 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 that experienced by free electrons.

9 pages, 4 figures, Proc. Natl. Acad. Sci. USA (2016)

References in corpus (11)

• Observation of the Magnon Hall Effect
• U(1) Gauge Theory of the Hubbard Model : Spin Liquid States and Possible Application to k-(BEDT-TTF)_2 Cu_2 (CN)_3
• Topological Nature of the Phonon Hall Effect
• Hall effect of triplons in a dimerized quantum magnet
• Thermal Hall Effect of Spins in a Paramagnet
• One-Third Magnetization Plateau with a Preceding Novel Phase in Volborthite
• Novel Pauli-paramagnetic quantum phase in a Mott insulator
• Magnetothermal transport in the spin-1/2 chains of copper pyrazine dinitrate
• Origin of the Phonon Hall Effect in Rare-Earth Garnets
• Phonon Hall thermal conductivity from Green-Kubo formula
• Coupled frustrated quantum spin-1/2 chains with orbital order in volborthite Cu3V2O7(OH)2(H2O)2

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