Quantum-disordered state of magnetic and electric dipoles in a hydrogen-bonded Mott system

Abstract

Strongly enhanced quantum fluctuations often lead to a rich variety of quantum-disordered states. A representative case is liquid helium, in which zero-point vibrations of the helium atoms prevent its solidification at low temperatures. A similar behaviour is found for the internal degrees of freedom in electrons. Among the most prominent is a quantum spin liquid (QSL), in which localized spins are highly correlated but fluctuate even at absolute zero. Recently, a coupling of spins with other degrees of freedom has been proposed as an innovative approach to generate even more fascinating QSLs such as orbital--spin liquids. However, such ideas are limited to the internal degrees of freedom in electrons. Here, we demonstrate that a coupling of localized spins with the zero-point motion of hydrogen atoms (proton fluctuations) in a hydrogen-bonded organic Mott insulator provides a new class of QSLs. We find that a divergent dielectric behaviour towards a hydrogen-bond order is suppressed by the quantum proton fluctuations, resulting in a quantum paraelectric (QPE) state. Furthermore, our thermal-transport measurements reveal that a QSL state with gapless spin excitations rapidly emerges upon entering the QPE state. These findings indicate that the quantum proton fluctuations give rise to a novel QSL --- a quantum-disordered state of magnetic and electric dipoles --- through the coupling between the electron and proton degrees of freedom.

arXiv comment10 pages, 8 figures

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• Boundary-limited and glassy-like phonon thermal conduction in EtMe${}_3$Sb[Pd(dmit)${}_2$]${}_2$
• Electronic State and Optical Response in a Hydrogen-Bonded Molecular Conductor
• Quantum valence bond ice theory for proton-driven quantum spin-dipole liquids
• Quantum transport of topological spin solitonsin a one-dimensional organic ferroelectric
• Electronic correlations and spin frustration in the molecular conductors $\kappa$-(BEDT-TTF)${}_2$X probed by magnetic quantum oscillations
• Itinerant and topological excitations in a honeycomb spiral spin liquid candidate
• Evidence of the impurity spin coupling with quantum paraelectric fluctuations in CaTi1-xRuxO3