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

## TL;DR

This study reveals a new quantum spin liquid state in a hydrogen-bonded Mott insulator, where proton fluctuations induce a quantum-disordered state of magnetic and electric dipoles, linking electron and proton degrees of freedom.

## Contribution

It demonstrates that coupling between localized spins and proton zero-point motion creates a novel quantum spin liquid in a hydrogen-bonded Mott system.

## Key findings

- Quantum proton fluctuations suppress hydrogen-bond order.
- Emergence of a gapless quantum spin liquid state.
- Observation of quantum paraelectric behavior.

## 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.