Evidence for a quantum dipole liquid state in an organic quasi-two-dimensional material

TL;DR

This study provides evidence for a quantum dipole liquid state in a molecular Mott insulator, where electric dipoles and spins fluctuate persistently without ordering at low temperatures.

Contribution

It reports the discovery of a quantum dipole liquid state in an organic Mott insulator, highlighting the coexistence and persistent fluctuations of charge and spin degrees of freedom.

Findings

Electric dipoles do not order at low temperatures.

Raman spectroscopy detects ongoing dipole fluctuations.

Heat capacity data supports persistent quantum fluctuations.

Abstract

Mott insulators are commonly pictured with electrons localized on lattice sites. Their low-energy degrees of freedom involve spins only. Here we observe emerging charge degrees of freedom in a molecule-based Mott insulator $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Br, resulting in a quantum dipole liquid state. Electrons localized on molecular dimer lattice sites form electric dipoles that do not order at low temperatures and fluctuate with frequency detected experimentally in our Raman spectroscopy experiments. The heat capacity and Raman scattering response are consistent with a scenario where the composite spin and electric dipole degrees of freedom remain fluctuating down to the lowest measured temperatures.