# Transition of a prestine Mott insulator to a correlated Fermi liquid: Pressure-dependent optical investigations of a quantum spin liquid

## TL;DR

This study investigates how applying pressure transforms a two-dimensional organic quantum spin liquid from a Mott insulator into a correlated Fermi liquid, revealing electronic, lattice, and anisotropic changes.

## Contribution

It provides the first detailed pressure-dependent infrared analysis of the Mott insulator-metal transition in a quantum spin liquid, highlighting the coupling between lattice vibrations and electronic states.

## Key findings

- Suppression of insulating state with increasing pressure
- Emergence of coherent quasiparticles above 0.6 GPa
- Strong electron-lattice coupling evidenced by vibrational mode shifts

## Abstract

Systematic pressure- and temperature-dependent infrared studies on the two-dimensional organic quantum spin-liquid $\beta^{\prime}$-EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ disclose the electronic and lattice evolution across the Mott insulator-metal transition. Increasing hydrostatic pressure continuously suppresses the insulating ground state; for $p>0.6$~GPa, a Drude-like component develops indicating the appearance of coherent quasiparticles at the Fermi level. In the vicinity of the Mott transition, not only the electronic state changes rapidly, but also the vibration modes exhibit a jump both in frequency and Fano constant, underlining the strong coupling between lattice and electrons. The anisotropy of the in-plane optical response becomes inverted above 0.6~GPa. The findings are discussed in detail and summarized in a phase diagram comprising different experimental approaches.