Bandwidth-tuning from insulating Mott quantum spin liquid to Fermi liquid via chemical substitution in $\kappa$-[(BEDT-TTF)$_{1-x}$(BEDT-STF)$_x$]$_2$Cu$_2$(CN)$_3$

TL;DR

This study demonstrates how chemical substitution in a molecular conductor can tune its electronic state from a Mott insulator with quantum spin liquid properties to a metallic Fermi liquid, revealing a first-order phase transition with percolative effects.

Contribution

It introduces a novel chemical substitution method to control bandwidth and induce a Mott transition in a molecular conductor, expanding the tools for studying correlated electron systems.

Findings

Observation of a Mott insulator-to-metal transition via chemical substitution.

Detection of phase coexistence and percolative effects near the transition.

Enhanced dielectric constant indicating inhomogeneous metallic regions.

Abstract

The electronic properties of molecular conductors can be readily varied via physical or chemical pressure as it increases the bandwidth W; this enables crossing the Mott insulator-to-metal phase transition by reducing electronic correlations U/W. Here we introduce an alternative path by increasing the molecular orbitals when partially replacing sulfur by selenium in the constituting bis-(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) molecules of the title compound. We characterize the tuning of the insulating quantum spin liquid state via a Mott transition to the metallic Fermi-liquid state by transport, dielectric, and optical measurements. At this first-order phase transition, metallic regions coexist in the insulating matrix leading to pronounced percolative effects most obvious in a strong enhancement of the dielectric constant at low temperatures.