Effects of Disorder on the Pressure-Induced Mott Transition in $\kappa$-BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl

TL;DR

This study investigates how controlled disorder introduced by x-ray irradiation affects the pressure-induced Mott transition in an organic charge-transfer salt, revealing that low disorder preserves the transition's first-order nature while higher disorder smears it.

Contribution

It provides experimental evidence on the impact of disorder on the Mott transition, showing the transition's persistence at low disorder and its smearing at higher disorder levels, aligning with theoretical predictions.

Findings

First-order transition persists at low disorder levels.

Critical pressure and temperature decrease with disorder.

High disorder levels smear the transition, reducing hysteresis.

Abstract

We present a study of the influence of disorder on the Mott metal-insulator transition for the organic charge-transfer salt $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl. To this end, disorder was introduced into the system in a controlled way by exposing the single crystals to x-ray irradiation. The crystals were then fine-tuned across the Mott transition by the application of continuously controllable He-gas pressure at low temperatures. Measurements of the thermal expansion and resistance show that the first-order character of the Mott transition prevails for low irradiation doses achieved by irradiation times up to 100 h. For these crystals with a moderate degree of disorder, we find a first-order transition line which ends in a second-order critical endpoint, akin to the pristine crystals. Compared to the latter, however, we observe a significant reduction of both, the critical pressure $p_c$ and the critical temperature $T_c$. This result is consistent with the theoretically-predicted formation of a soft Coulomb gap in the presence of strong correlations and small disorder. Furthermore, we demonstrate, similar to the observation for the pristine sample, that the Mott transition after 50 h of irradiation is accompanied by sizable lattice effects, the critical behavior of which can be well described by mean-field theory. Our results demonstrate that the character of the Mott transition remains essentially unchanged at a low disorder level. However, after an irradiation time of 150 h, no clear signatures of a discontinuous metal-insulator transition could be revealed anymore. These results suggest that, above a certain disorder level, the metal-insulator transition becomes a smeared first-order transition with some residual hysteresis.