Mott Transition, Compressibility Divergence and P-T Phase Diagram of Layered Organic Superconductors: An Ultrasonic Investigation

TL;DR

This study uses ultrasonic velocity measurements to map the phase diagram of a layered organic superconductor, revealing critical behavior and diverging compressibility near the Mott transition.

Contribution

It provides new ultrasonic data identifying crossover lines and critical points in the phase diagram of an organic superconductor, highlighting electronic compressibility divergence.

Findings

Identification of elastic anomalies indicating phase transitions

Observation of velocity softening near critical points

Evidence of diverging electronic compressibility

Abstract

The phase diagram of the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$Cl has been investigated by ultrasonic velocity measurements under helium gas pressure. Different phase transitions were identified trough several elastic anomalies characterized from isobaric and isothermal sweeps. Our data reveal two crossover lines that end on the critical point terminating the first-order Mott transition line. When the critical point is approached along these lines, we observe a dramatic softening of the velocity which is consistent with a diverging compressibility of the electronic degrees of freedom.