First-order Mott transition and its critical endpoint in a quasi-two-dimensional organic conductor, $κ-(BEDT-TTF)_2 Cu[N(CN)_2] Cl

Abstract

An organic Mott insulator, $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl, was investigated by resistance measurements under hydrostatic He gas pressure at zero and high magnetic fields. The resistance variation against pressure shows clear jump and hysteresis below 38 K, while it is continuous at the higher temperatures. This means that the first-order Mott transition has a critical endpoint at 38 K, where it changes into the crossover, analogous to the liquid-gas transition. The endpoint is characterized by critical divergence in pressure derivative of resistance, $|\frac{1}{R}\frac{\partial R}{\partial P}|$, vanishing of the resistance jump, and closing of the effective charge gap. The pressure-temperature phase diagrams under a zero field and a high magnetic field are presented.