Comparative transport and thermal expansion studies on quasi-2D organic superconductors close to the metal-to-insulator transition

A deuterated crystal of κ-(BEDT-TTF)2Cu[N(CN)2]Br has been studied by high-resolution thermal expansion and resistivity measurements. This crystal is found to be situated close to the critical pressure which marks the end point of the first-order metal-to-insulator transition in the generalized phase diagram of κ-(BEDT-TTF)2X. The in-plane a-axis expansion coefficient reveals a distinct anomaly at 30 K which is attributed to the anomalous lattice response upon approaching the first-order metal-to-insulator transition. This feature is shown to be intimately related to the anomaly at T∗ ≈ 40 K for the hydrogenated salt.