Lattice effects and entropy change at the Mott transition of a quasi-2D organic conductor

Layered organic charge-transfer salts of the κ-(BEDT-TTF)2X family have emerged as exemplary systems for exploring effects of strong electron-electron correlations. Here we report on high-resolution dilatometric investigations on a fully deuterated crystal of the X=Cu[N(CN)2]Br salt, where the Mott metal-insulator (MI) transition-a key phenomenon of strongly correlated electron systems-can be explored through temperature sweeps. Measurements of the uniaxial expansivities reveal discontinuous lattice changes at TMI≅14 K, accompanied by a striking in-plane anisotropy. The entropy change at TMI, determined from the volume change, is small, indicating that TMI coincides with the Neel-temperature TN at this point in the phase diagram