Anomalous Lattice Response at the Mott Transition in a Quasi-2D Organic Conductor

Discontinuous changes of the lattice parameters at the Mott metal-insulator transition are detected by high-resolution dilatometry on deuterated crystals of the layered organic conductor $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br. The uniaxial expansivities uncover a striking and unexpected anisotropy, notably a zero-effect along the in-plane c-axis along which the electronic interactions are relatively strong. A huge thermal expansion anomaly is observed near the end-point of the first-order transition line enabling to explore the critical behavior with very high sensitivity. The analysis yields critical fluctuations with an exponent $\tilde{\alpha }\simeq$ 0.8 $\pm$ 0.15 at odds with the novel criticality recently proposed for these materials [Kagawa $\mathit{\text{et}}\mathit{\text{al.}}$, Nature $\mathbf{\text{436}}$, 534 (2005)]. Our data suggest an intricate role of the lattice degrees of freedom in the Mott transition for the present materials.