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

Abstract

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 $\ensuremath{\kappa}\mathrm{\text{\ensuremath{-}}}(\mathrm{BEDT}\mathrm{\text{\ensuremath{-}}}\mathrm{TTF}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br}$. 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 us to explore the critical behavior with very high sensitivity. The analysis yields critical fluctuations with an exponent $\stackrel{\texttildelow{}}{\ensuremath{\alpha}}\ensuremath{\simeq}0.8\ifmmode\pm\else\textpm\fi{}0.15$ at odds with the novel criticality recently proposed for these materials [Kagawa et al., Nature (London) 436, 534 (2005)]. Our data suggest an intricate role of the lattice degrees of freedom in the Mott transition for the present materials.