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

TL;DR

This study reveals unexpected anisotropic lattice responses at the Mott transition in a layered organic conductor, highlighting the significant role of lattice degrees of freedom and critical fluctuations near the transition.

Contribution

It provides high-resolution dilatometry data showing anisotropic lattice anomalies and critical behavior, challenging recent theories of Mott criticality in these materials.

Findings

Anisotropic lattice anomalies at the Mott transition

Critical fluctuations with exponent ~0.8

Lattice degrees of freedom play a significant role

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 $\kappa$-(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 \textit{et al.}, Nature \textbf{436}, 534 (2005)]. Our data suggest an intricate role of the lattice degrees of freedom in the Mott transition for the present materials.