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 us to explore the critical behavior with very high sensitivity. The analysis yields critical fluctuations with an exponent alpha approximately 0.8+/-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.
de Souza
M
M
Physikalisches Institut, J.W. Goethe-Universität Frankfurt(M), FOR 412, D-60054 Frankfurt am Main, Germany.
Brühl
A
A
Strack
Ch
Ch
Wolf
B
B
Schweitzer
D
D
Lang
M
M