Phase-transition anomalies in the lattice response around 40 K in superconducting κ-(BEDT-TTF)₂X

High-resolution measurements of the coefficient of thermal expansion, α(T), on the superconductors κ-(BEDT-TTF)₂X with X = Cu[N(CN)₂]Br and Cu(NCS)₂ reveal distinct anomalies at T* around 40 K - the same temperature where the opening of a pseudo-gap has been inferred from anomalies in various thermal and magnetic properties. Judging from the shape of the α anomalies, we tend to assign these features to broadened second-order phase transitions. Employing a thermodynamic analysis, the signs of the uniaxial-pressure coefficients of T* can be determined and are found to be strictly anticorrelated to those of T_c. We propose that T* marks a density-wave (DW) instability involving only the small, quasi-1D portions of the Fermi surface (FS) as opposed to a pseudo-gap on the whole FS. Our scenario implies that the DW and superconductivity form on disjunct parts of the FS and compete for stability.