Thermal expansion in single crystals of ϰ-(BEDT-TTF)2Cu(NCS)2 in magnetic fields up to 6 tesla

The linear thermal expansion coefficients αi (i=a∗,b,c) of single crystals of the organic superconductor ϰ-(BEDT-TTF)2Cu(NCS)2 (Tc= 9.15K) have been measured in the temperature range from 2 to 300K, by means of capacitive dilatometry. At Tc, we resolved second order jumps Δαi, which disappear in magnetic fields above Hc2. From the initial slopes dHc2/dT parallel and perpendicular to the superconducting layers the Ginzburg-Landau coherence lengths ξ∥(0) = 67Å and ξ⊥(0) = 7.3Å were determined. Via Ehrenfestʹs relation it is possible to deduce the uniaxial stress dependence of Tc, which was found to be negative in the a∗-direction (∂Tc/∂σa∗=-3. 20 K/kbar), positive in the c-direction (∂Tc/∂σc = 1.46 K/kbar) and zero in the b-direction.