Comparative thermal-expansion study of beta ''-(ET)(2)SF5CH2CF2SO3 and kappa-(ET)(2)Cu(NCS)(2): Uniaxial pressure coefficients of T-c and upper critical fields

We report high-resolution measurements of the coefficient of thermal expans ion, alpha = l(-1) x (partial derivative l/partial derivative T), on single crystals of the organic superconductors beta "- (ET)(2)SF5CH2CF2SO3 and ka ppa-(ET)(2)Cu(NCS)(2). For both salts we find large and highly anisotropic phase-transition anomalies at T-c. Combining these data with literature res ults on the specific heat via the Ehrenfest relation, the uniaxial pressure coefficients of T-c can be determined. Most remarkably, a strikingly simil ar in-plane vs out-of-plane anisotropy is found for both compounds: the str ong suppression of T-c observed in hydrostatic-pressure experiments is domi nated by a huge negative uniaxial stress effect perpendicular to the conduc ting planes. Therefore we expect that an increase of T-c in this class of s uperconductors can be obtained by enlarging the distance between the conduc ting layers. Application of magnetic fields perpendicular to the planes for the beta"-(ET)(2)SF5CH2CF2SO3 salt were found to result in pronounced supe rconducting fluctuation effects and scaling behavior in alpha(T,B). Owing t o the pronounced phase-transition anomalies in alpha(T,B) at T-c, our measu rements allow for an accurate determination of the upper critical fields. W e find B-c2(perpendicular to)(0) = (1.4+/-0.2) T and B-c2(parallel to) (0) = (10.4+/-0.5) T for fields perpendicular and parallel to the conducting pl anes, respectively.