Comparative thermal-expansion study of ${\beta }^{″}-{\mathrm{}\left(\mathrm{ET}\right)\mathrm{}}_2{\mathrm{SF}}_5{\mathrm{CH}}_2{\mathrm{CF}}_2{\mathrm{SO}}_3$ and $\kappa -{\mathrm{}\left(\mathrm{ET}\right)\mathrm{}}_2{\mathrm{Cu}\left(\mathrm{NCS}\right)\mathrm{}}_2$: Uniaxial pressure coefficients of $T_c$ and upper critical fields

We report high-resolution measurements of the coefficient of thermal expansion, $\alpha {=l}^{-1}\times (\partial l/\partial T),$ on single crystals of the organic superconductors ${\beta }^{″}-{\mathrm{}\left(\mathrm{ET}\right)\mathrm{}}_2{\mathrm{SF}}_5{\mathrm{CH}}_2{\mathrm{CF}}_2{\mathrm{SO}}_3$ and $\kappa -{\mathrm{}\left(\mathrm{ET}\right)\mathrm{}}_2\mathrm{Cu}(\mathrm{NCS}{)}_2.$ For both salts we find large and highly anisotropic phase-transition anomalies at $T_c.$ Combining these data with literature results on the specific heat via the Ehrenfest relation, the uniaxial pressure coefficients of $T_c$ can be determined. Most remarkably, a strikingly similar in-plane vs out-of-plane anisotropy is found for both compounds: the strong suppression of $T_c$ observed in hydrostatic-pressure experiments is dominated by a huge negative uniaxial stress effect perpendicular to the conducting planes. Therefore we expect that an increase of $T_c$ in this class of superconductors can be obtained by enlarging the distance between the conducting layers. Application of magnetic fields perpendicular to the planes for the ${\beta }^{″}-{\mathrm{}\left(\mathrm{ET}\right)\mathrm{}}_2{\mathrm{SF}}_5{\mathrm{CH}}_2{\mathrm{CF}}_2{\mathrm{SO}}_3$ salt were found to result in pronounced superconducting fluctuation effects and scaling behavior in $\alpha \mathrm{}(T,B).$ Owing to the pronounced phase-transition anomalies in $\alpha \mathrm{}(T,B)\mathrm{}$ at $T_c,$ our measurements allow for an accurate determination of the upper critical fields. We find $B_{c_2}^{\perp }\mathrm{}\left(0\right)=(1.4\mathrm{}\pm 0.2)\hspace{0.5em}\mathrm{T}$ and $B_{c_2}^{\Vert }\mathrm{}\left(0\right)=(10.4\mathrm{}\pm 0.5)\hspace{0.5em}\mathrm{T}$ for fields perpendicular and parallel to the conducting planes, respectively.