Specific heat and critical fields of the organic superconductor ${\beta }^{″}-(\mathrm{BEDT}-\mathrm{TTF}{)}_2{\mathrm{SF}}_5{\mathrm{CH}}_2{\mathrm{CF}}_2{\mathrm{SO}}_3$

We report on specific-heat, magnetization, and ac-susceptibility measurements of ${\beta }^{″}-(\mathrm{BEDT}-\mathrm{TTF}{)}_2{\mathrm{SF}}_5{\mathrm{CH}}_2{\mathrm{CF}}_2{\mathrm{SO}}_3$, an organic superconductor with $T_c=4.5\mathrm{}$ K, where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene. The jump of the specific heat $C$ at $T_c$ and the exact form of the specific-heat difference between $C$ in the superconducting $(B=0\mathrm{}$ T) and in the normal $(B=3.5\mathrm{}$ T) state can be well described by BCS theory with strong coupling. We estimate an electron-phonon coupling parameter $\lambda \approx 1.1$. From measurements of $C$ in magnetic fields we extract the upper critical field $B_{c2\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$. Low-field dc-magnetization measurements were used to determine the temperature dependence of the lower critical field $B_{c1\mathrm{}}$. From $T_c$ down to $\sim 3$ K the characteristic suppression of $B_{c1\mathrm{}}^{\perp }\mathrm{}\left(T\right)\mathrm{}$ below the sensitivity limit is observed.