Specific he at and critical field s of the organic superconductor β"-(BEDT-TTF )2SF5CH2CF2SO3

We report on specific-heat, magnetization, and ac-susceptibility measurements of ${\ensuremath{\beta}}^{\ensuremath{''}}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}{\mathrm{SF}}_{5}{\mathrm{CH}}_{2}{\mathrm{CF}}_{2}{\mathrm{SO}}_{3}$, an organic superconductor with ${T}_{c}=4.5$ 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 $(B=0$ T) and in the normal $(B=3.5$ T) state can be well described by BCS theory with strong coupling. We estimate an electron-phonon coupling parameter $\ensuremath{\lambda}\ensuremath{\approx}1.1$. From measurements of $C$ in magnetic fields we extract the upper critical field ${B}_{c2}(T)$. Low-field dc-magnetization measurements were used to determine the temperature dependence of the lower critical field ${B}_{c1}$. From ${T}_{c}$ down to $\ensuremath{\sim}3$ K the characteristic suppression of ${B}_{c1}^{\ensuremath{\perp}}(T)$ below the sensitivity limit is observed.