Angular-dependent specific heat of the fully organic superconductor β′′-(ET)₂SF₅CH₂CF₂SO₃ - evidence for an FFLO phase

Since 1964 predicted, there is no clear-cut experimental proof of the existence of a field-induced superconducting state with finite center of mass momentum (q \neq 0) [1]. In 2007, Lortz et al. reported thermodynamic evidence for a FFLO state in κ-(ET)₂Cu(NCS)₂ [2], where ET stands for bisethylenedithio-tetrathiafulvalene. They found an emerging sharp double-peak structure in the specific heat, indicating first-order phase transitions at high magnetic fields applied parallel to the superconducting ET layers. Expecting similar novel results for β′′-(ET)₂SF₅CH₂CF₂SO₃, we performed high-resolution specific-heat measurements for different orientations with very fine steps around perfect parallel alignment. In conclusion, we found an upturn in the field-temperature phase diagram of the superconducting phase beyond the Pauli-Clogston limit and we could observe a double-peak structure above 9.4 Tesla but only by tilting the field 0.2 degree out of the ET sheets.