Nodal Superconductivity in FeSe single crystals from heat capacity

The superconducting gap in vapor-grown single-crystalline FeSe is studied via heat-capacity measurements down to 0.4 K and up to 14 Tesla. In our best crystals, i.e. with the highest T_c and RRR values, we consistently find a linear C/T term at low temperatures indicative of a nodal superconducting gap. This is supported by the magnetic field dependence of C/T. The electronic specific heat curve up to T_c is shown to be consistent with a recent gap determination using quasi particle interference only if one forces the gap to change sign at the minimum of one of the Fermi surface pockets. Finally, the specific heat of FeSe crystals grown under different conditions are compared to various C/T data taken from the literature.