Highly anisotropic superconducting gaps and possible evidence of antiferromagnetic order in FeSe single crystals

Specific heat has been measured in FeSe single crystals down to 0.414 K under magnetic fields up to 16 T. A sharp specific-heat anomaly is observed at the superconducting transition temperature T-c = 8.2 K. Another jump of specific-heat coefficient C/T is observed at about 1.08 K which is argued to be the long sought antiferromagnetic transition in bulk FeSe. Global fitting to electronic specific heat in a wide temperature region shows that the models with a single contribution with isotropic s-wave, anisotropic s-wave, and d-wave gaps all do not work well, nor the two isotropic s-wave gaps. We then fit the data by a model with two components in which one has the gap function of Delta(0)(1 + alpha cos 2 theta). To have a good global fitting and the entropy conservation for the low-temperature transition, we conclude that the gap minimum should be smaller than 0.15 meV (alpha = 0.9 to 1), indicating that the superconducting gap(s) are nodeless but highly anisotropic. Our results are quite consistent with the gap structure derived recently from scanning tunneling spectroscopy measurements and yield specific-heat contributions of about 32% weight from the hole pocket and 68% from the electron pockets.