A highly anisotropic superconducting gap is found in single crystals of FeSe by studying the London penetration depth $\mathrm{\Delta }\lambda$ measured down to 50 mK in samples before and after 2.5 MeV electron irradiation. The gap minimum increases with introduced pointlike disorder, indicating the absence of symmetry-imposed nodes. Surprisingly, the superconducting transition temperature $T_c$ increases by 0.4 K from $T_{c0}\approx 8.8$ K while the structural transition temperature $T_s$ decreases by 0.9 K from $T_{s0}\approx 91.2$ K after electron irradiation. We discuss several explanations for the $T_c$ enhancement and propose that local strengthening of the pair interaction by irradiation-induced Frenkel defects most likely explains the phenomenon.

• Received 19 June 2016
• Revised 5 August 2016

DOI:https://doi.org/10.1103/PhysRevB.94.064521