Fully - gapped anisotropic superconductivity robust against point - like disorder in FeSe single crystals.

A highly anisotropic but finite superconducting gap is found in single crystals of FeSe by studying the London penetration depth $\lambda (T)$ measured down to 50 mK in samples before and after 2.5 MeV electron irradiation. The gap minimum increases with the introduced point-like 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 scenarios for the $T_c$ enhancement and propose that local strengthening of the pair interaction by irradiation-induced Frenkel defects most likely explains the phenomenon.