Enhancement of superconducting transition temperature by pointlike disorder and anisotropic energy gap in FeSe single crystals

A highly anisotropic superconducting gap is found in single crystals of FeSe by studying the London penetration depth $\mathrm{\ensuremath{\Delta}}\ensuremath{\lambda}$ 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 ${T}_{c0}\ensuremath{\approx}8.8$ K while the structural transition temperature ${T}_{s}$ decreases by 0.9 K from ${T}_{s0}\ensuremath{\approx}91.2$ 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.