Enhancement of Tc by point-like disorder and anisotropic gap in FeSe
  A highly anisotropic superconducting gap is found in single crystals of FeSe by studying the London penetration depth, $\Delta \lambda$, measured down to 50 mK in samples before and after 2.5 MeV electron irradiation. The gap minimum increases with introduced point - like disorder, indicating the absence of symmetry - imposed nodes. Surprisingly, the superconducting transition temperature, $T_c$, \textit{increases} by 0.4 K from $T_{c0} \approx$ 8.8 K while the structural transition temperature, $T_s$, \textit{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. 
S. Teknowijoyo, K. Cho, M. A. Tanatar, J. Gonzales, A. E. B\"ohmer, O.   Cavani, V. Mishra, P. J. Hirschfeld, S. L. Bud'ko, ...
cond-mat.supr-con