Vortex dynamics in $\beta$-FeSe single crystals: effects of proton irradiation and small inhomogeneous stress

Abstract

We report on the critical current density Jc and the vortex dynamics of pristine and 3 MeV proton irradiated (cumulative dose equal to 2x10^16 cm^-2) $\beta$-FeSe single crystals. We also analyze a remarkable dependence of the superconducting critical temperature Tc, Jc and the flux creep rate S on the sample mounting method. Free-standing crystals present Tc =8.4(1)K, which increases to 10.5(1)K when they are fixed to the sample holder by embedding them with GE-7031 varnish. On the other hand, the irradiation has a marginal effect on Tc. The pinning scenario can be ascribed to twin boundaries and random point defects. We find that the main effect of irradiation is to increase the density of random point defects, while the embedding mainly reduces the density of twin boundaries. Pristine and irradiated crystals present two outstanding features in the temperature dependence of the flux creep rate: S(T) presents large values at low temperatures, which can be attributed to small pinning energies, and a plateau at intermediate temperatures, which can be associated with glassy relaxation. From Maley analysis, we observe that the characteristic glassy exponent {\mu} changes from ~ 1.7 to 1.35-1.4 after proton irradiation.