Hydrostatic pressure-induced huge enhancement of critical current density and flux pinning in Fe_1−xCo_xSe_0.5Te_0.5 single crystals

We performed a systematic study of the hydrostatic pressure (HP) effect on the supercon-ducting transition temperature (T _c), critical current density (J _c), irreversibility field (H _irr), upper critical field (H _c2), and flux pinning mechanism in un-doped and 3 at.% Co-doped FeSe_0.5Te_0.5 crystals. We found that T _c is increased from 11.5 to 17 K as HP increases from 0 to 1.2 GPa. Remarkably, the J _c is significantly enhanced by a factor of 3 to 100 for low and high temperature and field, and the H _irr line is shifted to higher fields by HP up to 1.2 GPa. Based on the collective pinning model, the δl pinning associated with charge-carrier mean free path fluctuation is responsible for the pinning mechanism of Fe_1−xCo_xSe_0.5Te_0.5 samples with or without pressure. A comprehensive vortex phase diagram in the mixed state is constructed and analysed for the 3 at.% Co-doped sample. (paper)