Effect of the proton irradiation on the thermally activated flux flow in superconducting SmBCO coated conductors

We investigate changes in the vortex pinning mechanism caused by proton irradiation through the measurement of the in-plane electrical resistivity for H//c in a pristine and two proton-irradiated (total doses of 1 × 1015 and 1 × 1016 cm−2) SmBa2Cu3O7-δ (SmBCO) superconducting tapes. Even though proton irradiation has no effect on the critical temperature (Tc), the resulting artificial point defect causes an increase in normal state electrical resistivity. The electrical resistivity data around Tc shows no evidence of a phase transition to the vortex glass state but only broadens with increasing magnetic field due to the vortex depinning in the vortex liquid state. The vortex depinning is well interpreted by a thermally activated flux flow model in which the activation energy shows a nonlinear temperature change $${\boldsymbol{U}}{\boldsymbol{(}}{\boldsymbol{T}},{\boldsymbol{H}}{\boldsymbol{)}}{\boldsymbol{=}}{{\boldsymbol{U}}}_{{\boldsymbol{0}}}{\boldsymbol{(}}{\boldsymbol{H}}{\boldsymbol{)}}{{\boldsymbol{(}}{\bf{1}}-{\boldsymbol{T}}{\boldsymbol{/}}{{\boldsymbol{T}}}_{{\boldsymbol{c}}}{\boldsymbol{)}}}^{{\boldsymbol{q}}}$$ U ( T , H ) = U 0 ( H ) ( 1 − T / T c ) q (q = 2). The field dependence of activation energy shows a $${{\boldsymbol{U}}}_{{\bf{0}}}{\boldsymbol{ \sim }}{{\boldsymbol{H}}}^{-{\boldsymbol{\alpha }}}$$ U 0 ~ H − α with larger exponents above 4 T. This field dependence is mainly due to correlated disorders in pristine sample and artificially created point defects in irradiated samples. Compared with the vortex pinning due to correlated disorders, the vortex pinning due to the appropriate amount of point defects reduces the magnitude of Uo(H) in the low magnetic field region and slowly reduces Uo(H) in high magnetic fields.