Long-Range Superconducting Transition Limited by Phase Slip or Vortex Glass Phase in SmFe1-xCoxAsO Polycrystalline Thin Films

The resistive transition, under an external magnetic field, was measured for two SmFe_1-xCo_xAsO superconducting films with different quality of intergranular coupling. It is observed that the sample with a low density of intergranular coupling has a semiconducting character and the transition to the long-range superconducting state is controlled by the thermal activated phase slip of the order parameter, described by the Ambegaokar-Halperin model. While in the sample with an intergranular coupling characterized by metallic connectivity, it is the freezing of the vortex lines into a vortex glass phase that controls the transition to the long-range superconducting state. A true superconducting state is observed only in the sample that shows a vortex glass phase. In contrast, in the sample in which the resistivity in the normal state indicates a semiconducting granular coupling, a broad transition that does not reach the zero-resistance state occurs. In fact, in this last sample at high applied fields, the long-range superconducting state is not reached because the intergranular coupling is broken, and the resistive signal associated with the thermal activated flux creep of the bulk vortices becomes dominant.