Annealing effects on superconductivity in Rb_(0.81)Fe_(1.72)Se_2 single crystal

摘要

The annealing effects on superconductivity in Rb_(0.81)Fe_(1.72)Se_2 are investigated. For as-grown single crystal and the one annealed with furnace cooling, magnetic susceptibility exhibits a weak diamagnetic transition at 30 K. Consistently, the resistivity curve only displays a small drop at 30 K and an insulating behavior at lower temperatures. These features together with scanning electron microscope (SEM) data indicate that the superconducting phase exists as clusters in insulating matrix. As for the crystal annealed with fast quenching, resistivity results reveal a T_c~(onset) ≈ 30 K and a T_c~(zero) ≈ 10 K but the susceptibility does not show diamagnetic transition until the temperature is cooled down to 10 K. SEM results show that the surface of quenched sample is more homogeneous than the one annealed with furnace cooling. We argue that the annealing at high temperature makes the coexistence of superconducting and insulating phases becomes homogeneous. Then, the fast quenching preserves this state in which percolative superconducting paths could be easily formed. In comparison, during the furnace cooling, the temperature drops slowly and results in the segregation of the superconducting phase from the insulating phase into isolated clusters.