Annealing effects on superconductivity in Rb0.81Fe1.72Se2 single crystal

The annealing effects on superconductivity in Rb0.81Fe1.72Se2 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 Tconset≈30 K and a Tczero≈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 percola...