Superconductivity in Re₆Se₈Cl₂, a Superatomic Two Dimensional Semiconductor

We report on low temperature transport measurements of Re₆Se₈Cl₂ - a synthesized van der Waal cluster solid with a hierarchical structure composed of covalently linked Re₆Se₈ clusters. Samples are prepared by mechanical exfoliation onto a PDMS stamp, then transferred to metallic leads pre-patterned on a SiO₂ substrate. As grown, this material is a weakly-doped semiconductor, with strongly insulating behavior observed at cryogenic temperatures. Using a current annealing technique we demonstrate the ability to induce an insulator to superconductor transition, in situ. This transition is accompanied by an increase in the carrier density by 3-4 orders of magnitude. The chemical (EDX) and structural (Raman) analysis after current annealing show no change in chemical composition or lattice structure. We conjecture that the transition results from a small loss of interplanar chlorine atoms through the application of large currents. Details of the superconducting state and its possible nature will be discussed.