Multiple superconducting transitions in alpha-Sn/beta-Sn mixed films grown by molecular beam epitaxy

We have tried to grow alpha-Sn films on two different substrates with different sample structures by molecular beam epitaxy. The mixture of an alpha phase with a beta phase in the Sn film has been confirmed. The electrical transport properties have been measured and multiple superconducting transitions have been observed in these alpha-Sn/beta-Sn mixed films. Enhanced critical temperatures and critical fields as well as shorter coherence lengths are observed, as compared to bulk beta-Sn. The two-dimensional nature is demonstrated by angle-dependent measurements and a type-II superconductor is indicated by the magnetization measurements. We attribute the multiple superconducting transitions to the beta-Sn islands formed within the film.