Multiple superconducting transitions in α-Sn/β-Sn mixed films grown by molecular beam epitaxy

We have tried to grow α-Sn films on two different substrates with different sample structures by molecular beam epitaxy. The mixture of an α phase with a β phase in the Sn film has been confirmed. The electrical transport properties have been measured and multiple superconducting transitions have been observed in these α-Sn/β-Sn mixed films. Enhanced critical temperatures and critical fields as well as shorter coherence lengths are observed, as compared to bulk β-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 β-Sn islands formed within the film.