Nanoscale superconductivity of γ-Ga islands grown by molecular beam epitaxy

We report on the preparation and superconductivity of metastable γ-Ga islands on Si (111) substrate. The Ga grows in a typical Volmer-Weber mode at a low temperature of 110 K, resulting in formation of Ga nanoislands at various sizes with the identical γ-phase. In-situ low temperature scanning tunneling spectra reveal quantized electronic states in ultrathin Ga islands. It is found that both the lateral size and thickness of the Ga islands strongly suppress the superconductivity. Due to substantial surface energy contribution, the transition temperature T c scales inversely with the island thickness and the minimum thickness for the occurrence of superconductivity is calculated to be two monolayers.