Metastable Face-Centered Cubic Structure and Structural Transition of Sn on 2H-NbSe_2 (0001)

Surface structures and properties of Sn islands grown on superconducting substrate 2H-NbSe_2(0001) are studied using low temperature scanning tunneling microscopy or spectroscopy. The pure face-centered cubic (fcc) structure of Sn surface is obtained. Superconductivity is also detected on the fcc-Sn(111) surface, and the size of superconducting gap on the Sn surface is nearly the same as that on the superconducting substrate. Furthermore, phase transition occurs from fcc-Sn(111) to \beta-Sn(001) by keeping the sample at room temperature for a certain time. Due to the strain relaxation on the \beta-Sn islands, both the in-plane unit cell and out-of-plane structures distort, and the height of surface atoms varies periodically to form a universal ripple structure.