Metastable Face-Centered Cubic Structure and Structural Transition of Sn on 2H-NbSe2 (0001)**Supported by the National Key Research and Development Program of China under Grant Nos 2016YFA0301003 and 2016YFA0300403, and the National Natural Science Foundation of China under Grant Nos 11521404, 11634009, U1632102, 11504230, 11674222, 11574202, 11674226, 11574201 and U1632272.

Abstract

Surface structures and properties of Sn islands grown on superconducting substrate 2H-NbSe2(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 β-Sn(001) by keeping the sample at room temperature for a certain time. Due to the strain relaxation on the β-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.