Phase transition-induced superstructures of β-Sn films with atomic-scale thickness* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61674045, 61911540074, and 21622304), the Fund from the Ministry of Science and Technology of China (Grant No. 2016YFA0200700), the Strategic Priority Research Program and Key Research Program of Frontier Sciences (Chinese Academy of Sciences) (Grant Nos. XDB30000000 and QYZDB-SSW-SYS031).

Abstract

The ultrathin β-Sn(001) films have attracted tremendous attention owing to its topological superconductivity (TSC), which hosts Majorana bound state (MBSs) for quantum computation. Recently, β-Sn(001) thin films have been successfully fabricated via phase transition engineering. However, the understanding of structural phase transition of β-Sn(001) thin films is still elusive. Here, we report the direct growth of ultrathin β-Sn(001) films epitaxially on the highly oriented pyrolytic graphite (HOPG) substrate and the characterization of intricate structural-transition-induced superstructures. The morphology was obtained by using atomic force microscopy (AFM) and low-temperature scanning tunneling microscopy (STM), indicating a structure-related bilayer-by-bilayer growth mode. The ultrathin β-Sn film was made of multiple domains with various superstructures. Both high-symmetric and distorted superstructures were observed in the atomic-resolution STM images of these domains. The formation mechanism of these superstructures was further discussed based on the structural phase transition of β to α-Sn at the atomic-scale thickness. Our work not only brings a deep understanding of the structural phase transition of Sn film at the two-dimensional limit, but also paves a way to investigate their structure-sensitive topological properties.