Phase transition-induced superstructures of β-Sn films with atomic-scale thickness

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.