Observation of oscillatory relaxation in the Sn-terminated surface of epitaxial rock-salt SnSe $\{111\}$ topological crystalline insulator*

Topological crystalline insulators have been recently observed in rock-salt SnSe $\{111\}$ thin films. Previous studies have suggested that the Se-terminated surface of this thin film with hydrogen passivation is a preferred configuration. In this work, synchrotron-based angle-resolved photoemission spectroscopy, along with density functional theory calculations, are used to demonstrate conclusively that a rock-salt SnSe $\{111\}$ thin film has a stable Sn-terminated surface. These observations are supported by low energy electron diffraction (LEED) intensity-voltage measurements and dynamical LEED calculations, which further show that the Sn-terminated SnSe $\{111\}$ thin film has undergone an oscillatory surface structural relaxation. In sharp contrast to the Se-terminated counterpart, the Dirac surface state in the Sn-terminated SnSe $\{111\}$ thin film yields a high Fermi velocity, $0.50\times10^6$m/s, which may lead to high-speed electronic device applications.