Abstract
Topological crystalline insulators have been recently predicted and observed in rock-salt structure SnSe thin films. Previous studies have suggested that the Se-terminated surface of this thin film with hydrogen passivation has a reduced surface energy and is thus a preferred configuration. In this paper, synchrotron-based angle-resolved photoemission spectroscopy, along with density functional theory calculations, is used to demonstrate that a rock-salt SnSe thin film epitaxially grown on 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 thin film has undergone a surface structural relaxation of the interlayer spacing between the Sn and Se atomic planes. In sharp contrast to the Se-terminated counterpart, the observed Dirac surface state in the Sn-terminated SnSe thin film is shown to yield a high Fermi velocity, , which suggests a potential mechanism of engineering the Dirac surface state of topological materials by tuning the surface configuration.
- Received 11 October 2016
DOI:https://doi.org/10.1103/PhysRevX.7.041020
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Topological insulators behave as electrical conductors on their surface but as insulators in their interior, which gives rise to intriguing phenomena that could be used in quantum computing and optoelectronics. Searches for new materials that could exhibit this behavior have led to a substance known as a topological crystalline insulator (TCI), whose properties arise from crystal symmetries. Unlike standard topological insulators, any modification to the surface properties of a TCI can critically compromise its unique behavior. In addition, in a typical TCI compound that comprises stacks of polar atomic planes, the divergence of electrostatic energy may destabilize the system and reconfigure the surface structure significantly. We examine the correlation between the internal structural configuration of a TCI and its surface behavior.
Our work makes use of the semiconductor tin selenide (SnSe), a structurally simple material that hosts rich material phases. Combining observations and calculations, we have identified the surface termination (the top atomic layer) of the metastable form of our SnSe thin films as well as their interatomic-plane distances. We also reveal the electronic structure associated with the corresponding surface termination. From our comprehensive battery of experimental and theoretical investigations, we find that the Sn-terminated surface is stabilized via an oscillatory surface relaxation, in which the interatomic distances along the direction perpendicular to the surface become alternately short and long while the topologically protected surface states are kept intact. We also find that the Sn-terminated surface has a high Fermi velocity, which, in addition, is large compared with its Se-terminated counterpart.
We believe that our findings will pave the way for eventual realization of controllable surface termination and surface states of metastable epitaxial topological materials, which can, in turn, permit modifications of the electrical and optical properties of the surface.
