We studied the response of the surface state spectrum of epitaxial Sb2Te3 thin films to applied gate electric fields by low temperature scanning tunneling microscopy. The gate dependence of the shift in the Fermi level and screening effect from bulk carriers were measured as function of film thickness. We observed a gap opening at the Dirac point for films thinner than four quintuple layers, due to the coupling of the top and bottom surfaces. Moreover, the top surface state band gap of the three quintuple layer films was found to be tunable by back gate, indicating the possibility of observing a topological phase transition in this system. Our results are well understood by an effective model of 3D topological insulator thin films with structure inversion asymmetry, and we conclude that three quintuple layer Sb2Te3 films are topologically nontrivial and belong to the quantum spin Hall insulator class.
Citation: Physical Review Letters
Pub Type: Journals
Topological insulator, scanning tunneling microscopy, cscanning tunneling spectroscopy