Topological limit of ultrathin quasi-free-standing Bi 2 Te 3 films grown on Si(111)

A fundamental issue for ultrathin topological films is the thickness limit below which the topological surface states become impacted by interfacial interaction. We show that for Bi2Te3 grown on Si(111) this limit is four quintuple layers based on angle-resolved photoemission measurements, using optimized photon energies and polarizations, of the Dirac cone warping and interaction-induced gap as a function of film thickness. The results are close to theoretical predictions for free-standing films. Evidence is presented to show why the substrate-film interaction is actually weak.