Topological limit of ultrathin quasi-free-standing Bi ₂Te 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 Bi ₂Te ₃ 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.