Non-trivial quantum magnetotransport oscillations in pure and robust topological $\alpha$-Sn films.

We report experimental evidence of topological Dirac fermion charge carriers in pure and robust $\alpha$-Sn films grown on InSb substrates. This evidence was acquired using standard macroscopic four-point contact resistance measurements, conducted on uncapped films with a significantly reduced bulk mobility. We analyzed and compared electrical characteristics of the constituting components of the $\alpha$-Sn/InSb sample, and propose a three-band drift velocity model accordingly. A surface band, with low carrier density and high mobility, is identified as the origin of the observed Shubnikov -- de Haas oscillations. The analysis of these quantum oscillations results in a non-trivial value of the phase shift $\gamma =0$, characteristic for topologically protected Dirac fermions. For the same uncapped samples we estimate the momentum relaxation time $\tau\approx 300\ \mathrm{fs}$, which is significantly larger in comparison with the previous reports on grown $\alpha$-Sn films.