Quantum transport properties of the topological Dirac Semimetal α-Sn

We report measurements of the electrical resistivity (ρ) and thermoelectric power (S) in a thin film of strained single-crystalline α-Sn grown by molecular beam epitaxy on an insulating substrate. The temperature (T) dependence of the resistivity of α-Sn can be divided into two regions:below T* ≈ 135 K ρ(T) shows a metallic-like behaviour, while above this temperature an increasing contribution from thermally excited holes to electrical transport is observed. However, it is still dominated by highly mobile electrons, resulting in a negative sign of the Seebeck coefficient above T = 47 K. In the low temperature limit, a small positive S likely reflects the persistent contribution from low-mobility holes or the positive phonon-drag thermopower. In the presence of the magnetic field (B) applied along an electric field or thermal gradient, we note a negative magnetoresistance or a negative slope of S(B), respectively. The theoretical prediction for the former (calculated using density functional theory) agrees well with the experiment. However, these characteristics quickly disappear when the magnetic field is deviated from an orientation parallel to the electrical field or the thermal gradient. We indicate that the behaviour of the electrical resistivity and thermoelectric power can be explained in terms of the chiral current arising from the topologically non-trivial electronic structure of α-Sn. Its decay at high temperature is a consequence of the decreasing ratio between the intervalley Weyl relaxation time to the Drude scattering time.