Evidence for massive bulk Dirac fermions in Pb1−xSnxSe from Nernst and thermopower experiments

Topological surface states protected by mirror symmetry are of interest for spintronic applications. Such states were predicted to exist in the rocksalt IV–VI semiconductors, and several groups have observed the surface states in (Pb,Sn)Te, (Pb,Sn)Se and SnTe using photoemission. An underlying assumption in the theory is that the surface states arise from bulk states describable as massive Dirac states, but this assumption is untested. Here we show that the thermoelectric response of the bulk states displays features specific to the Dirac spectrum. By relating the carrier density to the peaks in the quantum oscillations, we show that the first ( N =0) Landau level is non-degenerate. This finding provides robust evidence that the bulk states are indeed massive Dirac states. In the lowest Landau level, S xx displays a striking linear increase versus magnetic field characteristic of massive Dirac fermions. In addition, the Nernst signal displays a sign anomaly in the gap-inverted phase at low temperatures. Topological surface states in lead-doped tin selenide are assumed to arise from massive Dirac states in the bulk, but this has not been demonstrated to date. Using thermoelectric transport measurements, Liang et al. now close this gap, and further show a sign anomaly in the Nernst signal due to band inversion.