Anomalous thermoelectric power and polaronic transport in the vicinity of topological phase transition of Pb_1-xSn_xTe

Pb_1−xSn_xTe compounds exhibits the topological phase transition from trivial semiconductor (PbTe) to topological crystalline insulator (SnTe). In the vicinity of topological phase transition in the region of Pb_1−xSn_xTe (x= 0.2, 0.3, and 0.4), the band touching of conduction L₆ ⁻ and valence L₆ ⁺ bands gives rise to topological Dirac semimetal. Here we investigated thermoelectric properties in the region of topological Dirac semimetal in Pb_1−xSn_xTe compounds. From the temperature-dependent Hall R_H(T) and Seebeck coefficients S(T), we found the abnormal sign difference between Hall (positive R_H) and Seebeck (negative S) coefficients in the compounds of x= 0.3 and x = 0.4 at intermediate temperature range. We analyzed the abnormal sign difference between the Hall and Seebeck coefficients in terms of the Aharanov-Bohm effect of non-adiabatic 3-site polaron hopping. The polaronic hopping in the compounds can be justified by strong electron-phonon coupling. The temperature-dependent behavior of Hall mobility μ_H(T) and carrier density n_H(T) indicates the strong electron-phonon scattering. From the temperature-dependent thermal conductivity κ(T) measurement, the broad peaks at low temperature, driven by freezing-out of Umklapp scattering, are systematically suppressed by increasing Sn concentration, which consistent with the strong electron-phonon interaction in the system.