UC Riverside

This thesis summarizes our work in the past four years in the field of transport studies of the topological insulator materials and thermoelectric properties of graphene. The first half of the thesis is focused on the transport properties of topological insulator material-Bi₂Se₃. In our research, we systematically tune the position of the chemical potential in p-type Ca-doped Bi₂Se₃ thin devices first by eliminating excess holes with controlled post-fabrication electron beam irradiation that results in an insulating bulk state. In spite of the fact that the energetic electron beam creates defects to localize the bulk carriers and inevitably to cause additional scattering, we find a tenfold increase in carrier mobility associated with the extended states in the band gap. In addition, the resistance undergoes a fivefold increase and passes the maximum as the chemical potential is further tuned by electrostatic gating. A cusp-like low-field magnetoresistance feature also emerges which is indicative of strong spin-orbit interaction. The observed gate-tunable high-mobility is a signature of massless Dirac fermions in the band gap of Bi₂Se₃.

The second half of this thesis is focused on graphene. Our work first reported the thermoelectric study of graphene and demonstrated the anomalous thermoelectric transport of massless Dirac fermions. As a direct consequence of the linear dispersion of massless particles, we find that the Seebeck coefficient Sxx diverges with 1 / |n_2D|^½, where n_2D is the carrier density. We observe a very large Nernst signal S_xy (~ 50 μV/K at 8 T) at the Dirac point, and an oscillatory dependence of both S_xx and S_xy on n_2D at low temperatures. Our results underscore the anomalous thermoelectric transport in graphene, which may be used as a highly sensitive probe for impurity bands near the Dirac point.