Unconventional thermoelectric behaviors and enhancement of figure of merit in Rashba spintronic systems

Thermoelectric transport in strongly spin-orbit coupled two-dimensional Rashba systems is studied using the exact solution of the linearized Boltzmann equation. Some unusual transport behaviors are revealed. We show that the electrical conductivity as a function of the Fermi energy ${\ensuremath{\epsilon}}_{F}$ behaves differently between the two sides of the band crossing point (BCP). The deviation from the Mott relation occurs when ${\ensuremath{\epsilon}}_{F}$ lies in the vicinity of the BCP and is attributed to the topological change of Fermi surface varying across the BCP. It is shown that the thermopower and thermoelectric figure of merit are strongly enhanced when ${\ensuremath{\epsilon}}_{F}$ downs below the BCP. This enhancement is attributed to not only the one-dimensional-like density of state, but also the unconventional intraband elastic scattering below the BCP. The differences between these results and those obtained by the relaxation time approximation (RTA) are discussed in detail, revealing the necessity of going beyond the RTA in the considered system.