Thermoelectric transport of mesoscopic conductors coupled to voltage and thermal probes

We investigate the basic properties of the thermopower (Seebeck coefficient) of phase-coherent conductors under the influence of dephasing and inelastic processes. Transport across the system is caused by a voltage bias or a thermal gradient applied between two terminals. Inelastic scattering is modeled with the aid of an additional probe acting as an ideal potentiometer and thermometer. We find that inelastic scattering reduces the conductor's thermopower and, more unexpectedly, generates a magnetic field asymmetry in the Seebeck coefficient. The latter effect is shown to be a higher-order effect in the Sommerfeld expansion. We discuss our result by using two illustrative examples. First, we consider a generic mesoscopic system described within random matrix theory and demonstrate that thermopower fluctuations disappear quickly as the number of probe modes increases. Second, the asymmetry is explicitly calculated in the quantum limit of a ballistic microjunction. We find that asymmetric scattering strongly enhances the effect and discuss its dependence on temperature and Fermi energy.

Figure 1

Sketches of (a) a chaotic cavity attached to voltage and thermal reservoirs and (b) a microjunction with an asymmetric scatterer coupled to a probe as a model of a quantum wire with incoherent scattering.

Figure 2

Numerical results for the thermopower (a),(b) and thermopower $B$ asymmetry (c),(d) of a microjunction with an asymmetric scatterer [Fig. 1b] as a function of Fermi energy (a),(c) and temperature (b),(d). Solid (open) symbols correspond to the presence (absence) of the voltage and thermal probe. Solid (dashed) lines in (b) are obtained using the Sommerfeld expansion to lowest order with (without) the probe. Note the strong deviation at moderately low temperatures. Solid line in (d) shows the $\Phi \propto {\theta }^3$ dependence of the $B$ asymmetry at low temperature. Parameters: Magnetic length $\hslash c/eB=L^2$, temperature $k_B\theta ={\hslash }^2/m{L}^2$ [(a),(c)], and Fermi energy $E_F=10{\hslash }^2/m{L}^2$ [(b),(d)].