Abstract
It is theoretically demonstrated that the figure of merit (ZT) of quantum dot (QD) junctions can be significantly enhanced when the degree of degeneracy of the energy levels involved in electron transport is increased. The theory is based on the the Green-function approach in the Coulomb blockade regime by including all correlation functions resulting from electron-electron interactions associated with the degenerate levels (L). We found that electrical conductance (G_e) as well as electron thermal conductance (κ_e) are highly dependent on the level degeneracy (L), whereas the Seebeck coefficient (S) is not. Therefore, the large enhancement of ZT is mainly attributed to the increase of G_e when the phonon thermal conductance (κ_ph) dominates the heat transport of QD junction system. In the serially coupled double-QD case, we also obtain a large enhancement of ZT arising from higher L. Unlike G_e and κ_e, S is found almost independent on electron inter-dot hopping strength.