Keldysh field theory approach to electric and thermoelectric transport in quantum dots

We compute the current and the noise power matrix in a quantum dot connected to two metallic reservoirs by using the Keldysh field theory approach, a nonequilibrium quantum field theory language in the functional integral formalism. We first show how this technique allows one to recover rapidly and straightforwardly well-known results in the literature, such as the Meir-Wingreen formula for the average current, which is extremely effective in dealing with the quantum transport problem. We then discuss in detail the electric and thermoelectric properties due to transport of electrons in the case of a single-level and two-level noninteracting quantum dot. In particular, we derive the optimal conditions for maximizing the thermoelectric current, finding an upper limit for the thermoelectric coefficient. Moreover, in the two-level system we show that the zero-temperature linear conductance drops rapidly to zero through a symmetrical removal of the degeneracy at the Fermi energy.