Transport through a metallic quantum dot in the limit of strong Coulomb interaction

In the limit of strong interaction where a two-charge state approximation i s applicable, the current through a metallic quantum dot is studied based o n the Keldysh formalism, We performed diagrammatic analyses by describing t he operators relevant to charge in terms of spin 1/2 operators and by descr ibing the spin 1/2 operators in terms of the product of two Fermion operato rs. Two transport processes through a quantum dot are found: (i) The first one satisfies the current continuity independent of the configuration of th e electrochemical potentials (ECPs) in the subsystems, and corresponds to t he resonant tunneling process in double-barrier structures. Schoeller and S chon give an approximate expression for this process. (ii) The second one c orresponding to the sequential process does not satisfy the current continu ity regardless of the ECPs. It is also shown that, the currents through the left and the right junctions are continuous only in an appropriate configu ration of the ECPs.