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 is applicable, the current through a metallic quantum dot is studied based on the Keldysh formalism, We performed diagrammatic analyses by describing the operators relevant to charge in terms of spin 1/2 operators and by describing the spin 1/2 operators in terms of the product of two Fermion operators. Two transport processes through a quantum dot are found: (i) The first one satisfies the current continuity independent of the configuration of the electrochemical potentials (ECPs) in the subsystems, and corresponds to the resonant tunneling process in double-barrier structures. Schoeller and Schon give an approximate expression for this process. (ii) The second one corresponding to the sequential process does not satisfy the current continuity regardless of the ECPs. It is also shown that, the currents through the left and the right junctions are continuous only in an appropriate configuration of the ECPs.