Nonequilibrium charge-Kondo transport through negative- U molecules

Low-temperature transport through molecules with effectively negative charging energy U exhibits a charge-Kondo effect. We explore this regime analytically by establishing an exact mapping between the negative- U and the positive- U Anderson models, which is suitable for the description of nonequilibrium transport. We employ this mapping to demonstrate the intimate relation between nonequilibrium transport in the spin-Kondo and charge-Kondo regimes, and derive analytical expressions for the nonlinear current-voltage characteristics as well as the shot noise in the latter regime. Applying the mapping in the opposite direction, we elucidate the finding of super-Poissonian noise in the positive- U Anderson model at high temperatures, by relating the correlations between spin flips to pair-tunneling processes at negative U.