Abstract
We study how bias and tunneling asymmetries affect nonlinear current through a quantum dot with N discrete levels in the Fermi liquid regime, using an exact low-energy expansion of the current derived up to terms of order V³ with respect to the bias voltage. The expansion coefficients are described in terms of the phase shift, the linear susceptibilities, and the three-body correlation functions, defined with respect to the equilibrium ground state of the Anderson impurity model. In particular, the three-body correlations play an essential role in the order V³ term, and their coupling to the nonlinear current depends crucially on the bias and tunnel asymmetries. The number of independent components of the three-body correlation functions increases with N the internal degrees of the quantum dots, and it gives a variety in the low-energy transport. We calculate the correlation functions over a wide range of electron fillings of the Anderson impurity model with the SU(N) internal symmetry, using the numerical renormalization group. We find that the order V³ nonlinear current through the SU(N) Kondo state, which occurs at electron fillings of 1 and N-1 for strong Coulomb interactions, significantly varies with the three-body contributions as tunnel asymmetries increase. Furthermore, in the valence fluctuation regime toward the empty or fully occupied impurity state, a sharp peak emerges in the coefficient of V³ current in the case at which bias and tunneling asymmetries cooperatively enhance the charge transfer from one of the electrodes.