Multiterminal Anderson impurity model in nonequilibrium: Analytical perturbative treatment

We study the nonequilibrium spectral function of the single-impurity Anderson model connecting with multiterminal leads. The full dependence on frequency and bias voltage of the nonequilibrium self-energy and spectral function is obtained analytically up to the second-order perturbation regarding the interaction strength U. High- and low-bias voltage properties are analyzed for a generic multiterminal dot, showing a crossover from the Kondo resonance to the Coulomb peaks with increasing bias voltage. For a dot where the particle-hole symmetry is not present, we construct a current-preserving evaluation of the nonequilibrium spectral function for arbitrary bias voltage. It is shown that finite-bias voltage does not split the Kondo resonance in this order, and no specific structure due to multiple leads emerges. Overall bias dependence is quite similar to finite-temperature effect for a dot with or without the particle-hole symmetry.