  We examine the recently proposed imaginary-time formulation for strongly
correlated steady-state nonequilibrium for its range of validity and discuss
significant improvements in the analytic continuation of the Matsubara voltage
as well as the fermionic Matsubara frequency. The discretization error in the
conventional Hirsch-Fye algorithm has been compensated in the Fourier
transformation with reliable small frequency behavior of self-energy. Here we
give detailed discussions for generalized spectral representation ansatz by
including high order vertex corrections and its numerical analytic continuation
procedures. The differential conductance calculations agree accurately with
existing data from other nonequilibrium transport theories. It is verified
that, at finite source-drain voltage, the Kondo resonance is destroyed at bias
comparable to the Kondo temperature. Calculated coefficients in the scaling
relation of the zero bias anomaly fall within the range of experimental
estimates.
