A New Mean-Field Theory of the Kondo Resonance at Finite Bias

We introduce a new slave-boson mean-field treatment of the Kondo effect in a quantum dot attached to the leads, when the bias voltage across the leads is finite. The model employs two slave boson and two pseudo-fermion operators to express the localized electron. The solution of the mean-field equations gives in general two resonance peaks pinned to the chemical potential of each lead. The Kondo temperature is shown to scale as $min(\tstar, (\tstar)^2/V)$, where $\tstar$ is the Kondo temperature at equilibrium, and $V$ is the chemical potential difference of the leads.