Sergey Smirnov, Milena Grifoni
We present a theory for the Kondo spin-1/2 effect in strongly correlated quantum dots at any temperature. The theory is based on an infinite family of quadratic Keldysh effective actions parameterized, as we demonstrate, by a universal function of the temperature and voltage. The tunneling density of states is given in an analytic form through this universal function whose temperature dependence in equilibrium is obtained from comparison with numerical renormalization group calculations of the linear conductance. We show that in the strong coupling limit it is equal to the Kondo energy, in the weak coupling limit it is zero while in the crossover region it exhibits a strong nonmonotonic behavior. The nonequilibrium universal function is obtained from comparison with empirical laws describing experiments with high accuracy. It turns out that at zero temperature and at low voltages its voltage dependence is linear with the proportionality coefficient equal to pi/2.
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http://arxiv.org/abs/1203.4360
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