AC Bias Spectroscopy of the Kondo Singlet in a Single Electron Transistor

We have measured the nonlinear differential conductance, G, of a single electron transistor in the spin 1/2 Kondo regime in presence of an oscillating source voltage. In two distinct regimes, hf $>$ k$_B$T$_K$ and hf $\ll$ k$_B$T$_K$, where f is the oscillation frequency and T$_K$ is the Kondo temperature, comparison to the static model of Kondo transport reveals agreement at very low frequencies and an increasing systematic departure at high frequencies. When hf > k$_B$T$_K$, the G defined as the derivative of the time averaged current through the device with respect to the average bias drastically differs from the static model. We show that the effect cannot be explained by an increase in the electron temperature.