  We have studied the influence of microwave radiation on the transport
properties of a semiconductor quantum dot in the Kondo regime. In the entire
frequency range tested (10--50 GHz), the Kondo resonance vanishes by increasing
the microwave power. This suppression of the Kondo resonance shows an
unexpected scaling behavior. No evidence for photon-sideband formation is
found. The comparison with temperature-dependence measurements indicates that
radiation-induced spin dephasing plays an important role in the suppression of
the Kondo effect.
