Chaos in a dc-bias Josephson junction in the presence of microwave radiation

We report the observation of chaotic noise in Josephson tunnel junctions in the presence of a dc current and irradiated with microwave radiation. Four different chaotic phenomena have been measured associated to either anomalies in the $I-V$ characteristics and/or high noise temperatures, with chaotic noise temperatures as high as ${10}^6$ K. A new chaotic mode is reported consisting of a recurrent smoothing of large sections of the $I-V$ characteristic for certain values of microwave power. The resistively shunted junction model is shown to contain most of the observed phenomena.