Stochastic resonance phenomenon in a superconducting surface state of single-crystal Nb

We report an experimental observation of a stochastic resonance pheomenon i n single-crystal Nh. Measurements of the nonlinear susceptibility of Nb (T( c)approximate to 9.15 K) have been performed as a function of temperature, and applied de and ac fields. The latter consist of a high-frequency signal with an amplitude modulation at low-frequency. Nonmonotonic changes of the nonlinear response have been found as a function of the de and ac fields i n a surface superconducting state of Nb. The theoretical analysis based on the two level model of a stochastic resonance phenomenon provides a qualita tive explanation of the experimental results.