Current noise in high Tc granular superconductors under non-stationary conditions of current and magnetic field

摘要

We present a set of experimental results concerning the power spectrum ofcurrent noise, detected on a granular high Tc superconductor submitted eitherto a slowly varying magnetic field or to a varying current intensity.Experiments were performed on a YBCO specimen suitably treated in order toweaken the weak links without affecting the oxygen content of grains. Theweakening of the intergrain region allowed the use of very small magneticfields and currents to induce the resistive transition of the specimen and toobserve current noise. The induced noise is of the 1/f^2 type and will beinterpreted in terms of two different models. One of the model is based on theenhancement of the noise due to the clustering of the resistive transition ofthe weak links, produced by correlation effects related to the strongnonlinearity of their Josephson type I-V characteristics. This model has beenthe object of a computer simulation based on a 3D-network of Josephson-likeelements and seems suitable to explain the noise produced by current variation.The second model explains the excess noise as produced by discontinuouspenetration of the magnetic flux inside the intergrain region. Thisdiscontinuity is related to the field screening effect of rings made of severalsuperconducting weak links connecting different grains, which are alternativelybroken and restored by the current induced during flux variation, and seemssuitable to explain the larger noise produced by a varying magnetic field.