Current noise in HTC polycrystalline superconductors: A comparison between experiments and different types of percolation models

Experimental results concerning current noise in ceramic high-T_c superconductors under different conditions of temperature and magnetic field are reported and compared with the results of different models based on percolation theory. It is assumed that under a magnetic field the specimens undergo a first stage transition where the grains become superconducting while the weak links are resistive. Further lowering of the temperature gradually increases the number of weak links that become superconducting, until a percolation threshold is reached which gives rise to macroscopic superconductivity. Two noise-generation models based on a switching model recently proposed by Kiss and Svedlindh, where a small number of weak links near a critical state undergo dynamic transitions from the normal to the superconducting state, are numerically developed and discussed. These models seem to give a much better account of the behavior of the noise intensity than a classical model, where the resistance of the weak links fluctuates. Comparison with the experiments is also given.