Resistive transition process in granular superconducting MgB2 films by analysis and simulation of the generated current noise

An original model for the interpretation of the noise produced during the resistive transition of disordered granular MgB2 superconductive films is presented and tested by comparison with extended experiments on these types of films. Both the amplitude and frequency behavior of the noise power spectrum, simulated on the basis of this model, are in very good agreement with the experimental results, practically without the introduction of adjustable parameters. The model is based on the onset of correlated transitions of large sets of grains, forming resistive layers through the film cross-section area during the transition process. The strong nonlinear behavior and correlation of the grains produces abrupt resistance variations, giving rise to the large noise, of the 1/f^3 type, observed in experiments. These results show that, even in the case where the grain interface behaves as a strong link, as in MgB2, if the grains are randomly oriented and are strongly anisotropic in regard to their critical current density, the transition models based on fluxoids depinning and motion, used for single crystals or grain oriented films, cannot be applied, at least when the transition takes place near the critical temperature at low bias currents. These models would justify a current noise power spectrum 2 or 3 orders of magnitude lower than the observed one in the low frequency range. Since the intrinsic superconductive properties of the material do not enter in its development, it is believed that this model may be generally applicable to disordered granular high-temperature superconductors, at least when the conditions of temperature and bias current specified above are met.