Resistive transition process in granular superconductingMgB2films by analysis and simulation of the generated current noise

Abstract

An original model for the interpretation of the noise produced during the resistive transition of disordered granular MgB 2 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 MgB 2 , 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.

Current noise in stationary conditions in MgB<inf>2</inf> thin films

“…In a subsequent paper [12] the specimen was treated as a network of superconducting grains, characterized by a random distribution of critical currents, and solved by Kirchoff equations during the temperature change. These calculations confirmed the theoretical model proposed in [11].…”

“…The resistive transition is, in this condition, characterized by the presence of an intermediate state constituted by normal and superconducting domains. Measurements of non-stationary noise on polycrystalline MgB 2 thin films, reported in a previous paper, evidenced this situation [11]. The experimental results were interpreted by assuming that the specimen resistance increases when subsequent resistive layers form across the specimen.…”

“…When the specimen resistance is larger than 4 Ω, the noise behaves as in a MgB 2 film in its normal state, shunted by a noiseless resistance, which changes its value from zero to infinite. Using a simplified model we can assume that the specimen is in an intermediate state, as suggested in [11,12] and that it can be represented as a series of resistive layers separated by superconducting regions. The reported results show that at the end of the transition the relative resistance fluctuations of the specimen are the same as during the transition, when the intermediate state is reached.…”

Dual stage resistive transition of MgB2 evidenced by noise analysis

“…The specimen is assumed to be in the intermediate state, as the experimental results on the transition noise reported in Ref. 18 suggest. In a simplified picture, the specimen can be represented as a series of resistive layers separated by superconducting regions that shrink during the transition, to disappear at its end.…”

“…18 The experimental results were interpreted by means of a simplified model, assuming that, during this process, the resistance increase is generated by the formation of resistive layers across the specimen. These assumptions were confirmed by modeling the polycrystalline film as a network of superconducting grains, characterized by a random distribution of critical currents.…”