Transition between the Bragg glass and the disordered phase in Nb3Sn detected by third harmonics of the ac magnetic susceptibility.

We report on experimental evidence of the presence of a universal behavior in the field-temperature phase diagram of type II superconductors, characterized by a phase transition in the vortex matter between the disordered and the Bragg glass phase. The experimental detection of a peak effect phenomenon has been proved to be strictly connected to the existence of this phase transition. In this paper, we show an observation of a peak effect in the compound Nb3Sn, by using first harmonics of the ac magnetic susceptibility. Peak effect has been detected at fields between 3 and 13 T, whereas it is not observable at higher fields. This seems to be in contrast with the theoretical predictions of such a phase transition at all fields and, therefore, with the predicted universality in the magnetic behavior of the type II superconductors. Nevertheless, by measuring the third harmonics of the ac susceptibility, this phase transition has been detected up to our highest available field (19 T), thus demonstrating the necessity of the higher harmonics analysis in studying these topics and moreover proving the validity of the theoretical predictions.