DATE / TIME: Monday, April 24th 2017, 4:00 p.m.
LOCATION: Erwin Schrödinger Lecture Hall, 5th floor, Boltzmanngasse 5, 1090 Vienna
ABSTRACT: Despite its age, the Brezinskii-Kosterlitz-Thouless (BKT) transition remains one of the most fascinating examples of topological phase transitions. Its universality class describes several phenomena ranging from the quantum metal-insulator transition in one dimension to the Columb-gas screening transition in 2D, and of course the metal-to-superﬂuid transition in 2D. Among real systems, the case of quasi-two-dimensional superconductors (SC) is particularly interesting because of the interplay between the vortex-unbinding transition and the electron inhomogeneity, which spontaneously emerges in thin SC ﬁlms. Indeed, although the BKT transition is usually protected against disorder, its ﬁngerprints in real system, like e.g. the universal superﬂuid-density jump, are often at odd with this expectation. In this work we have shown, by means of Monte Carlo simulations, that the disorder-induced granularity of the superconducting state modiﬁes the nucleation mechanism for vortex-antivortex pairs leading to a considerable smearing of the universal superﬂuid-density jump as compared to the paradigmatic clean case, in agreement with experimental observations. Finally, we have extended our study in the presence of a transverse magnetic ﬁeld focusing both on its eﬀects on the fragmentation of the SC order parameter (eﬀect partially similar to increasing the disorder strength) and on the superﬂuid density transition.
 arXiv:1702.05930 (2017) I. Maccari, L. Benfatto, C. Castellani. Broadening of the Berezinskii-Kosterlitz-Thouless transition by correlated disorder