Martinoli, Piero

2007, Cubero García, Oscar Jesús, Martinoli, Piero

The aim of this thesis is to observe the Berezinskii-Kosterlitz-Thouless (BKT) transition on a magnetically shielded array of Josephson Junctions using an inductive technique. A major component of this work was the design, fabrication and investigation of the dynamics of a Superconductor-Normal-Superconductor (SNS) Josephson junction embedded in a coplanar and gradiometric superconducting coil. The fabrication process involved a series of steps combining different thin film techniques, such as, photolithography, sputtering, Plasma Enhanced Chemical Vapor Deposition (PECVD) and Reactive Ion Etching (RIE). Furthermore, to extract physical magnitudes, namely the sample coil current, we carried out exact calculations of the self and mutual inductances between the coils of the measurement system. In order to understand the response of our device we started with a lumped circuit model consisting of a Josephson junction in series with a superconducting ring of self inductance L. This model leads to a useful graphical representation which explains qualitatively the principal features of the DC and AC measurements. In particular, it has been demonstrated that the source of the observed dissipation in the AC measurements is a series of jumps in the total flux and a series of hysteresis cycles that the sample undergoes when subjected to an alternating magnetic field. A lumped circuit model that also takes into account the normal resistance of the junction, and then the dissipation, has been implemented. The numerical solution of the corresponding differential equation is in very good quantitative agreement with the AC measurements. On the other hand, it was possible, for the first time with a contactless technique, to extract the critical current of a Josephson junction. Two principal features were observed using the device consisting of a single Josephson junction embedded in a superconducting ring. Firstly, a sudden fall in the critical current near the temperature at which dissipation appears, and secondly, a dependence of this transition temperature on the thickness of the junction’s normal metal. Both behaviors were also observed when the single junction was substituted for an array of Josephson junctions. As a result, the possible contribution to dissipation due to the unbinding of vortex-antivortex pairs, as predicted by the BKT theory, may be hidden by the aforementioned dissipation mechanism. No clear signature of the BKT transition was found.