Vortices are topological defects accounting for many important effects in superconductivity, superfluidity, and magnetism. Here we address the stability of a small number of such excitations driven by strong external forces. We focus on Abrikosov-Josephson vortex that appears in lateral superconducting S/S’/S weak links with suppressed superconductivity in S’. In such a system the vortex is nucleated and confined in the narrow S’ region by means of a small magnetic field and moves under the effect of a force proportional to an applied electrical current with a velocity proportional to the measured voltage. Our numerical simulations show that when a slow moving Abrikosov-Josephson vortex is driven by a strong constant current it becomes unstable with respect to a faster moving excitation: the Josephon-like vortex. Such a current-driven transition explains the structured dissipative branches that we observe in the voltage-current curve of the weak link. When vortex matter is strongly confined phenomena as magnetoresistance oscillations and reentrance of superconductivity can possibly occur. We experimentally observe these phenomena in our weak links.

No abstract

The results of Point Contact Andreev Reflection Spectroscopy on polycrystalline RuSr2GdCu2O8 pellets are presented. The wide variety of the measured spectra are all explained in terms of a modified BTK model considering a d-wave symmetry of the superconducting order parameter. Remarkably low values of the energy gap ∆ = (2.8 ± 0.2)meV and of the 2∆/kBTc ≃ 2 ratio are inferred. From the temperature evolution of the dI/dV vs V characteristics we extract a sublinear temperature dependence of the superconducting energy gap. The magnetic field dependence of the conductance spectra at low temperatures is also reported. From the ∆ vs H evolution, a critical magnetic field Hc 2 ≃ 30T is inferred. To properly explain the curves showing gap-like features at higher voltages, we consider the formation of a Josephson junction in series with the Point Contact junction, as a consequence of the granularity of the sample.

We report an experimental and numerical study of the vortex matter moving in a very thin type II superconducting strip with asymmetrically nanostructured profile. The asymmetric thickness profile generates a geometrical force landscape that sets a preferential direction for vortex motion, resulting in an uncommon voltage-current curve of the strip, with vanishingly small asymmetry in the positive and negative critical currents but appreciably large asymmetry in the voltages. Experimental results as well as the geometrical force affecting the moving vortex matter are discussed in the framework of time dependent Ginzburg-Landau model for superconductors with variable thickne

The high temperature reaction between the superconducting Nd123(x) (Nd 1+x Ba 2−x Cu 3 O 7−δ+x/2 ) phase and Nd422(z) (Nd 4−2z Ba 2+2z Cu 2−z O 10−2z ), leading to substitutions of Nd in the Ba sites of Nd123, was analysed. For this purpose, differential thermal analysis (DTA) was performed on powders with different Nd422 addition. Measurement results were compared with the values, reported in the literature, of the peritectic temperatures in function of Nd123 substitution. The superconducting transition critical temperature analysis of melt-textured samples shows that the use of Barium rich Nd422 allows significant improvements of the superconducting parameters of produced samples, avoiding the occurrence of the Nd-Ba substitutions.