L. Maritato

Disorder and vortex dynamics in high- superconductors

We have investigated V - I characteristics in the microvolt region of c-axis-oriented films in the temperature range 20 - 70 K. A change in the curvature of the log V - log I curves was observed and to explain it we have used a model based on the Anderson - Kim theory of thermally activated flux creep for the washboard-type pinning potential in the presence of a pinning strength distribution. This model gives quantitative agreement with our experimental data.

Melting of the vortex lattice in perforated Nb/CuMn multilayers

Abstract Transport properties of Nb/CuMn multilayers with a regular array of antidots have been measured at different temperatures in the presence of external perpendicular magnetic fields. Two series of samples, which differ for the Mn percentage, for the diameter, and for distance of the antidots have been analyzed. In both the series, hysteresis in current–voltage characteristics has been observed, which disappears when approaching the upper critical magnetic field. We have related the onset of this hysteresis to the presence of an irreversibility line in the H – T plane, due to the quantum melting of the vortex lattice.

Tunnel junctions based on superconducting/magnetic multilayers

Abstract Several high quality tunnel cross junctions based on Nb/CuMn (superconducting/spin-glass) multilayers were realized to investigate microscopic properties of superconducting/magnetic layered structures in view of potential electronic applications. The Mn concentration (2.35 at.%), which is related to the magnetic coupling, as well as the Nb layer thickness (∼250 A) were kept constant. The CuMn layer thickness dCuMn was varied from 0 to ∼50 A, where a non monotonic behaviour of the superconducting temperature vs. dCuMn is observed, as a manifestation of the system entering the so-called π-phase. The geometry of the Nb/CuMn base electrode was defined by standard optical lithography. The oxide barrier (Al2O3) and counterelectrode (Nb) were defined by lift-off and fabricated without breaking the vacuum to insure a good quality of the insulating barrier. Superconducting properties were investigated in the temperature range 2–10 K.

Tunnel measurements on Nb/CuMn multilayer based planar junctions

Abstract Recently, transport properties of Nb/CuMn (superconducting/spin glass) multilayers have been largely investigated with a particular interest to their vortex dynamics behaviour. A better understanding of the properties of these systems requires the analysis of microscopic properties such as the superconducting energy gap Δ and the density of states. For this reason we have realized and characterized high quality planar tunnel junctions based on Nb/CuMn multilayers. When changing the CuMn layer thickness dCuMn from 4 to 45 A, keeping constant Mn concentration (2.35%) and Nb layer thickness (250 A), a BCS-like temperature dependence with values of the zero temperature energy gap Δ(0) in the range 0.79–1.05±0.02 meV is always observed. Moreover, Δ(0) decreases when dCuMn increases, revealing a suppressing effect of the CuMn layers thickness.

Realization and characterization of tunnel junctions based on Nb/CuMn multilayers

High quality planar tunnel junctions based on Nb/CuMn (superconducting/spin glass) multilayers are realized and investigated. When changing the CuMn layer thickness dCuMn from 4 to 45 A, keeping constant Mn concentration (2.35%) and Nb layer thickness (250 A), a temperature dependence for the gap in accordance with the Bardeen-Cooper-Schrieffer (BCS) theory, with zero temperature values Δ(0) in the range 0.79-1.05±0.02 meV, is observed. Moreover, Δ(0) decreases when dCuMn increases in the same fashion as the critical temperature Tc, revealing a similar effect of the CuMn layers in suppressing both Tc and Δ(0).

Scaling of Hc2⊥(T) in Nb/CuMn multilayers

Measurements of the perpendicular upper critical magnetic field Hc2⊥(T) are reported for several Nb/CuMn multilayers. It is found that, despite the magnetic nature of the samples, the data for samples with low Mn percentage in the CuMn layers are simply described by the Werthamer–Helfand–Honenberg theory for conventional type-II superconductors, neglecting both Pauli spin paramagnetism and spin orbit impurity scattering. For high Mn concentration a different theoretical approach is needed.