C. Attanasio

Interface transparency and proximity effect in Nb/Cu triple layers realized by sputtering and molecular beam epitaxy

We have investigated, in the framework of the proximity effect theory, the interface transparency T between Nb and Cu in the case of high quality Nb/Cu trilayers fabricated by molecular beam epitaxy (MBE) and sputtering deposition techniques. The obtained T values do not seem to be strongly influenced by the fabrication methods but more by the intrinsic properties of the two metals; a slightly higher value for T has even been deduced for the MBE prepared samples. The proximity effect in these samples has also been studied in the presence of an external magnetic field. In the parallel configuration a significant shift towards lower values of the 2D–3D crossover temperature has been observed for MBE samples, in good agreement with very recent theoretical predictions. In the perpendicular case a positive curvature of the temperature dependence of the upper critical field has been detected, which was less pronounced for sputtered samples. Both the effects have been observed only for trilayers with low Nb thickness (<600 A) which confirms the crucial influence of the interface transparency on the values of the upper critical field in such samples.

Magnetic field depedennce of pinning mechanisms in Bi2Sr2Ca1Cu2O8+x thin films

Abstract We performed investigation of the pinning mechanisms in Bi 2 Sr 2 CaCu 2 O 8+ x (BSCCO) thin films as a function of the temperature, magnetic field and bias current. The specimens were prepared by molecular beam epitaxy deposition technique with a subsequent “ex-situ” annealing in air atmosphere. Transition temperatures T c exceeded 86 K and critical current densities J c were of the order of 10 5 A/cm 2 at T = 4.2 K. Analyzing the temperature dependence of the resistivity ϱ ( T ) data we show that they could be well described by an activation process. The absoute value of the activation energy U depends crucially on the elaboration procedure, while the obtained magnetic field dependence U ( H ) does not depend on the selected model. Introducing a pinning strength distribution we were able to describe the U ( H ) dependence taking into account a crossover from an individually pinned vortex regime to a collective behavior. This crossover was also confirmed by the current dependence of U . In fact, below the crossover field H 0 the curves U ( J ) were well described by a spatial washboard-type variation of the pinning energy, while for H > H 0 the U ( J ) curves showed a logarithmic behavior.

Surface impedance measurements of superconducting (NbTi)N films by a ring microstrip resonator technique

In the framework of the search for new materials for sputter coated superconducting rf cavities for particle accelerators, an innovative method to measure the rf properties of superconducting films, based on an inverted microstrip ring resonator technique, is presented. Test measurements on well characterized Nb sputtered films show excellent agreement with the Bardeen, Cooper and Schrieffer theory and prove the good resolution of the method. New results obtained by this technique on the surface impedance of (NbTi)N reactively sputtered films, including the temperature dependence and the rf field amplitude dependence of the surface resistance and the temperature dependence of the magnetic field penetration depth, are then presented and discussed.

Static and dynamic properties of the vortex lattice in superconductor/weak ferromagnet bilayers

By measuring I–V characteristics as a function of the temperature and the external magnetic field, we have analyzed the static and dynamic properties of the vortex lattice in Nb/Pd0.84Ni0.16 bilayers. In particular, the critical current density Jc for the onset of the vortex motion and the dynamic instability of the moving vortex lattice at high driving currents have been studied and compared to the results obtained in a single Nb film. We find that Jc is smaller in the bilayers than in the single superconducting film due to the smaller value of the superconducting order parameter in the bilayers. The critical velocity v* for the occurrence of the instability is larger in the S/F bilayers than in the single S layer. However, the quasiparticle energy relaxation rate extracted from v* is almost temperature-independent, implying that a different relaxation mechanism plays a role in the Nb/Pd0.84Ni0.16 bilayers.

New aspects of microwave properties of Nb in the mixed state

We present a study of the frequency dependence of the vortex dynamics in a conventional superconductor. We have employed a swept frequency, Corbino disk technique to investigate the temperature (3.6 K -T c ) and high-field (from H c2 /2 to H c2 ) microwave complex resistivity in Nb thin (20-40 nm) films as a function of the frequency (1-20 GHz). We have found several previously unnoticed features: (i) a field-dependent depinning frequency in the GHz range; (ii) deviations from the accepted frequency dependence [1], than can be ascribed to some kind of vortex creep; and (iii) the presence of switching phenomena, reminiscent of vortex instabilities. We discuss the possible origin of the features here reported.

Interface transparency of Nb/Pd layered systems

Abstract.We have investigated, in the framework of proximity effect theory, the interface transparency

Proximity effect and interface transparency in Nb/Cu multilayers

{\cal T}

Evaluation of the specific boundary resistance of superconducting/weakly ferromagnetic hybrids by critical temperature measurements

of superconducting/normal metal layered systems which consist of Nb and high paramagnetic Pd deposited by dc magnetron sputtering. The obtained

Nonequilibrium fluctuations as a distinctive feature of weak localization

{\cal T}

Properties of Bi2+xSr2−xCuO6+δ thin films obtained by MBE

value is relatively high, as expected by theoretical arguments. This leads to a large value of the ratio