The aim of this work is to investigate the mechanisms controlling the current-carrying capability of YBa 2 Cu 3 O 7Ϫ␦ thin films. A comparison between the magnetic properties of a film with intrinsic grainboundary defects and two films crossed by columnar defects with different densities is presented. Such properties have been studied by means of ac susceptibility measurements, resistivity measurements, and structural characterizations. The Clem and Sanchez model ͓Phys. Rev. B 50, 9355 ͑1994͔͒ is used to extract critical current values from the susceptibility data. In the virgin film, correlated grain-boundary defects were created among islands with homogeneous size, by means of the appropriate modifications in the growth process. Columnar defects were produced through 0.25-GeV Au-ion irradiation. The central issue concerns the investigation of the plateaulike features characterizing the log-log field dependence of the critical current density, the analysis of the J c temperature dependence, and of the irreversibility line. An analytical expression of J c vs B is given in order to compare the main issues with the experimental data. This model suggests that the intergrain pinning dominates in the high-current/low-temperature regime through a network of frustrated Josephson junctions, while the intragrain pinning is effective near the irreversibility line.
The properties of symmetric 24 • and asymmetric 45 • [100] tilt YBa 2 Cu 3 O 7−x bicrystal grain boundary junctions (GBJs) have been extensively investigated. A large number of junctions, with dimensions ranging from 2 to 20 µm, have been fabricated and characterized at 4.2 K. Experimental data have been compared with [001] tilt bicrystal and [100] tilt biepitaxial YBa 2 Cu 3 O 7−x junctions. [100] GBJs show high I c R n products in the range 5-10 mV at T = 4.2 K, these being slightly sensitive to the critical current density values. A direct tunnelling mechanism for both Cooper pair and quasiparticles has been evidenced. The reduced I c R n values of asymmetric 45 • with respect to symmetric 24 • [100] tilt GBJs have been accounted for by considering a d-wave symmetry of the order parameter using the Sigrist-Rice equation.
Submicron YBa2Cu3O7−x
bicrystal grain boundary junctions have been fabricated, for the first time, by a focused ion
beam process. Although such a process has always been considered detrimental to the
YBa2Cu3O7−x
because of gallium contamination, high quality
24°
[001] tilt junctions characterized by RSJ current–voltage characteristics,
ICRN products of
the order of 1–4 × 104 A cm−2
at 77 K and Fraunhofer-like modulation patterns have been obtained. No significant
degradation has been observed over more than 3 months.
The critical current density JC
and the characteristic voltage ICRN
show a clear maximum for widths of the order of the Josephson
penetration depth. The asymptotic normal resistance shows a typical
(width)−1
dependence, indicating that the FIB process does not increase the grain boundary
resistivity of submicron junctions.
Experimental results clearly show that FIB is a very powerful tool for the fabrication of
high critical temperature superconducting circuits, requiring a small number of submicron
Josephson junctions, and for fundamental physics analysis. It also allow the final tuning or
repair of superconducting or more complex integrated superconducting–semiconducting
devices.
We present Nb/AlOx/Nb–Al/AlOx/Nb stacked Josephson junctions which we propose as three-terminal elements to control the quantum behavior of Josephson complex systems. A bias current can be independently injected in the “top” junction, providing a fine control of the critical current of the “bottom” junction. The reported characterization of the device refers to data in the classical limit including measurements of the switching dynamics between metastable states of the system at different temperatures and bias conditions. At low temperature, the effective dissipation of the bottom junction remains substantially unchanged when different injection currents flow in the top junction. These results are interesting to project new configurations of macroscopic quantum experiments.
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