C. Camerlingo

Probing of current-tailoring mechanisms in YBa2Cu3O7−δ films by means of heavy ion irradiation

Abstract This paper deals with the origin of the plateau-like feature of the critical current density vs. logarithm of the field in YBCO films. By means of a suitable model, we check the hypothesis that this trend depends on a particular grain boundary (GB) network modulated by intrinsic correlated defects, coherently trapping the vortices. Such network consists of a 1D row of high- J c intergrain Josephson junctions (JJ). Irradiation with gold ions provides extrinsic columnar defects. The fit of the data with the model indicates that the columnar defects change the length and distribution of the JJ. In interpreting our data, it results that at least in a range of temperature up to 0.5 T / T c , J c depends on the defect-induced modulation (either intrinsic or extrinsic) of the JJ network.

Low 1/f flux noise in sputtered YBa2Cu3O7−x bicrystal dc-superconducting quantum interference devices

YBa2Cu3O7−x bicrystal dc-superconducting quantum interference devices (SQUIDs) and single layer magnetometers working at 77 K have been fabricated and analyzed. Samples have been made in situ by cylindrical magnetron dc sputtering on [001] SrTiO3 bicrystal substrates. Magnetic flux noise levels and magnetic field sensitivities by standard flux-locked-loop electronics have been measured out on dc-SQUIDs and magnetometers, respectively. Flux noise levels as low as a few μΦ0Hz−1/2 in the white noise regime have been measured in SQUIDs, whereas the magnetic field sensitivity of the magnetometers was influenced by external sources. A comparison of experimental data with analytical expressions modelling the behavior of high Tc SQUIDs has been carried out, with special reference to the flux-to-voltage transfer parameter and noise characteristics. Finally, a reduction of 1/f noise of the SQUIDs has been achieved by a wide band bias reversal electronics.

Effect of the loop capacitance on resonant modes in superconducting two‐junction interferometers

Self‐resonant current steps in superconducting interferometers are investigated. The case of interferometers with non‐negligible loop capacitance is considered. A theoretical analysis based on an equivalent electrical circuit is performed to get the resonance frequencies in the current‐controlled case. Careful measurements on a slightly asymmetrical (C1≠C2) interferometer are reported. The magnetic field dependence of the maximum amplitude of the resonant steps and their voltage position have been studied. A good agreement between theory and experiments is obtained.

Study of the effect of intrinsic and induced defects on the I-V characteristics of YBCO films

Abstract The current–voltage ( I – V ) characteristics of YBCO films in zero applied magnetic field were investigated. Different conditions of fabrication have been considered in order to face with different structural defect configurations. The measurements were performed in a wide range of temperature, in the regime of thermal activation motion of magnetic vortices. The current-dependent activation energy for dissipative behavior was evaluated, and a correlation to the structural sample configuration was attempted. The results were compared to those obtained in Au + ion irradiated samples, where a regular distribution of artificial columnar defects is added to the background superconductor crystal defects. Two levels of irradiation were considered for the same sample.

High-quality YBCO d.c. SQUIDs based on bicrystal substrates

SummaryAn analysis of superconducting transport properties and magnetic behaviour of d.c. SQUIDs employing YBCO bicrystal grain boundary junctions (GBJs) has been performed. GBJs have been obtained by deposition of ac-axis-oriented YBCO film on a SrTiO3 bicrystal substrate by ICM sputtering technique. Experimental measurements on a YBCO d.c. SQUID with a misorientation angle θ=20° are reported. The SQUID shows a critical temperatureTc∼89 K and a high critical current densityJc∼3·106 A/cm2 atT=4.2 K. Current-voltage characteristics are close to the behaviour predicted by the resistively shunted junction (RSJ) model and the temperature dependenceJc(T) shows a linear behaviour at small reduced temperatures and a depressedJc value close toTc. High-quality flux-voltage curves have been found upT=87 K over a large range of magnetic field. The high reproducibility and the good control of transport properties by the variation of θ make YBCO bicrystal GBJs very useful for applications in electronics.

Current direction dependence of vortex pinning in (103)/(013) oriented YBCO films

Abstract The electrical and the superconducting properties of (103)/(013) oriented YBCO films were investigated as a function of the angle of incidence of the current with respect to the in-place orientation of the film lattice. Unidirectional twin boundary defects are expected to occur in the film, resulting in the observed anisotropy γ of the resistivity in normal state. Also the current-voltage (V-I) characteristics are affected by the current direction. In this case CuO layer intrinsic pinning is argued to be the origin of the observed behavior.

Effect of vortex-pair fluctuations on zero-field current-voltage characteristics of YBCO films

Abstract The current-voltage characteristics of YBCO films were investigated on both c-axis and a/b axis oriented samples. The films, about 100 nm thick, were fabricated by inverted cylindrical dc magnetron sputtering on (100) and (110) SrTiO 3 substrates. In c-axis oriented films the observed behavior is consistent with a model based on two-dimensional pair vortex fluctuations in the CuO 2 planes. Preliminary measurements on the a/b axis oriented samples are reported, for different angles between incident bias current and c-axis planes.

Proximity effects in all refractory Josephson tunnel junctions

Abstract The theoretical approach to proximity effect based on the thermodynamic Greens functions is considered to investigate the behaviour of all refractory Josephson tunnel junctions. The experimental dependence of the maximum dc Josephson current on temperature is analyzed. Two junction configurations are studied: NbAl/AlO x /Nb structures with a rather thick Al film and high quality Nb/Nb junctions with either a semimetallic or a metallic back-layer (Nb/AlO x /NbBi, Nb/AlO x /NbAl). A satisfying agreement between theoretical calculations and experimental data is found.

HTSC grain boundary weak links employing high-quality ICM sputtered YBCO films

SummaryBicrystal grain boundary junctions of YBCO have been fabricated by using the Inverted Cylindrical Magnetron d.c. sputtering technique. The main characteristics of the junction behaviour are reported. Some aspects of the experimental results are discussed in the framework of existing models developed for describing weak-link structures.