M. A. Navacerrada

Properties of [100] tilt Y-Ba-Cu-O bicrystal grain boundary junctions

We have fabricated and measured high-quality [100] Y-Ba-Cu-O grain boundary junctions in both symmetric and asymmetric configurations. High values of the critical current times normal resistance I/sub c/R/sub n/ products have been systematically observed. The high I/sub c/R/sub n/ values found in these junctions may be of interest for the fabrication of high-sensitive superconducting quantum interference devices.

Sound Absorption Properties of Aluminum Sponges Manufactured by Infiltration Process

In this paper, the sound absorption properties of aluminum sponges manufactured by infiltration process have been studied. The results show that the sound absorption factor α changes in function of pore size. Additionally, the sound absorption factor α was sensitive to the frequency range measured and it was depending of the thickness sample. It was found that the interconnected porosity plays an important role increasing the sound absorption properties of the aluminum sponges studied.

Structural Image of the Barrier of

In general, information about the structure of the barrier in YBa2Cu3O7(YBCO) grain boundaries Josephson junctions (GBJJs) has been mainly derived by means of destructive techniques. In this line, any structural data directly derived from the analysis of the electromagnetic and transport parameters would be very useful. For GBJJs based on three bicrystalline geometries (24deg [001] asymmetric, 45deg [100] asymmetric and 24deg [001]symmetric + 45deg [100] asymmetric) we present a structural study of the barrier based on the transport and electromagnetic parameters deduced from the current-voltage (I-V) curves. The dependence of the critical current (IC) on the junction resistance (RN) shows an approximately ICRN -1 power law type dependence. This result is in accordance with a SNINS or SIS description of the barrier. Concerning the electromagnetic parameters, we have calculated the capacitance (C) by means of Fiske resonance positions and hysteresis observed in the I-V curves. While C values calculated with both methods are similar in the case of YBCO 24deg [001] asymmetric GBJJs, the ones deduced from Fiske resonances are always higher in the case of the other two misorientation angles, where a [100] component is present. This discrepancy allows us to estimate the length of the normal N regions at both sides of the barrier.

{\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7}

We have studied the frequency dependence of the dielectric constant of YBa₂Cu₃O₇ Josephson junctions fabricated on bicrystalline substrates with different angles tilted around alpha and <i>c</i> axis. The ratio of the dielectric constant to the thickness of the barrier, epsiv/<i>t</i>, can be deduced by measuring the voltage of Fiske steps V_eta = etaOslash₀c macr<i>/ </i>omega, where eta is the resonance number, omega the junction width and Oslash₀ the magnetic flux quantum. Changing a technological parameter as omega we are modifying V_eta, so the resonant frequency <i>f</i> _n = <i>V</i> _eta/Oslash₀ for each fixed eta . This makes possible to generate experimentally a dispersion relation of the dielectric constant of the barrier, epsiv(<i>f</i> _n) = epsiv(omega) . For all the bicrystalline geometries investigated, data can be fitted to the expression of epsiv(omega) that describes the behavior of the dielectric constant close to a resonance in a dielectric medium with losses. Consistent with the analysis of transport parameters, the values deduced for the resonance frequency and damping constant show a tendency to a more semiconductive behavior with the increase of the misorientation angle. In terms of the equivalent circuit RLC, we can obtain additional information on the inductive response of the barrier.

Grain Boundary Josephson Junctions Based on the Analysis of the Electromagnetic Parameters

We have studied the effects of irradiation with He+ at 80 keV on the properties of YBa2Cu3O7-x (YBCO) grain boundary Josephson junctions (GBJs) and superconducting quantum interference devices (SQUIDs) made on bicrystalline substrates. He+ irradiation produces a shift in the position of the Fiske and flux-flow resonances and consequently changes of the Swihart velocity and the ratio between the dielectric constant and the barrier thickness. Interestingly IC of GBJs can be raised over 10% of its value by irradiation. The modification of the barrier properties depends on the dose and on the particular distribution of the current inside the barrier. By selecting the appropriate doses from the detailed analysis of the shape of the critical current versus magnetic field curve, irradiation offers the possibility to induce a posteriori controlled variations in the properties of YBCO GBJs. These variations imply a decrease in the critical temperature of the electrodes. At this stage of our study, this is the most important inconvenience in using helium irradiation for applications related with nitrogen cooled devices based in GBJs. However, as an interesting example from a scientific point of view, we show the evolution by irradiation of dc SQUID voltage modulation curves, the shape of which are altered by the presence of Fiske steps.

Dispersion Relation of the Dielectric Constant of

Since soundscapes are strongly related to the human perception, the sound pressure level does not seem to be a sufficient representative of a soundscape by itself. Therefore, the characterization of a soundscape might be improved by using psychoacoustic parameters. A study has been conducted and the relationship between psychoacoustic parameters and the subjective perception has been analyzed, for a collection of urban spaces. For that purpose, several locations at the city of Madrid have been selected, based on their main use and geometrical features. Then, binaural recordings have been made for several days and different seasons, which allow to observe the relevance of the behavioral differences for the same locations under different conditions. Psychoacoustic parameters, as well as sound pressure level, have been calculated for both the complete recordings and selected parts of them. A semantic differential listening test have been carried out to look for correlations between the calculated parameters ...

{\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7}

We have developed a simple model, in the framework of the Kulik theory of Fiske steps in Josephson junctions, for the electromagnetic resonances observed in the current voltage characteristics of certain high temperature superconductor grain boundary junctions. Some preliminary results are illustrated.