On the suppression of the sidelobes of the supercurrent in small Josephson tunnel junctions

The critical currents of Nb/Al, Al-oxide, Al/Nb tunnel junctions of various shapes have been measured as a function of the applied magnetic field. For the square junction and for some special shapes like the diamond, ¿1 + cosine¿ and quartic junctions the Ic(B) pattern falls off theoretically as 1/Bn, with n respectively equal to 1, 2, 3 and 4. In general the measurements are in good agreement with the theoretical predictions. For the "1 + cosine" and quartic shapes we found a sidelobe suppression that is even larger than that obtained in theory. For the quartic junction the first sidelobe is only 0.3% of the zero-field current. An Ic(B) modulation with a small, only slowly decreasing amplitude is observed for the diamond, "1 + cosine", and quartic junctions, that can be explained by rounding of the sharp edges of the junction shapes, due to the fabrication process

X-ray response of tunnel junctions with a trapping layer

The use of trapping layers in superconductive tunnel junctions may drastically improve their functioning as X-ray detectors. Information about these trapping layers can be obtained from I/V-curves and X-ray spectra. The application of a magnetic field causes a substantial reduction of the bandgap in the trapping layer

Injection-detection experiments to study quasiparticle losses in Nb, using a series array of Nb/Al junctions

Injection-detection experiments have been performed to study quasiparticle diffusion and quasiparticle losses in Nb strips using a series array of Nb/Al junctions. The I-V curve of the detector junction was measured as a function of the spacing between the (high-quality) detector junction and the simultaneously biased injector junction. For our sample geometry, the quasiparticle gas is described in terms of a one-dimensional diffusion model. It was found that trapping losses at the interface between the Nb base electrode and the anodised Nb2O5 dominate. In addition, a very low quasiparticle diffusion constant at 1 K, D = 1.2 × 10−4 m2/s has been found. We estimate that this corresponds to an electron mean free path of l ≈ 7 nm. Due to the difference between the bandgaps of the base and top electrode and to the peak in the density of states at the gap edge, the excess detector current shows a characteristic voltage dependence for bias voltages below 0.5 mV

Field dependence of the Josephson current and Fiske resonances in specially shaped Josephson junctions

We investigated the magnetic-field dependence of the Josephson current and Fiske resonances in specially shaped Josephson junctions. In order to be able to use junctions for high-resolution X-ray spectroscopy, a very good suppression of the sidelobes of both the Josephson current and the Fiske resonances must be achieved. In a theoretical argument we show that a properly chosen junction shape leads to the sidelobe suppression of both the critical current and Fiske resonance amplitudes. The Josephson current and Fiske resonance amplitudes were measured as a function of the magnetic field, for junctions fabricated in Nb/Al technology. As expected, a very good sidelobe suppression was obtained for quartic-shaped junctions. For junctions with anodized structures within the tunneling area, the shape of the internal structures is reflected in the field dependence of both the Josephson current and the Fiske resonances. Finally, Fiske modes in these junctions have been imaged with low-temperature scanning electron microscopy, and we conclude that a quartic junction can be approximated by a rectangle, to describe the lower-order Fiske modes, whereas the high-order modes are specific to the exact shape of the junction

Superconductive junctions with trappinglayers for the detection of X-rays

Addition of a trapping layer to an SIS junction improves its performance as an X-ray detector. In this article X-ray induced pulse height and decay time spectra will be presented as a function of bias voltage. These measurements are in good agreement with a description based on the time constants for trapping, excitation and tunneling calculated by means of a model for proximity layers developed by Golubov et al..1,2The interpretation of the data doesn't require an initial fast loss process for the created quasi-particles as discussed by Van Vechten

Superconductive tunnel junctions for x-ray spectroscopy

Superconductive tunneljunctions are under development as detectors for X-ray astronomy in the 0.5-10 keV energy range, because of their potentially high energy resolution (E < 10 eV) in combination with high detection efficiency. Especially absorber-junction combinations offer the prospect of high energy resolution detectors with a high detection efficiency and a reasonable ( 1 cm2) size. The proximity effect between the Nb absorber and the Al trapping layer plays a dominant role. A study of the proximity effect in Nb/Al/Al2 03/Al/Nb junctions with different Al-layer, the trapping layer, thicknesses is presente