Evert Pieter Houwman

Quasiparticle relaxation rates in a spatially inhomogeneous superconductor

Effective quasi-particle relaxation rates in reduced gap regions of a dirty superconductor (S) at low temperatures are calculated from microscopic theory.

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. n nIn 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. n nThe 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.

Fabrication and properties of Nb/Al,AlOx/Nb Josephson tunnel junctions with a double oxide barrier.

High‐quality Nb/Al, Alox/Nb Josephson tunnel junctions using double‐oxide layers as barriers have been fabricated. The critical current density is controlled by the thickness of the second Al layer. This layer has to be oxidized completely through in order to obtain high‐quality junctions. Typically, gap voltages of 2.8–3.0 mV and Vm up to 70 mV at 4.2 K were obtained.

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.

Superconductive tunnel junctions for X-ray spectroscopy

In order to investigate the influence of quasi-particle trapping on the performance of superconductive tunnel junctions as X-ray detectors, a series of Nb/Al/Al/sub 2/O/sub 3//Al/Nb junctions have been produced with different Al-layer thicknesses in the bottom electrode. The proximity effect between the Nb absorber and the Al trapping layer plays a dominant role, because it greatly influences the trapping time of excess quasi-particles from the Nb electrode to the Al trapping layer. A study of the influence of Al layer thickness on the operation of tunnel junctions as X-ray detectors is presented. Quasi-particle trapping is shown to be quite an efficient process in Nb junctions with Al-layers of 10 nm and thicker. The proximity model of A.A. Golubov and E.P. Houwman can explain the present data quite well. Nb junctions suffer from the bandgap reduction at oxidized and anodized surfaces. Without any special attention, extremely fast loss processes, about 30 ns, are present.<<ETX>>

Design and construction of a 19-channel DC-SQUID neuromagnetometer

The 19-channel DC-SQUID neuromagnetometer which is under construction at the University of Twente is described. Several aspects of this development are considered: the DC-SQUID sensor, the design of the SQUID module, the arrangement of the 19 gradiometers, and the electronics, including the output transformer, the preamplifier and the control and detection section. The completed system will be installed in our magnetically shielded room.

A 19-channel d.c. SQUID magnetometer system for brain research

A 19-channel d.c. SQUID magnetometer system for neuromagnetic investigations is under constuction. The first-order gradiometers for sensing the signal are placed in a hexagonal configuration. D.c. SQUIDs based on niobium/aluminium technology have been developed, leading to a field sensitivity of about 5 fT/ Hz. SQUID read-out is realized with a resonant transformer circuit at 100 kHz. The multichannel control and detection electronics are compactly built.

Field dependence of Fiske resonances in Nb-AlO/sub x/ based Josephson junctions

Fiske resonances have been measured in a rectangular Nb-Al, AlO/sub x/, Al-Nb Josephson junction as a function of the magnetic field applied parallel to a junction side. Due to the high quality factor of the junction, many resonant modes could be measured, using a special measuring technique. Each mode shows more lobes than reported before. The measured curves are in very good agreement with the high Q theory of I.O. Kulik (1967), and Q-values ranging from 40 to 450 have been obtained. The surface resistance of the Al/Nb barrier probably dominates the microwave losses in the junction barrier at 4.2 K. Losses due to the quasi-particle tunneling current can be neglected. Fiske resonances in a square junction at a field angle of pi /4 rad were also measured. Two-dimensional resonant modes have been found. The field dependence of the

Quasiparticle trapping, excitation, and tunneling times in Josephson junctions with spatially inhomogeneous electrodes

On the basis of a microscopic model of the proximity effect in an SS-sandwich we have calculated effective trapping, excitation, and tunneling rates of the reduced gap region in an SS′IS″S junction as a function of temperature, voltage over the junction and strength of the proximity effect. We developed a simple model to describe the effect of magnetic fields on the gap of the superconductor, and calculated the trapping rate as function of the field. The experimentally determined gap reduction and quasiparticle loss times as function of the field were described in terms of this model.

Josephson junctions and DC SQUIDS based on Nb/Al technology

A process for fabricating high-quality Josephson junctions and DC SQUIDs on basis of Nb/Al technology has been developed. DC magnetron sputtering is used for the deposition of the metal layers and the barrier is formed by thermal oxidation of the Al-layer. The junction area of 5 microns x 5 microns is obtained using anodisation. Three types of Josephson tunnel junctions have been prepared: standard Nb/Al, AlO kappa/Nb, symmetric Nb/Al, AlO kappa, Al/Nb and Nb/Al, AlO kappa/AlO kappa/Nb, the latter having a double oxide layer. We performed current-voltage and conductance-voltage measurements at different temperatures and special attention was paid to the noise behaviour. Gap and sub-gap parameters as well as barrier parameters are presented. Three different DC SQUID configurations were developed on basis of the Nb/Al Josephson junctions. The measured characteristics of the standard Tesche-Clarke DC SQUID, the resistively shunted SQUID and the inductively shunted SQUID are compared with special attention being paid to the noise properties. A 19-channel DC SQUID magnetometer with standard and/or resistively-shunted DC SQUIDs is under construction.