G.C.S. Brons

Four layer monolithic integrated high Tc dc SQUID magnetometer

YBa2Cu3O7−x based monolithic integrated dc SQUID magnetometers, consisting of a dc SQUID integrated with a flux transformer on a single bicrystalline substrate, have been fabricated and characterized. The devices consist of four layers, including two superconducting layers, and first realizations operate up to 73 K. A maximum voltage modulation of 32 μV is observed at 40 K. A field sensitivity of 0.17 pT/√Hz is obtained above 200 Hz at 45 K and 0.49 pT/√Hz at 1 Hz and 65 K.

Double relaxation oscillation superconducting quantum interference devices with gradiometric layout

Double relaxation oscillation superconducting quantum interference devices (DROSs) with a gradiometric signal SQUID and either a reference SQUID or a reference junction will be presented in this article. The devices are user friendly, particularly those with a reference junction. Because of the large flux-to-voltage transfer of ∂V/∂Φ=0.7–1 mV/Φ0, the devices can be operated in a flux locked loop based on direct voltage readout without loss of sensitivity. The typical white flux noise of the DROSs amounts to √SΦ=5–6μΦ0/√Hz, which corresponds to an energy resolution e=SΦ/2Lsq≃200 h. Coupled to an external planar first-order gradiometer, a white magnetic field sensitivity of √SB<2 fT/√Hz was measured inside a magnetically shielded room.

Laser scanning imaging and local characterization of superconducting properties in high-Tc thin film multiturn coil

Low-temperature scanning laser microscopy has been used to investigate the spatial variation of the critical temperature Tc and critical current Ic in thin-film high-Tc multilayer structures that include dielectric layers. The method is described and measurements are presented on an YBa2Cu3O7-x-based multiturn coil with SrTiO3 insulating layer. We found that the critical temperature Tc of the YBa2Cu3O7-x top layer, from which the return strip of the coil is formed, is higher than that of the YBa2Cu3O7-x base layer. The critical current of the coil is limited by the quality of the YBa2Cu3O7-x base layer and not by the edges of the crossovers.

Smart SQUIDs based on relaxation oscillation SQUIDs

Smart SQUIDs based on double Relaxation Oscillation SQUIDs (DROS) and a superconducting up-down counter have been developed. DROS and counter form a flux locked loop on one single chip. The DROS output consists of a series of pulses that controls the two up and down write gates of the counter. The pulsed output structure of the DROS constitutes the internal clock for this single-chip device. Several prototypes were built with a clock frequency of 100 MHz, a linear operation flux range of about 2.5 /spl Phi//sub 0/, and a white noise level of 6.5 /spl mu//spl Phi//sub 0///spl radic/Hz. The smart SQUID is in principle a promising device for application in multichannel SQUID systems.

Low input coil inductance SQUIDs for cryogenic current comparator applications

Dc SQUIDs with an optimal input coil inductance have been developed for a Cryogenic Current Comparator (CCC) that is used for the calibration of electrical standards. We studied a series of SQUIDs with input inductances in the range from 20-160 nH. The electrical properties like input current noise and flux to voltage transfer have been investigated. The CCC is an overlapping tube configuration and the tube itself is used as the pick-up coil of the flux transformer circuit of the SQUID. The coupling between CCC and flux transformer is in this case ideal and should have an optimal value when the effective overlapping tube inductance, typically in the range from 10-100 nH, equals that of the SQUID input coil (flux transformer theory). To compare with theory, sensitivity measurements on the SQUID-CCC have been performed in a special set-up where the effective overlapping tube inductance can be modified placing the CCC in a superconducting shield at various distances.

Quasiparticle relaxation rates in Nb/AlOx/Nb tunnel junctions due to Abrikosov vortices

Superconducting tunnel junctions are promising detector elements in next generation x-ray, UV, and optical detectors. Quasiparticles which approach the neighborhood of the normal core may scatter and emit a phonon, thus being trapped in the Abrikosov vortex (AV). In this way the existence of AVs reduces the lifetime of quasiparticles. In this experiment excess quasiparticles in a Nb/AlOx/Nb junction are created by means of a low temperature scanning electron microscope. The trapping rate of quasiparticles into AVs is determined by measuring the quasiparticle lifetime as a function of the AV concentration. This results in an effective trapping radius of the AV of 14.8±1.4 nm, which is slightly less than theoretical expectations.

Frequency readout of relaxation oscillation superconducting quantum interference devices in the GHz regime

The output of relaxation oscillation superconducting quantum interference devices (ROSs) consists of a sequence of voltage pulses with a frequency that depends on the flux that is applied to the ROS. In this paper, a theoretical model for the flux‐to‐frequency conversion of a ROS is presented, and this model is validated in practice for oscillation frequencies up to 7 GHz. The experiments have been performed on more than ten different ROSs and the model was able to fit all measured data, which illustrates the versatility of the model. Furthermore, a simple flux locked loop based on frequency readout of a ROS in the GHz regime is presented. The measured flux noise, √SΦ=2.5μΦ0/√Hz, corresponding to an energy resolution e≊600h, is probably not intrinsic to the ROSs, but due to the readout electronics.

A 1-MHz low noise preamplifier based on double relaxation oscillation SQUIDs

A low noise and wideband preamplifier based on Double Relaxation Oscillation Superconducting Quantum Interference Devices (DROSs) has been realized. A major advantage of a DROS is that it can be operated in a simple flux modulation. So far, biomagnetic measurements performed in our group required only a limited bandwidth smaller than 100 kHz. Other applications, like for instance readout of radiation and particle detectors, demand a larger bandwidth. In this paper, we will discuss our efforts aimed at increasing the operational bandwidth of a DROS in flux locked loop. Presently, a flux locked loop scheme with a -3 dB bandwidth of 1.45 MHz has been built. With this system a white flux noise of 8 /spl mu//spl Phi//sub 0///spl radic/Hz was measured with a 1/f-corner frequency of 10 Hz. The slew rate was 2.5/spl middot/10/sup 5/ /spl Phi//sub 0//s. With the mutual input inductance of 6.7 nH, an input current noise of the preamplifier of 2.5 pA//spl radic/Hz was found and a current slew rate of 80 mA/s. We will discuss the suitability of our DROS-based preamplifier for readout of cryogenic particle detectors based on superconducting tunnel junctions.

Spatial inhomogeneities in the energy response of a tunnel junction detector due to penetration of Abrikosov vortices

For the application of superconductive tunnel junctions (STJs) as high resolution X-ray detectors the homogeneity of the detector response is of utmost importance. In this article it is shown how this homogeneity is degraded by the penetration of Abrikosov vortices (AVs) into the junction electrodes. These AVs may exist because of small misalignments in the parallel magnetic field, which is applied to suppress the Josephson current and Fiske steps. By means of Low Temperature Scanning Electron Microscopy (LTSEM) the response of the tunnel junction to local energy deposition is mapped and found to be reduced in areas where AVs are present. On the basis of these experiments the threshold perpendicular field at which vortices start to penetrate the junction is derived.

Geometry effects on the response of a superconducting tunnel junction detector

Superconductive tunnel junction detectors are promising as next generation X-ray detectors. By means of Low Temperature Scanning Electron Microscopy, the inhomogeneities in the response of this type of detectors are investigated. Spatial variations in the quasiparticle life time and the quasiparticle tunnel time are reasons for inhomogeneity in the response. In this article junctions with two different counter electrode geometries are compared. It is shown that the response homogeneity of a junction with a wiring layer fully covering the counter electrode is better than the response of a junction in which the counter electrode is contacted by means of a contact pad.