Dick Veldhuis

Superconducting quantum interference device based on MgB2 nanobridges

The superconductor MgB2, with a transition temperature of 39 K, has significant potential for future electronics. An essential step is the achievement of Josephson circuits, of which the superconducting quantum interference device ~SQUID! is the most important. Here, we report Josephson quantum interference in superconducting MgB2 thin films. Modulation voltages of up to 30 mV are observed in an all-MgB2 SQUID, based on focused-ion-beam patterned nanobridges. These bridges, with a length scale ,100 nm, have outstanding critical current densities of 73106 A/cm2 at 4.2 K.

Preparation and properties of all high T/sub c/ SNS-type edge DC SQUIDs

High-T/sub c/ SNS-type Josephson junctions and DC SQUIDs were successfully fabricated using hetero-epitaxially grown multilayers of YBa/sub 2/Cu/sub 3/O/sub x/ and PrBa/sub 2/Cu/sub 3/O. These layers are c-axis oriented, and hence edges of the multilayers give rise to a current flow in the ab-plane between the electrodes of a Josephson junction. The necessary structuring was done by Ar ion beam etching. The individual junctions exhibit a supercurrent up to 80 K. The I/sub c/R/sub n/ product of these junctions usually has a lower limit of 8 mV at 4.2 K. Voltage modulation of the first DC SQUIDs can be observed up to 66 K. The voltage modulation for various bias currents investigated at 4.2 K noise measurements were performed. Details on the fabrication and measurements are presented.

SQUID magnetometer operating at 37K based on nanobridges in epitaxial MgB2 thin films

Superconducting quantum interference devices SQUIDs and magnetometers are fabricated from nanoconstrictions in epitaxial MgB2 films. The nanobridges are contained within single-crystalline grains, resulting in clean transport, a large critical current density of 5107 A/cm2 at 4.2 K, and stable SQUID voltage modulation up to 38.8 K. The magnetometer is realized with an inductively coupled pickup loop, giving rise to a field sensitivity of 1 pT Hz?1/2 down to 1 Hz. The device properties are governed by the two-band superconducting nature of MgB2, posing, however, no problems to a successful development of boride magnetic field sensing devices. The MgB2 zero-temperature London penetration depth is measured to be 62 nm, close to theoretical predictions.

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.

The road to magnesium diboride thin films, Josephson junctions and SQUIDs

The remarkably high critical temperature at which magnesium diboride (MgB2) undergoes transition to the superconducting state, Tc approx 40 K, has aroused great interest and has encouraged many groups to explore the properties and application potential of this novel superconductor. For many electronic applications and further basic studies, the availability of superconducting thin films is of great importance. Several groups have succeeded in fabricating superconducting MgB2 films. An overview of the deposition techniques for MgB2 thin film growth will be given, with a special focus on the in situ two-step process.

Multichannel heart scanner based on high-T/sub c/ SQUIDs

A 7-channel magnetometer for magnetocardiography based on high-T/sub c/ SQUIDs has been realized. This magnetometer is used for test experiments in the development of a multichannel high-T/sub c/ SQUID based heart-scanner for clinical applications. The intrinsic noise level of the channels in the 7-channel system is typically 120 fT//spl radic/(Hz) down to 1 Hz. Magnetocardiograms were recorded inside a magnetically shielded room. Introductory experiments were performed on the suppression of noise by combining magnetometers to form planar gradiometers. The noise suppression that can be established appeared to be limited by the imbalance of the gradiometric configuration, which is roughly 2%. This relatively poor balance of the system is caused by inaccuracies in the transfer functions of the individual SQUID magnetometers, and by deviations from the planar geometry.

YBa/sub 2/Cu/sub 3/O/sub 7/ nano-bridge junctions and dc SQUIDs made by focused ion beam milling

YBa/sub 2/Cu/sub 3/O/sub 7/ nano-bridges with widths ranging from 200 to 800 nm were made using Focused Ion Beam milling. The I-V characteristics of the narrowest nano-bridges show, under microwave irradiation, pronounced Shapiro steps up to the transition temperature. An inductively shunted single layer SQUID, using these nano-bridges, has been realised by a combination of Focused Ion Beam milling and selective epitaxial growth. Flux to voltage modulation up to 82 K was observed, with a maximum modulation depth of 3.7 /spl mu/V at 78 K.<<ETX>>

Characterization of different types of Nb-AlO/sub x/ based Josephson tunnel junctions

Three types of Josephson tunnel junctions, standard Nb/Al,AlO/sub x//Nb, symmetric Nb/Al,AlO/sub x//Al/Nb, and Nb/Al,AlO/sub x//AlO/sub x//Nb containing a double-oxide layer were investigated by means of temperature-dependent I-V measurements, conductance-voltage measurements, noise analysis, and Auger electron spectroscopy scanning across the edge of a sputtered crater profile. In standard junctions, frequently small leakage currents have been observed as well as resistance fluctuations, leading to telegraph noise. Both effects can be related to the direct contact between the AlO/sub x/ and the Nb counter electrode. Leakage currents larger than 0.01% of the theoretical maximum critical current have not been observed in any of the symmetric junctions. The sub-gap current of these junctions is dominated by single- and two-particle tunneling. The SNAP process that was used to define the junction areas affects the tunnelling mechanisms below the sum-gap voltage, probably by the introduction of barrier inhomogeneities at the edges of the junctions. The AlO/sub x/ barrier in symmetric and asymmetric junctions cannot completely be represented by a trapezoidal barrier shape. The metal-insulator interface between Al and AlO/sub x/ in both junction types is probably not very sharp, which might be due to oxygen diffusion. The metal-insulator interface between AlO/sub x/ and Nb in standard junctions can be represented by a step-wise increase of the potential barrier, indicating that this interface is very distinct. The AlO/sub x/ barrier in double-oxide layer junctions is not homogeneous and probably contains low barrier channels.

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.

Direct Measurement of Inter-Filament Resistance in Superconducting Multifilamentary NbTi and

A quantitative knowledge of inter-filament transverse resistance will allow us to describe current redistribution processes inside strands more accurately. This is particularly important for the analysis of the influence of strain and crack distribution patterns in Nb3Sn filaments on the shape of the voltage-current curves. Several indirect methods are commonly used to assess inter-filament resistance. Here we use a direct method to measure transverse inter-filament resistance and filament-to-matrix contact resistance. Two four-probe voltage-current methods are developed for measurements below 10 K at various background magnetic fields. In addition to FEM (Finite Element Method) simulation, also a new 3D strand model is developed to simulate the current- and voltage distributions. The experimental methods, first results as well as the simulations using the FEM method and new 3D strand model are described.