F. Bömmel

Multichannel DC SQUID sensor array for biomagnetic applications

A biomagnetic multichannel system for medical diagnosis of the brain and heart has been developed. 37 axial first order gradiometers (manufactured as flexible superconducting printed circuits) are arranged in a circular flat array of 19 cm in diameter. Additionally, three orthogonal magnetometers are provided. The DC SQUIDs are fabricated in all-Nb technology, ten on a chip. The sensor system is operated in a shielded room with two layers of soft magnetic material and one layer of Al. The everyday noise level is 10 fT/Hz/sup 1/2/ at frequencies above 10 Hz. Within two years of operation in a normal urban surrounding, useful clinical applications have been demonstrated (e.g., for epilepsy and heart arrhythmias). For the first time current sources of sporadic events causing epilepsy or ventricular extrasystoles have been localized from coherent recordings of complete biomagnetic field distributions with spatial resolution of millimeters and temporal resolution of 1 ms.

Boundary Element Solution of Biomagnefic Problems

A boundary element formulation for biomagnetic problems is described. The use of an analytical element integration and tuning of the boundary element method (BEM) forward solution for implementation in the inverse problem algorithm results in a far better relationship between accuracy and computational effort. The computational accuracy of the realistic heart and torso model compared with the simplified models commonly used in the literature is shown. >

Noise properties of YBaCuO step edge DC-SQUIDs with different inductance

Autonomous step edge DC-SQUIDs (superconducting quantum interference devices) with inductances between 20 pH and 140 pH have been prepared on one chip. The noise properties of these devices have been determined by operating them in the flux locked loop. The best energy sensitivity of 6*10/sup -31/ J/Hz was obtained for a 20 pH SQUID and frequencies above 100 Hz. The frequency-dependent noise for the best devices was about 2*10/sup -29/ J/Hz at 1 Hz and could be reduced by applying an AC bias.<<ETX>>

Ultra low noise all niobium DC-SQUIDs

The noise and signal properties of SQUIDs with amorphous silicon barriers and Al/sub 2/O/sub 3/ barriers are studied. The barrier material is found to be of great importance for the value of the 1/f noise component. The best results were obtained for SQUIDs with Al/sub 2/O/sub 3/ barriers and a 1/f noise level at 1 Hz of about 1*10/sup -6/ Phi / square root Hz was found. After integration of coupling coils onto the SQUIDs, a signal limitation and a dramatic increase of the noise were found. Implementation of a damping circuitry over the coupling coil results in optimized signals ( Delta V( Phi /sub 0//2) approximately=I/sub c/R) and a white noise level comparable to the white noise level without a coupling coil. The 1/f noise component for SQUIDs with a damped coupling coil is higher than for 1/f noise component of SQUIDs without a coupling coil. For SQUIDs with Al/sub 2/O/sub 3/ barriers, the 1/f noise level keeps below 3*10/sup -6/ Phi /sub 0// square root Hz at 1 Hz. For SQUIDs with an amorphous silicon barrier the 1/f noise component changes per cooling cycle in an irregular way. The stability for thermal cycling and room-temperature storage is very good for all the devices.

Transport properties of YBaCuO step edge Josephson junctions

Step edge Josephson junctions have been prepared on

Analytical calculation of the I - V characteristics of SQUIDs with parasitic elements

An approximate analytical calculation of the current - voltage characteristics of SQUIDs with parasitic capacitances and inductances is presented. The analytical treatment of the I - V curves is based on the approximation of harmonic signals. For bias currents (critical current of the SQUID) the approximated and the numerically calculated I - V curves are in very good agreement. From the analytical treatment of the characteristics we have derived analytical formulae for three voltages, where there is no flux modulation at the I - V characteristic. The comparison of numerically calculated and experimental characteristics shows that the equation of the upper point without flux modulation of the I - V curves is suitable for calculating parameters such as inductances and capacitances of SQUIDs with parasitic elements.

Preparation and characterization of planar YBa/sub 2/Cu/sub 3/O/sub x/ flux transformers

Planar magnetometer and gradiometer flux transformers, which can be used for high-temperature superconductor superconducting quantum interference devices (SQUIDs), were fabricated from epitaxial, excimer-laser-deposited YBa/sub 2/Cu/sub 3/O/sub x/ three-layer stacks. The first YBa/sub 2/Cu/sub 3/O/sub x/ layer and the SrTiO/sub 3/ insulator layer were deposited through silicon shadow masks. As the edges of these layers are very smooth, a second YBa/sub 2/Cu/sub 3/O/sub x/ film can grow epitaxially onto the device without forming grain boundaries. It was subsequently patterned by argon ion milling to magnetomer and gradiometer flux transformers with up to 12 turns and 25- mu m linewidth. The flux transformers were characterized by resistance and persistent current measurements, using a conventional SQUID magnetometer. At 77 K a persistent current of about 1 mA was measured in the flux transformer with 50- mu m linewidth.<<ETX>>

Noise in All-Niobium DC-SQUIDs

The equivalent flux noise of DC-SQUIDs fabricated with different technologies has been measured. The white noise is fitted very well with a thermal noise model. The coloured noise depends strongly on the preparation technique and the presence of a coupling coil. For Al2O3-barriers, for instance, we find an extremely low coloured noise level of typically 1×10−6 O0/(Hz)1/2 at 1 Hz for autonomous 100 pH-SQUIDs and about 3×10−6 O0/(Hz)1/2 at 1 Hz for SQUIDs with a coupling coil. For amorphous silicon barriers on the other hand these values are about one order of magnitude higher. The different behaviour of SQUIDs with amorphous silicon barriers compared to SQUIDs with Al2O3-barriers will be discussed.

A niobium planar SQUID gradiometer operating in an unshielded environment

We present an integrated thin-film gradiometric SQUID sensor in technology. The dc-SQUID converter is designed as a parallel inductance double-washer SQUID. It is inductively coupled to a planar parallel inductance gradiometer antenna with 15 mm baseline by two series input coils with eight turns each. The field gradient sensitivity is . The intrinsic field gradient resolution down to 1 Hz is , measured in a shielded environment. The sensor can be operated reliably without any magnetic shielding. Under these conditions the transfer function is decreased by about a factor of three, resulting in a field gradient resolution of in the white noise region. Coherent noise sources produced by a power line and railway were suppressed by means of electronic subtraction using a reference channel and selective filtering. The resolution of the system is demonstrated by MCG measurements without any shielding.

YBa2Cu3Ox Flip-Chip SQUID Magnetometer

YBa2Cu3Ox SQUID magnetometer and gradiometer were fabricated by coupling YBa2Cu3Ox bicrystal SQUIDs to multiturn flux transformers in flip-chip configuration. The flux transformers were made from epitaxial, excimer laser deposited YBa2Cu3Ox-SrTiO3-YBa2Cu3Ox three layer stacks, which were patterned with shadow masks as well as with ion milling. A maximum transformer gain of 60 was achieved, leading to a sensitivity of 0.8 nT/Φ0. Performing noise measurements we found, that in our best devices the field resolution is limited by the SQUID noise and not by the transformer noise; but we als prepared flux transformers, which made the noise of the magnetometer worse by one order of magnitude. We achieved a magnetic field resolution of 50 fT/√Hz in the white frequency region and about 150 fT/√Hz at 10 Hz.