Thin-film dc SQUID gradiometer using a single YBa2Cu3O7−x layer

Abstract

A dc SQUID gradiometer with a large effective pickup area using only a single YBa2Cu3O7−x layer and step-edge Josephson junctions on SrTiO3 substrate has been prepared. The planar gradiometer consists of two loops coupled directly to a small area SQUID in the center of the structure. The main advantage in comparison to magnetometers is the possibility of direct measurements in a magnetically unshielded environment. At 77 K without bias reversal technique we obtain a white noise level of about 4.5×10−5 Φ0 Hz−1/2 or 8×10−13 T cm−1 Hz−1/2 down to 10 Hz. At 1 Hz noise increases up to 6×10−12 T cm−1 Hz−1/2.

High-transition-temperature superconducting quantum interference devices

“…This configuration has the disadvantage that large supercurrents are induced around the perimeter when the device is rotated in an ambient field. Knappe et al (1992), Zakosarenko et al (1994), Daalmans et al (1995), Schultze, Stolz et al (1997), have all made single-layer, first-derivative gradiometers of this kind, using dc SQUIDs with either step-edge or bicrystal junctions. The baselines are limited by the size of the substrate to about 5 mm, and the best gradient sensitivities are about 50 pT m Ϫ1 Hz Ϫ1/2 .…”

Spatial frequency response of conventional and non-conventional SQUID gradiometers

“…With the development of alternative coupling schemes [4], [5], the SQUID, the input coil and additional elements can also play an active role in the spatial discrimination, in view of the lack of shielding. Consequently, an enhancement of the spatial filtering model is required to incorporate the extra factors that affect the output signal.…”

“…Considering that the origin of the gradiometer coordinate system is placed at the center of the pick-up coil, we have that the flux through the SQUID at a generic point is given by (4) where is the fraction of the SQUID inductance through which the external flux is coupled to the device.…”

“…For single-turn rectangular coils with sides of length and , the Fourier transform of the stepwise function is given by (8) Expressing (4) in the spatial frequency domain and substituting (7) and (8), we get (9) where the gradiometer spatial frequency response is given by (10) For the gradiometer configuration described in [5], we have: mm (an estimation allowing for flux focusing effect), mm, mm, mm, and mm; nH, nH, nH and nH. Fig.…”

Long baseline hardware gradiometer based on HTS rf-SQUIDs with substrate resonators

“…However, a hardware gradiometer has the advantage that it can be operated in presence of external noise with an amplitude exceeding the dynamic range of the SQUID magnetometer. Many attempts have been made to develop HTS thin film planar gradiometers based on dc-and rf-SQUIDs [3]- [6]. The main problem of dc-SQUID based gradiometers is that the dc-SQUID itself has a nonzero response to magnetic field which reduces the gradiometer balance.…”