HTS SQUIDs and GRADIOMETERS: CURRENT STATUS and PERSPECTIVES


Colin Pegrum, Alex Eulenburg, Ally Millar, Ed Romans, and Chris Carr

Department of Physics and Applied Physics
University of Strathclyde
Glasgow G4 0NG, UK


The first HTS SQUIDs were made well over ten years ago, and in that time
substantial improvements in their design and fabrication have contributed to
matching improvements in performance. As a result, the best HTS SQUIDs made
currently have an intrinsic flux sensitivity at 77K which begins to come close
to that of some conventional niobium SQUIDs at 4.2K. This is good news for
SQUID applications, but real applications need more than just a SQUID.  
Gradiometric field-sensing coils are required, either first order or preferably
second-order. Often these can only be made in just one HTS layer and they must
be coupled effectively to the SQUID. A high degree of balance is essential and
in addition the system usually must tolerate movement in the Earth's field.

In this presentation we will first outline some possible approaches to these
design issues. We will then describe our recent work on the fabrication and
evaluation of large-area YBCO first-order gradiometers on bicrystal substrates,
which use a novel two-SQUID arrangement to attain a balance of $5 \times
10^{-5}$. They have a field gradient sensitivity of
50\,fT\,cm$^{-1}$Hz$^{-1/2}$ at 1 kHz, the best reported worldwide to date, and
we show that this is adequate for recording the adult magnetocardiogram.  We
will also describe our alternative type of gradiometer, which uses step-edge
junctions and a novel type of gradiometric SQUID. We will conclude by looking
at techniques for making high-performance second-order gradiometers, and the
applications that will then be possible.