As well as the Josephson Junction shift register fabricated out of high temperature superconducting materials (CI No 1,638), Conductus Inc, Sunnyvale, California has developed an instrument that detects extremely weak magnetic fields in partnership with the Lawrence Berkeley Laboratory and the University of California at Berkeley. They describe it as the most complicated device yet to be made out of the high-temperature superconducting compounds. The magnetometer is sufficiently sensitive to have practical applications in laboratory instruments, geophysical surveying, and non-destructive testing and is seen as demonstrating conclusively that high-temperature superconductors can be used in electronic circuits. The magnetometer involves two components, one is a superconducting quantum interference device or Squid, the other a superconducting flux transformer chip that collects magnetic signals over a comparatively large area and concentrates them in a multiturn coil that is pressed tightly to the Squid. The Squid is a thin-film loop of Yttrium Barium Copper Oxide interrupted by two weak link Josephson Junctions, and in the presence of a current, the Squid produces a voltage signal in response to a tiny magnetic field. The Squid was developed and fabricated at Conductus, using a technique that produces grain-boundary Josephson Junctions in the superconducting films deposited on precisely controlled substrates, and the technique is applicable to more complex circuits. The flux transformers involve an insulating layer of Strontium Titanate sandwiched between two layers of the Yttrium Barium Copper Oxide ceramic. All three layers are laser-deposited on a heated chip so that they grow epitaxially – with aligned crystal structures. The devices operate at minus 190oC, well above the boiling point of liquid Nitrogen, so cooling is cheap.
