MAGNETIC CONTROL OF CRITICAL CURRENTS IN SUPERCONDUCTING QUANTUM INTERFERENCE DEVICES OPERATING AT 77 K

THE SQUID (SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE) MAKES POSSIBLE MAGNETIC MEASUREMENTS OF UNEQUALLED SENSITIVITY. HERETOFORE, PRACTICAL SQUID SYSTEMS HAVE BEEN COOLED BY LIQUID HELIUM. THE PROBLEM OF RELIABLY FABRICATING PRACTICAL, LOW NOISE SQUIDS THAT OPERATE IN LIQUID NITROGEN REMAINS UNSOLVED. THE EXTREMELY SHORT COHERENCE LENGTH IN THE HIGH CRITICAL TEMPERATURE SUPERCONDUCTORS (ABOUT 20 ANGSTROM) MAKES FABRICATION OF WEAK LINKS FROM MULTI-LAYER THIN FILMS UNRELIABLE. THIN FILM SQUIDS MADE THUS FAR OPERATE NOISILY AT 77 K. SQUIDS MADE BY FORMING CRACK JUNCTIONS IN BULK RINGS OF SUPERCONDUCTING MATERIAL OPERATE WITH LOW NOISE BUT ARE TOO FRAGILE FOR PRACTICAL APPLICATION. THIS PROJECT INVOLVES A NOVEL APPROACH TO THE MANUFACTURE OF SQUIDS THAT CAN OPERATERELIABLY AND WITH LOW NOISE IN LIQUID NITROGEN. WEAK LINKS IN THE SINGLE LAYERS OR BULK FORMS OF THE OXIDE SUPERCONDUCTOR ARE MADE IN TWO STEPS. FIRST, A PHYSICAL CONSTRICTION IS MADE TO ATTAIN A CRITICAL CURRENT OF A FEW MILLIAMPERES. NEXT, THE CRITICAL CURRENT IS REDUCED TO THE TARGET VALUE OF A FEW MICROAMPERES MAGNETICALLY, USING THE PROPERTIES OF OXIDE SUPERCONDUCTOR GRAIN BOUNDARIES. PHASE I OF THIS PROJECT WILL DEMONSTRATE THE FEASIBILITY OF MAGNECICALLY CONTROLLING THE CRITICAL CURRENT IN A WEAK LINKIN BULK OXIDE. THE DEMONSTRATION WILL CLEAR ONE OF THE MAJOR OBSTACLES ON THE PATH TO PRODUCTION OF PRACTICAL SQUIDSYSTEMS OPERATING IN LIQUID NITROGEN.