DETECTION OF TRAPPED FLUX QUANTA IN SUPERCONDUCTORS BY A HYDRODYNAMICALLY CONTROLLED SQUID PICK-UP COIL

The purpose of this study is to detect the flux quanta trapped in a large area superconductor by scanning a SQUID pick-up coil. For this purpose a gas floating technique has been developed. In this technique the supplied gas flow supports the coil while the coil scans the superconductor surface in liquid helium. First, the applicability of the gas floating technique is examined, and it is shown that this technique can be utilized even in liquid nitrogen or liquid helium. Second, measurements of magnetic field distribution over several superconductors in liquid helium are reported. By scanning the pick-up coil over the superconductor, changes in the magnetic field, which come from the flux trapped in the superconductor, are observed. Furthermore the background variation during sample rotation and displacement is reduced to about 2.4-PHI-0. The minimum detectable flux is shown to be less than PHI-0. These results show the possibility of our technique in the detection of trapped flux quanta in superconductors.