MEASUREMENTS AND MODEL-CALCULATIONS OF FORCES BETWEEN A MAGNET AND GRANULAR HIGH-T-C SUPERCONDUCTOR

Several central aspects of the phenomenon of magnetic levitation with high-T-c superconductors have been studied. We report high-resolution measurements of the repulsive vertical force and its associated stiffn ess as well as the horizontal stabilizing force and the stiffness gove rning lateral vibrations. The results obtained at 77 K using a granula r YBa2Cu3O7-delta sample and Nd-Fe-B magnet in a rectangular levitatio n configuration are compared with theoretical predictions. The calcula tions, which are based on the critical state model with the assumption that it applies to the grains individually, give closed-form expressi ons for all the measured quantities. We find that the present model ex plains all features of our observations in a consistent way. Using onl y three adjustable parameters a very good agreement exists also al a q uantitative level.