Unusually large shielding fields in single-grain YBa₂Cu ₃O_7-δ

Magnetic field trapping and shielding by several unirradiated single-grain YBa₂Cu₃O_7-δ disks have been investigated between 4.2 and 71 K up to 20 T. Flux avalanches due to thermal instabilities limited the trapping field to ∼5 T below 45 K and often caused microcracks in the disks. However, a shielding field greater than 19 T was detected at 4.2 K in the same sample assembly even after it was damaged by flux avalanches. This violates the expected symmetry between shielding and trapping fields. The shielding field of ≳19 T exceeds the total quench field of 10 T predicted by the Swartz-Bean model. The observations suggest that thermomechanical properties play an important role in field shielding and trapping and mechanical stress may be effectively used to enhance the field trapped by disks of high temperature superconductors.