A persistent-current experiment maintained for 12 days with a sample of ${\mathrm{Ba}}_{0.4}{\mathrm{Y}}_{0.6}\mathrm{Cu}{\mathrm{O}}_{3-y}$ is reported. The estimated upper limit of resistivity is 2×${10}^{-18\mathrm{}}$ Ω cm. A logarithmic decay at the beginning is detected, which is in good agreement with the flux-creep theory. Variations in decay rate due to changes of trapped field and temperature are measured.

• Received 11 May 1987

DOI:https://doi.org/10.1103/PhysRevB.36.2414