Persistent currents in Tl-Ba-Ca-Cu-O superconductors

We have demonstrated the presence of circulating circumferential currents in toroids of Tl-Ba-Ca-Cu-O superconductors at 77.3 and 90.2 K. The nominal composition was Tl/sub 1.5/Ba2Ca2Cu3O/sub x/; however, both the x-ray diffraction pattern and the Meissner effect indicate the presence of more than one phase. The circulating current was found to decrease by 27% in 3.8 x 103 s. The nature of the above time dependence is not consistent with the presence of a simple ohmic resistance but rather indicates that a process such as flux creep is the cause of this decay. An estimated upper limit of the resistivity is 1.3 x 10-14 Ω cm at 77.3 K