NONACTIVATED MAGNETIC-RELAXATION IN A HIGH-TC SUPERCONDUCTOR

IN the copper oxide superconductors the induced magnetic moments due to circulating supercurrents decay much more quickly than they do in most conventional superconductors1. This inability to sustain strong persistent currents is an important limitation to technological applications of these materials. The quasi-logarithmic relaxation has been generally ascribed to thermally activated movements of magnetic flux lines2. Here we report the results of relaxation measurements made at temperatures as low as 0.1 K in the critical state; we find that the decay process does not come to a halt as the temperature tends to zero. This means that a non-activated relaxation process dominates at low temperatures. Possible explanations include the tunnelling of flux lines through pinning barriers3, or the existence of an inhomogeneous distribution of critical current density in the sample4. ö © 1990 Nature Publishing Group.