Irreversibility line and flux flow noise in superconducting Nb-Ta

It has been found that the critical current density, J/sub c/, in high-T/sub c/ oxide superconductors is reduced to zero at high temperatures and/or high magnetic fields even in a superconducting region. The irreversibility line was investigated in weakly pinned superconducting Nb-Ta with low T/sub c/ and was compared with that in oxide superconductors. Large flux flow noise with a peak around 0.3 Hz was observed in Nb-Ta specimens. The noise power integrated in the frequency range of 0.1-1.0 Hz had a sharp peak, when it was plotted against the magnetic field. The mechanism of such a characteristic noise is examined.