Real-time observation of individual vortices in high-T-c superconductors by Lorentz microscopy

The dynamics of quantized vortices in superconductors are closely related t o practical applications of superconductors, Using the phase information of a field-emission electron beam transmitted through these vortices, we dire ctly observed the motion of individual vortices in Bi2Sr2CaCu2O8+delta (Bi- 2212) thin films at the depinning threshold when a driving force was applie d to them by changing the magnetic field. We found that the manner of vorte x motion greatly depended on the sample temperature and the magnetic field. For example, at temperatures below 25 K, all the vortices migrated slowly and smoothly, while above 25 K, vortices began to be pinned at some prefere ntial points and to move plastically. The migration motion below 25 K is at tributed to the collective pinning due to extremely tiny defects such as ox ygen defects, and this collective pinning effect is thought to practically vanish above 25 K due to an increase in thermal vibration of vortices, a be havior which is peculiar to high-T-c superconductors. (C) 2000 Elsevier Sci ence B.V. All rights reserved.