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

Abstract

The dynamics of quantized vortices in superconductors are closely related to practical applications of superconductors. Using the phase information of a field-emission electron beam transmitted through these vortices, we directly observed the motion of individual vortices in Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) thin films at the depinning threshold when a driving force was applied to them by changing the magnetic field. We found that the manner of vortex 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 preferential points and to move plastically. The migration motion below 25 K is attributed to the collective pinning due to extremely tiny defects such as oxygen defects, and this collective pinning effect is thought to practically vanish above 25 K due to an increase in thermal vibration of vortices, a behavior which is peculiar to high- T c superconductors.