Real-time Observation of Vortices in Superconductors by Lorentz Microscopy

The dynamics of individual quantized vortices in superconducting thin films became observable using coherent Lorentz microscopy with our field-emission transmission electron microscopes (1). The observation principle is based on the Aharonov-Bohm effect (2) Since a phase shift of 2$\pi $ is produced between two electron beams enclosing a magnetic flux of $h$/$e$, a vortex having magnetic flux of $h$/(2$e)$ is a phase object of $\pi $ for an illuminating electron beam, which cannot be observed by in--focus electron microscopy. However, the vortices are observable by holographic interference microscopy (3) and defocused Lorentz microscopy$^{ }$(4). Using Lorenz microscopy, various kinds of vortex motions in superconductors with pinning centers were observed. The vortex motions in niobium thin films were elastic, plastic and even rectified (5) depending on the sample temperature, and also on the distributions and strengths of the pinning centers. In high-$T_{c}$ superconductors, when the sample temperature decreased. The vortex motion changed from hopping to slow migration due to increasing pinning effect of atomic-size defects,which was so strong that even the pinning effect of columnar defects was hidden behind it. (1) A. Tonomura: ?Electron Holography? 2$^{nd}$ Edition, Springer, Heidelberg (1999) (2) M. Peshkin and A. Tonomura : ? The Aharonov-Bohm Effect? Lecture Notes in Physics,\textbf{ 340} (Springer-Verlag, Heidelberg, 1989). (3) J. E. Bonevich \textit{et al}. ? Electron holography observation of vortex lattices in a superconductor? Phys. Rev. Lett. \textbf{70} No. 19 (1993) p. 2952-2955. (4) K. Harada \textit{et al}. ? Real-time observation of vortex lattices in a superconductor by electron microscopy? Nature \textbf{360} ( 5 November 1992) p. 51-53. (5) Y. Togawa \textit{et al. } to be submitted to Phys. Rev. Lett.