Direct observation of the interaction between a vortex lattice and dislocations in a superconducting Nb crystal

Using a cryo-Lorentz electron microscope, we have succeeded in observing di rectly the interaction between a vortex lattice and dislocations in a Nb fi lm, to which magnetic fields were externally applied up to 200 G at 5 K. At 100 G, vortices were clearly pinned by dislocations and a vortex lattice w as locally formed around the pinned sites. Its extension and the density of the vortices increase with the increase of magnetic-held strength to form "the Abrikosov lattice." At 200 G, it is not perfect yet but consists of "d omains" with disordered areas at their boundaries. When the specimen temper ature was raised above the critical temperature (9.2 K), with keeping the m agnetic field at 200 G, the vortex lattice disappeared. When the temperatur e was successively decreased back to 5 K, a similar vortex lattice appeared again but with a slightly changed orientation. The reorientation suggests that the interaction between vortices and dislocations pinning them is rath er weak. (C) 1998 American Institute of Physics. [S0003-6951(98)00535-X].