LORENTZ MICROSCOPY OF VORTEX LATTICES (FLUX LINES) IN NIOBIUM

Observations of the flux-line lattice in tilted superconducting specim ens by standard methods of Lorentz microscopy can be understood in ter ms of a simple model which (i) first approximates the individual vorte x line by a flux tube of negligible radius, thereby obtaining an analy tical expression for the two-dimensional phase shift, and then (ii) ta kes into account the finite flux core structure by convoluting the for mer expression with the assumed model for the core. By taking the Lond on model for the magnetic-field distribution of the core, thus having the vortex described by a single parameter, i.e., the London penetrati on depth, the most relevant contrast features observed in the experime ntal out-of-focus images of a Nb superconductor are interpreted.