Lorentz microscopy of vortex lattices (flux lines) in niobium

Abstract

Observations of the flux-line lattice in tilted superconducting specimens by standard methods of Lorentz microscopy can be understood in terms of a simple model which (i) first approximates the individual vortex line by a flux tube of negligible radius, thereby obtaining an analytical expression for the two-dimensional phase shift, and then (ii) takes into account the finite flux core structure by convoluting the former expression with the assumed model for the core. By taking the London model for the magnetic-field distribution of the core, thus having the vortex described by a single parameter, i.e., the London penetration depth, the most relevant contrast features observed in the experimental out-of-focus images of a Nb superconductor are interpreted.