Neutron Scattering from Vortex Lattices in Superconductors

Abstract

When a magnetic field is applied to a superconductor the magnetic field can penetrate as an array of flux lines. These lines are often ordered to form a lattice, the orientation and geometry of which reflects the anisotropy of the underlying interactions between the lines. Neutron scattering gives an image of the mean magnetic field contrast due to this lattice in the bulk of a crystal, while the weak nature of the scattering means that the charge-neutral neutron does not disturb the system being measured. At the simplest level, small-angle neutron scattering (SANS) provides a direct determination of the geometry and orientation of the flux-line lattice (FLL), which can in some special circumstances provide a sensitive probe of the underlying symmetry of the superconductivity itself. The scattered intensities give further quantitative information about the spatial form of the magnetic field distribution in the sample, usually expressed as the field profile normalised per vortex line. Even with only the most limited data, the field dependence of the intensities of the lowest order Bragg reflections is sufficient to estimate the two fundamental length scales that characterise a superconductor, namely the penetration length (λ) and coherence length (ξ). From the intensities of higher order reflections, the field profile at smaller length scales, and ultimately the vortex cores, can be probed.