As well as investigations of the atomic-level magnetism which may be found in many superconducting materials, muons can be used to explore the flux-line lattice generated when a field is applied to a type II material. The implanted muons are distributed randomly with respect to the flux lattice, with each muon experiencing a slightly different local field and so precessing at its own frequency. This results in a relaxation of the muon polarisation, which can be measured and used to determine fundamental superconducting parameters such as the penetration depth, coherence length, superconducting carrier density and effective mass. Temperature and field dependencies can yield information on the nature of the pairing mechanism. The field distribution inside the material can also be extracted from the muon response and used to follow the behaviour of the flux line lattice as it changes with temperature or applied field, or moves under the influence of an applied current. SANS studies of the flux line lattice benefit from being combined with muon investigations, particularly in superconductors with long penetration depths.

Picture: Molecule of an organic superconductor superimposed on the magnetic field distribution in the flux line lattice