  Muon spin rotation (muSR) has emerged as the leading experimental probe of
the effective size of magnetic vortices in type-II superconductors. muSR data
on several different classes of type-II superconductors shows that the inner
structure of a vortex can depend quite strongly on temperature and the strength
of the external magnetic field. In this paper it is shown that these behaviours
are related to the quasiparticle excitation spectrum both inside and outside of
the vortex cores. Here we establish that the vortex-core size determined by
muSR is particularly sensitive to the nature of the superconducting energy
gap(s) and the symmetry of the Fermi surface. A survey of results for different
superconductors arrives at the conclusion that the large vortex-core size
observed by muSR in YBa2Cu3O6.95 is due to CuO chain superconductivity.
