Substituting the fitted parameters into
expression (5.2) reveals the spatial profile of the
B(r) within the sample.
Figure 5.3 displays the profile around a flux line generated by
an external field
The inset contains the internal field distribution n(B) belonging to this spatial profile and, for comparison, the real amplitude of the FFT of the recorded precession signal . The two distributions are similar; however, as alluded to in Section 5.1, the finite time span of the SR data broadens the FFT and creates rapid oscillations in it. This ringing is especially visible in the high field B tail of the FFT, despite the FFT having undergone apodisation to smooth it. Apodisation effectively convolutes the FFT with a Gaussian function, and so broadens the distribution still further. Nuclear dipolar fields and slight vortex lattice disorder also broaden this distribution. The small peak in the FFT at field arises from muons that miss the sample. The maximum peak in the FFT, and in the fitted field distribution n(B), occurs for the field B located at the midpoint between nearest neighbour vortices. The shoulder corresponding to fields B weaker than this, expected for a square vortex lattice according to the local London model, is absent here in both the fitted field distribution n(B) and the FFT of the recorded polarisation . This lack of a sizeable low field shoulder is a consequence of nonlocality. The effect of nonlocality on the field distribution n(B) appears in more detail in the next chapter. The very high fields B in the field distribution n(B) of a flux line lattice derive from the vortex core region.
A useful definition of the radius
of a vortex core is the
distance r from the core centre to the point where the supercurrent
density J(r) is greatest. Applying one of Maxwell's equations,