It is often convenient to fit the measured asymmetry spectrum in a ``rotating reference frame'' (RRF). To do this, one multiplies the complex muon polarization by a function . The RRF frequency is chosen to be slightly lower than the average Larmor-precession frequency of the muon in the sample. There are two important benefits from this procedure. The first is that the quality of the fit can be visually inspected. The precession signal viewed in this rotating reference frame has only low frequency components on the order of , where is the average precession frequency in the lab frame. Second and most important, it allows the data to be packed into much fewer bins, greatly enhancing the speed of fitting.
Further details of the SR technique may be found elsewhere (e.g. see Refs. [47,46,49,50]). The essential point is that the muon accurately probes the local distribution of magnetic fields in the bulk of the superconductor. The resulting SR line shape contains considerable information. Of particular interest, are the magnetic penetration depth , the coherence length and the vortex-lattice structure. Unfortunately, the SR line shape also contains information not generally wanted--such as the effects of flux lattice disorder and additional fields such as those due to nuclear dipolar moments. Furthermore, extracting quantities such as and from the data requires some modelling of the internal field distribution. This is the major difficulty in employing the SR technique.