Magnetic-field variance in layered superconductors

In highly anisotropic or layered high-${\mathit{T}}_{\mathit{c}}$ superconductors with magnetic field along the c axis, both pinning-caused and thermal fluctuations of the highly flexible vortex cores, or of chains of vortex dots, threading the cuperconducting layers may drastically reduce the variance σ of the magnetic-field fluctuation. In contrast, the fluctuations of the averaged flux lines (smoothed over the in-plane penetration depth λ) increase σ. As a consequence, muon-spin-rotation lines may be sharpened not only the thermal motional narrowing, but also by pinning-caused static disorder.