Investigation of flux line lattices by SANS with unpolarized and polarized neutrons

We have investigated flux lines in low- and high-TcTc superconductors using unpolarized and polarized neutron techniques. In high-TcTc materials, we observe transitions from triangular to square flux line lattices (FLLs) which probably represent the effects of d-wave pairing. In niobium, a supposedly ‘conventional’ superconductor, even more FLL phase transitions can be observed, into structures which spontaneously break the crystal symmetry. Observations of polarized neutron scattering intensities from niobium with a high density of pinning centers show the effects of pinning on the spin-dependence of the signal. Other possible applications of polarized neutrons scattering from the FLL are mentioned.