Small angle neutron scattering studies of the vortex lattice in the UPt₃ mixed state: Direct structural evidence for the B->C transition

Small angle neutron scattering studies of the flux line lattice (FLL) in UPt3 for fields H perpendicular to c provide direct microscopic evidence for the 5 kOe B --> C transition. We find a pronounced maximum in the longitudinal correlation length of the FLL at the transition and an abrupt change in the field dependence of the scattered intensity which can be interpreted as a 15% decrease in the coherence length and a 9% increase in the penetration depth, consistent with discontinuities in the critical fields. Finally, in the low field phase, the FLL distortion evolves roughly linearly with field, while in the high field phase it appears to be less field dependent.