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 evi dence for the 5 kOe B --> C transition. We find a pronounced maximum i n the longitudinal correlation length of the FLL at the transition and an abrupt change in the field dependence of the scattered intensity w hich can be interpreted as a 15% decrease in the coherence length and a 9% increase in the penetration depth, consistent with discontinuitie s in the critical fields. Finally, in the low field phase, the FLL dis tortion evolves roughly linearly with field, while in the high field p hase it appears to be less field dependent.