FERMI-SURFACE OF UPT3 FROM 3-T TO 30-T - FIELD-INDUCED QUASI-PARTICLE BAND POLARIZATION AND THE METAMAGNETIC TRANSITION

The magnetic-field dependence of the Fermi surface Of UPt3 has been studied up to 30 T from 17 to 150 mK using the quantum oscillatory magnetoresistance. Near the 20-T metamagnetic transition, rapid changes in the frequency spectrum indicate strong nonlinear magnetic splitting of the quasiparticle bands; weaker nonlinear splitting is seen at all fields. Above the transition the mass enhancement remains very large, and masses exceeding 100m(e) have been observed. Below the transition a surface with mass 170m(e) has been found.