A change of the Fermi surface in UGe₂across the critical pressure

Abstract

We carried out the de Haas-van Alphen (dHvA) experiment under pressure for a ferromagnet UGe2. The dHvA frequency of a main dHvA branch named α, that corresponds to a majority up-spin band 40-hole Fermi surface, decreases monotonously with increasing pressure, but in the pressure range from pc* (≃1.2 GPa) to pc (≃1.5 GPa) the dHvA signal disappears completely, where pc and pc* correspond to critical pressures for a Curie temperature TC and the second phase transition temperature T* (<TC), respectively. For p>pc we observed new dHvA branches with large cyclotron masses of 19-64m0, which correspond to main Fermi surfaces in the paramagnetic state.