Multicritical points in a model for 5f-electron systems under pressure and magnetic field.

We investigate the evolution of multicritical points under pressure and magnetic field in a model described by two 5fbands (calledαandβ) that hybridize with a single itinerant conduction band. The interaction is given by the direct Coulomb and the Hund's rule exchange terms. Three types of orderings are considered: two conventional spin density waves (SDWs) and an exotic SDW, i.e., with no magnetic moment formation. The conventional SDWs phases, are characterized bymfβ>mfαandmfα>mfβ, respectively, wheremfαandmfβare the intraband staggered magnetizations. The exotic SDW, which has time reversal symmetry, is described by a purely imaginary order parameter. This phase is related to a band mixing given by the spin-flip part of the Hund's rule exchange interaction. As result, without magnetic field, the phase diagrams of temperature (T) versus pressure (given by the variation of the bandwidth (W)) shows a sequence of phase transitions involving the three phases which gives rise to multicritical points. The presence of the magnetic field (hz) has drastic effects on part of the phase diagram and the location of the multicritical points.