Unfolding of antiferromagnetic phases and multicritical points in a two-orbital model for Uranium compounds under pressure and magnetic field

We investigate the occurrence of multicritical points under pressure and magnetic field in a model that describes two 5f bands (of either $α$ or $β$ characters) which hybridize with a single itinerant conduction band. The 5f-electrons interact through Coulomb and exchange terms. The AF order parameter is a Néel vector, which is assumed to be fixed by an Ising anisotropy. The applied magnetic field is transverse to the anisotropy axis. Without field, our results for the temperature - pressure phase diagram show that, at low temperatures, a first-order phase transition occurs between two distinct antiferromagnetic phases, AF$_1$ and AF$_2$, as the pressure is increased. The two phases are characterized by the gaps of bands $α$ and $β$ given by $Δ_α$ and $Δ_β$, respectively. The AF$_1$ phase occurs when $Δ_β>Δ_α>0$, while in the AF$_2$ phase, the gaps satisfy $Δ_α>Δ_β>0$. The application of a magnetic field produces a drastic change in the phase diagram. The AF1 and AF2 phases separate with the latter acquiring a dome shape which is eventually suppressed for large values of the applied field. The evolution of the phase diagram under pressure, without and with magnetic field, shows the presence of multicritical points. Our results show that the evolution of these multicritical points by the simultaneous application of pressure and field is also drastic with the suppression of some multicritical points and the emergence of others ones. We believe that these results may have relevance for the growing field of multicritical points (classical and quantum) in the physics of Uranium compounds.

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