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

摘要

We investigate the occurrence of multicritical points under pressure and magnetic field in a model that describes two 5f bands (of either alpha or beta characters), which hybridize with a single itinerant conduction band. The 5f electrons interact through Coulomb and exchange terms. The antiferromagnetic (AF) order parameter is a Neel 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 alpha and beta given by Delta(alpha) and Delta(beta), respectively. The AF(1) phase occurs when Delta(beta) > Delta(alpha) > 0, while in the AF(2) phase the gaps satisfy Delta(alpha) > Delta(beta) > 0. The application of a magnetic field produces a drastic change in the phase diagram. The AF(1) and AF(2) phases separate, with the latter acquiring a dome shape that 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 magnetic field is also drastic, with the suppression of some multicritical points and the emergence of others. We believe that these results may have relevance for the growing field of multicritical points (classical and quantum) in the physics of uranium compounds.