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In this study, we explore the effect of a single flat band in the electronic properties of a ferromagnetic two-dimensional Lieb lattice using the multiband Hubbard model with polarized carriers, spin-up and spin-down. We employ the self-consistent dynamical mean field theory and a Green functions cumulant expansion around the atomic limit to obtain the correlated densities of states while varying the intra- and interband interactions. Our findings demonstrate a renormalization of the correlated density of states in both the spin-up and spin-down carriers as we varied the intra- and interband interactions. We conclude that the presence of a flat band enables the system to maintain a metal state with itinerant ferromagnetism in the spin-up carrier., (© 2024 IOP Publishing Ltd.)

We investigate the evolution of multicritical points under pressure and magnetic field in a model described by two 5 f bands (called α and β ) 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 ( h z ) has drastic effects on part of the phase diagram and the location of the multicritical points., (© 2021 IOP Publishing Ltd.)