Electronic and magnetic transitions in a multiband model for ${\mathrm{La}}_2$${\mathrm{NiO}}_4$

The transition-metal oxide ${\mathrm{La}}_2$${\mathrm{NiO}}_4$ is studied using a multiband Hubbard model with a Hund coupling J. We obtain its mean-field phase diagram in the U- (correlation strength on Ni sites) Δ (charge-transfer energy) plane. For J=0 we have a continuous transition from a paramagnetic metal (PMM) to a paramagnetic insulator (PMI). On increasing J, the magnetic moment M and the band gap ${\mathrm{E}}_{\mathrm{G}}$ change discontinuously at J=${\mathrm{J}}_{\mathrm{c}}$(U,Δ). For J=1, the insulating phase (AFI) is antiferromagnetic, but we have, in addition, both paramagnetic (PMM) and antiferromagnetic (AFM) metallic phases. The insulator-metal (AFI-AFM and AFI-PMM) phase boundaries are first order, but the metal-metal (AFM-PMM) boundary is second order; and it seems to meet the first-order boundary at a critical end point. A reentrant PMM-AFM-PMM sequence is also seen.