A $U\left(1\right)$ slave-spin representation is introduced for multiorbital Hubbard models. As with the $Z_2$ form of de’Medici et al. [Phys. Rev. B 72, 205124 (2005)], this approach represents a physical electron operator as the product of a slave spin and an auxiliary fermion operator. For nondegenerate multiorbital models, our $U\left(1\right)$ approach is advantageous in that it captures the noninteracting limit at the mean-field level. For systems with either a single orbital or degenerate multiple orbitals, the $U\left(1\right)$ and $Z_2$ slave-spin approaches yield the same results in the slave-spin-condensed phase. In general, the $U\left(1\right)$ slave-spin approach contains a $U\left(1\right)$ gauge redundancy, and properly describes a Mott insulating phase. We apply the $U\left(1\right)$ slave-spin approach to study the metal-to-insulator transition in a five-orbital model for parent iron pnictides. We demonstrate a Mott transition as a function of the interactions in this model. The nature of the Mott insulating state is influenced by the interplay between the Hund's rule coupling and crystal-field splittings. In the metallic phase, when the Hund's rule coupling is beyond a threshold, there is a crossover from a weakly correlated metal to a strongly correlated one, through which the quasiparticle spectral weight rapidly drops. The existence of such a strongly correlated metallic phase supports the incipient Mott picture of the parent iron pnictides. In the parameter regime for this phase and in the vicinity of the Mott transition, we find that an orbital selective Mott state has nearly as competitive a ground-state energy.

• Received 25 April 2012

DOI:https://doi.org/10.1103/PhysRevB.86.085104