A two-orbital $t$-$J$ model over the square lattice that describes low-energy electronic excitations in iron-pnictide high-$T_c$ superconductors is analyzed with Schwinger-boson-slave-fermion mean-field theory and by exact numerical diagonalization on a finite system. When interorbital hole hopping is suppressed, a quantum critical point (QCP) is identified that separates a commensurate spin-density wave (cSDW) state at strong Hund's rule coupling from a hidden half metal state at weak Hund's rule coupling. Low-energy spin waves that disperse anisotropically from cSDW momenta are predicted at the QCP. Nested Fermi surfaces similar to those observed experimentally in iron-pnictide materials are also predicted in such case.

• Received 13 July 2011

DOI:https://doi.org/10.1103/PhysRevB.84.224504