Ferromagnetism in doped excitonic insulators

The theory of doped excitonic insulators is reinvestigated in light of rece nt experiments on hexaborides. For the appropriate valley-degenerate X-3,X- 3' band structure, "intravalley" condensation is energetically favored. Fer romagnetism occurs upon doping due to the quenching of kinetic energy at th e otherwise direct first-order excitonic insulator-metal transition. The ph ase diagram includes states of spatially inhomogeneous density and magnetiz ation at low temperatures.