EI state lies intermediate between band and Mott insulators, doping the EI is a sort of interpolation between doping a conventional band insulator and doping an antiferromagnetic insulator. The latter problem is of course at the crux of the physics of high-temperature superconductivity, so that perhaps the experimental and theoretical insights gained in the hexaborides will be helpful more generally. At any rate, doping the EI can be shown by very simple arguments to favor ferromagnetism in strong coupling. Essentially, the physics of this behavior is similar to the “Nagaoka effect”[@Nagaoka] in a doped antiferromagnet – ferromagnetic alignment of the excitonic order parameters allows for more coherent propagation of the doped electrons, and hence a lowering of their kinetic energy. This mechanism is actually [*stronger*]{} in the EI than in the antiferromagnet, because of the global [*coherence*]{} of the excitonic condensate, and the near degeneracy (due to approximate SU(4) symmetry) of ferromagnetic and paramagnetic states.

![Partial phase diagram of the strong-coupling model as a function of $E_{\rm\scriptscriptstyle G}$ (half the bare splitting between $a$ and $b$ orbitals), and $\mu$, the chemical potential. The shaded region is not analyzed in this paper. Thick lines indicate the boundary between the undoped region 