  Excitonic insulators are usually considered to form via the condensation of a
soft charge mode of bound electron-hole pairs. This, however, presumes that the
soft exciton is of spin-singlet character. Early theoretical considerations
have also predicted a very distinct scenario, in which the condensation of
magnetic excitons results in an antiferromagnetic excitonic insulator state.
Here we report resonant inelastic x-ray scattering (RIXS) measurements of
Sr3Ir2O7. By isolating the longitudinal component of the spectra, we identify a
magnetic mode that is well-defined at the magnetic and structural Brillouin
zone centers, but which merges with the electronic continuum in between these
high-symmetry points and which decays upon heating concurrent with a decrease
in the material's resistivity. We show that a bilayer Hubbard model, in which
electron-hole pairs are bound by exchange interactions, consistently explains
all the electronic and magnetic properties of Sr3Ir2O7 indicating that this
material is a realization of the long-predicted antiferromagnetic excitonic
insulators phase.
