Direct measurement of the longitudinal exciton dispersion in h-BN by resonant inelastic x-ray scattering

We report resonant inelastic x-ray scattering (RIXS) measurements on the prototypical hexagonal boron nitride (h-BN) layered compound. The RIXS results at the B and N K edges have been combined with electron energy loss spectroscopy (EELS) experiments and ab initio calculations within the framework of the Bethe-Salpeter equation of many-body perturbation theory. By means of this tight interplay of different spectroscopies, the lowest longitudinal exciton of h-BN has been identified. Moreover, its qualitatively different dispersions along the K and the M directions of the Brillouin zone have been determined. Our study advocates soft x-ray RIXS and EELS to be a promising combination to investigate electronic excitations in materials.