The dispersion of the A-valence-band in GaN has been deduced from the observation of high-index magnetoexcitonic states in polarized interband magnetoreflectivity and is found to be strongly nonparabolic with a mass in the range 1.2-1.8me. It matches the theory of Kim et al. [Phys. Rev. B 56, 7363 (1997)] extremely well, which also gives a strong k-dependent A-valence-band mass. A strong phonon coupling leads to quenching of the observed transitions at about an LO-phonon energy above the band gap and a strong nonparabolicity. The valence band was deduced from subtracting from the reduced dispersion the electron contribution with a model that includes a full treatment of the electron-phonon interaction.
Shields, P. A., Nicholas, R. J., Peeters, F. M., Beaumont, B., & Gibart, P. (2001). Free-carrier effects in gallium nitride epilayers: Valence-band dispersion. Physical Review B, 64(8), 812031-812034. . https://doi.org/10.1103/PhysRevB.64.081203