  The dispersion of the A-valence-band in GaN has been deduced from the
observation of high-index magneto-excitonic states in polarised interband
magneto-reflectivity and is found to be strongly non-parabolic with a mass in
the range 1.2-1.8 m_{e}. 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 an LO-phonon energy above the band gap and a strong
non-parabolicity. 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.

Materials Science