Brandt, J.; Fröhlich, D.; Sandfort, C.; Bayer, M.; Stolz, H.; Naka, N. (2007), "Ultranarrow absorption and two-phonon excitation spectroscopy of Cu2O paraexcitons in a high magnetic field", Phys. Rev. Lett., 99: 217403, doi:10.1103/PhysRevLett.99.217403