Two-phonon bound states

A sharp peak in the two-phonon Raman spectrum of diamond was first observed by Krishnan and has recently been shown to be at a frequency higher than twice the maximum single-phonon frequency by Solid and Ramdas. We interpret this peak as evidence for the existence of bound states of the two-phonon system. The repulsive anharmonic phonon-phonon interaction which splits the bound states off the two-phonon continuum is estimated for diamond.