Phonon spectrum in a plasma crystal

(Received 27 February 2002; published 25 June 2002)The Fourier spectra of longitudinal and transverse waves corresponding to random particle motionwere measured in a two-dimensional plasma crystal. The crystal was composed of negatively chargedmicrospheres immersed in a plasma at a low gas pressure. The phonons were found to obey a dispersionrelation that assumes a Yukawa interparticle potential. The crystal was in a nonthermal equilibrium,nevertheless phonon energies were almost equally distributed with respect to wave number over theentire first Brillouin zone.