Hypersound observations of glass fluctuations

Correlations of fourth order in polarization fluctuations, the quantity whose ordering is frustrated, are observed in a structural glass via strictive coupling to sound. The results were obtained on a Rb1-x(NH4)xH2PO4 crystal with Brillouin scattering. The existence of an Edwards-Anderson order parameter much above Vogel-Fulcher freezing is revealed. This suggests that the observed susceptibility slowing-down might, in many glasses, be controlled by a much slower hidden competing network. © 1985 The American Physical Society.