Critical fluctuations at the phase transition in benzil

Abstract

New Brillouin scattering data are presented for benzil single crystals near the phase transition at 83.5 K. These data demonstrate that for the c11 governed longitudinal acoustic (LA) mode at ∼15 GHz, critical fluctuations are quite large near the phase transition and dominate the behavior of this mode within +40 K of the transition. These observations are analyzed in terms of four contributing soft modes; an optical soft mode and two transverse acoustic (TA) soft modes at the zone center and a zone boundary M-point soft mode. It is argued that the zone boundary mode is the major contributor to the width and elastic constant anomalies of the LA mode. Calculations of these properties support these conclusions. Critical exponents are evaluated for Δc11 and ΔΓa, the critical contributions to the elastic constant and width of the c11 governed LA mode, based on the experimental data.