Critical behavior in annealed and unannealed crystals of benzil

摘要

Simultaneous Brillouinhyphen;correlation light scattering results obtained under an external applied stress pulse are presented for annealed and unannealed (as grown) crystals of benzil. Critical fluctuations are characterized for the LAahyphen;axis mode governed by the elastic constantc11and are shown to contribute to this elastic anomaly in the high temperature phase. Both a central peak intensity anomaly and a critical relaxation time anomaly are characterized for unannealed crystals. These effects disappear in wellhyphen;annealed crystals and without the external stress pulse even in as grown crystals. The critical relaxation behavior in benzil is coupled to the external stress compression wave generated by the mechanical refrigerator used to cool the sample. The latter interaction and relaxation anomaly are observed in polarized scattered light with the stress applied along theb(ora) axis as required for ac11coupling. These observations are qualitively analyzed employing the theory of anelastic solids with higher order fluctuation terms incorporated into the free energy expansion. The relationship of this description to the Halperinndash;Varma theory of defect induced central peaks is outlined.