Central peak in an isotopically disordered KH2PO4(KDP) -type crystal studied by a coupled-operators continued-fraction expansion

A continued-fraction calculation, for coupled operators, of an isotopically disordered K${\mathrm{H}}_2$P${\mathrm{O}}_4$ (KDP)-type crystal is used to calculate the order-parameter fluctuation spectrum. The intensity and width of the central peak are calculated as a function of deuterium content. The results are compared to a calculation presented before where a decoupled approximation was used in the continued-fraction expansion. It is shown that the decoupled procedure although being quantitatively wrong gives qualitatively correct results.