PREASYMPTOTIC 3D XY CRITICAL-BEHAVIOR OF HEAT-CAPACITY AT NORMAL-INCOMMENSURATE PHASE-TRANSITION IN K2ZNCL4 AND K2SEO4

Heat capacity of K2ZnCl4 and K2SeO4 has been measured using a high-res olution ac calorimeter. Critical behavior of the heat capacity anomali es associated with the normal-incommensurate (N-INC) phase transitions has been analyzed with the preasymptotic renormalization-group expres sion which includes correction terms up to the second order. It is rev ealed that the heat capacity anomalies of K2ZnCl4 and K2SeO4 are descr ibed well with the three dimensional (3D) XY model; the critical expon ent, the critical amplitude ratio and the dimensionless ratio derived from the first-order correction terms agree well with the theoreticall y expected values of 3D XY model. These results are consistent with ou r previous experimental result for Rb2ZnCl4 and the Cowley and Bruce's prediction that the N-INC phase transition is classified into the 3D XY system.