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-resolution ac calorimeter. Critical behavior of the heat capacity anomalies associated with the normal-incommensurate (N-INC) phase transitions has been analyzed with the preasymptotic renormalization-group expression which includes correction terms up to the second order. It is revealed that the heat capacity anomalies of K2ZnCl4 and K2SeO4 are described well with the three dimensional (3D) XY model; the critical exponent, the critical amplitude ratio and the dimensionless ratio derived from the first-order correction terms agree well with the theoretically expected values of 3D XY model. These results are consistent with our 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.