The critical invariant Q for triglycine selenate and deuterated triglycine selenate

Abstract

The quantity Q=χ(0T_m )/χ(E, T_m ), was measured experimentally for a number of TGSe samples and a few deuterized TGSe (DTGSe) samples at the temperature T _m = T _m (E) of maximum of susceptibility chi(E, T) for various values of electric field E. It was shown that Q is independent of E and T in the case of continuous phase transitions, i.e., for deuterium content c=0, c=0.35 and c = 0.55. The universal constant Q decreases when deuterium content c increases. The experimental values of Q for c=0 and c=0.35 lie between the values 2 and 4/3 for Landau critical and tricritical point, respectively, while the value Q for c=0.55 is less than 4/3. If c=0.95 there is a first order phase transition and Q is not a universal constant. It has a value less than unity and increases when electric field and temperature T _m =T _m (E) are increased.