THE CRITICAL-TEMPERATURE DEPENDENCE OF BIREFRINGENCE NEAR THE NORMAL INCOMMENSURATE PHASE-TRANSITION IN RB2ZNBR4

Anomalous (non-classical) temperature behaviour of birefringence at the second-order normal-incommensurate phase transition in Rb2ZnBr 4 crystals is studied experimentally. An attempt is undertaken to discuss it using the existing theoretical approach. A critical review of some theoretical results concerning the temperature dependence of the local mean square of an order parameter is given both for the region of small corrections to the results of the Landau theory and for the scaling region. It is shown that, in the region of 'small corrections' (at mod tau mod approximately=10 -1), qualitative agreement of the experiment with the results of fluctuation theory can be recognised. Near the critical point, at mod tau mod <10-3, the deviation from the critical behaviour of the XY model is found; the nature of this deviation is not clear.