The nematic critical point in the molecular field approximation

Abstract
The classical spin system with biquadratic exchange interaction and single spin anisotropy is analysed in the molecular field approximation. The temperature dependence of important thermodynamic quantities versus anisotropy strength is studied and the crossover from ordinary critical point behaviour to nematic point behaviour is discussed. The power-law parameters describing the temperature dependence are numerically estimated in the experimentally available temperature region of 10-4 ≤ |ε = (T - Tt)/Tt| ≤ 10 -2 both below and above the transition point Tt. The estimated values of the gap and the correlation exponents at the nematic critical point are : Δ+ = 1.32 ± 0.005, Δ- = 1.29 ± 0.01 and ν+ = 0.321 ± 0.001, ν- = 0.357 ± 0.004 respectively