Effects of fluctuations in the orientational order parameter in the cyanobiphenyl (n CB) homologous series

Photopyroelectric measurements of the anisotropy in the thermal conductivit y Delta k vs temperature in the nCB (n=5,...,9) series are reported. The da ta have been used to deduce the behavior of the orientational order paramet er Q close to the nematic-isotropic (N-I) and smectic A-nematic (A-N) phase transitions, respectively. It has been shown that near the N-I transition the data for 5CB and 6CB are consistent with the so-called "tricritical hyp othesis," which predicts beta=0.25. This is not true for 7CB and 8CB in whi ch the order parameter exhibits a behavior that could be caused by the pres ence of fluctuations that become increasingly important when the transition temperature is approached. A very simple model, which takes into account t he contribution of fluctuations to the orientational order, has been develo ped close to the A-N transition and it has been shown that it is in good ag reement with the experimental results. A semiquantitative explanation for t he observed behavior in compounds with different nematic range has been als o given.