M. Marinelli et F. Mercuri, Effects of fluctuations in the orientational order parameter in the cyanobiphenyl (n CB) homologous series, PHYS REV E, 61(2), 2000, pp. 1616-1621
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.