OBSERVATION OF FLUCTUATIONS IN A 3-DIMENSIONAL HEISENBERG-SYSTEM WITHPOSITIONAL DISORDER

Several years ago de Gennes predicted that orientational director fluc tuations in a nematic liquid crystal, in zero field, fulfill the condi tions of a 3D Heisenberg system. The verification of this prediction b y neutron scattering represents a challenge, since the relevant time s cale is macroscopic and the substance has no positional order. We have looked for real-time fluctuations of the liquid diffraction peaks and analyzed the observed time series by the Hurst-exponent (H) method. I n zero magnetic field we find H similar to 1, indicating infinite temp oral correlations between orientational fluctuations of the director. With increasing magnetic field we observe a cross-over to H similar to 0.5, corresponding to zero temporal correlations. A similar cross-ove r is found by the application of a temperature gradient, or by heating to the isotropic liquid phase.