Complex dynamics and the stimulated gain spectrum of pentylcyanobiphenyl in the isotropic phase

A mode coupling theory description of the short- and long-time dynamics of nematogens in the isotropic phase

Optical heterodyne-detected optical Kerr effect (OHD-OKE) experimental data are pre-sented on nematogens 4-(trans-4-n-octylcyclohexyl)isothiocyanatobenzene (8-CHBT), and 4-(4-pentyl-cyclohexyl)-benzonitrile (5-PCH) in the isotropic phase. The 8-CHBT and 5-PCH data and previously published data on 4-pentyl-4-biphenylcarbonitrile (5-CB) are analyzed using a modification of a schematic mode coupling theory (MCT) that has been successful in describing the dynamics of supercooled liquids. At long time, the OHD-OKE data (orientational relaxation) are well described with the standard Landau-de Gennes (LdG) theory. The data decay as a single exponential. The decay time diverges as the isotropic to nematic phase transition is approached from above. Previously there has been no theory that can describe the complex dynamics that occur at times short compared to the LdG exponential decay. Earlier, it has been noted that the short-time nematogen dynamics, which consist of several power laws, have a functional form identical to that observed for the short time behavior of the orientational relaxation of supercooled liquids. The temperature-dependent orientational dynamics of supercooled liquids have recently been successfully described using a schematic mode coupling theory. The schematic MCT theory that fits the supercooled liquid data does not reproduce the nematogen data within experimental error. The similarities of the nematogen data to the supercooled liquid data are the motivation for applying a modification of the successful MCT theory to nematogen dynamics in the isotropic phase. The results presented below show that the new schematic MCT theory does an excellent job of reproducing the nematogen isotropic phase OHD-OKE data on all time scales and at all temperatures.

Mode-selective dynamic light scattering spectroscopy: application to the isotropic phase of liquid crystals

We have developed a new method of "mode-selective" dynamic light scattering spectroscopy. This method allows us to study dynamic modes usually hidden behind large fluctuations of a critical mode. Here we apply this method to measurements of three dynamic modes of the isotropic phase of liquid crystals near the isotropic-nematic transition. In conventional methods detecting thermally excited modes, the thermal diffusion and sound modes are usually overwhelmed by the orientational relaxation mode since the intensity of the last mode diverges toward the isotropic-nematic critical point. Thanks to the phase-coherent detection ability of our method, we have successfully observed the thermal diffusion and sound modes without the interference of incoherent critical orientational fluctuations.

Complex dynamics of isotropic 4-cyano-4-n-pentylbiphenyl (5CB) in linear and nonlinear dielectric relaxation studies

A comprehensive presentation of the temperature evolution of "linear" and "nonlinear" dielectric relaxation in the isotropic phase of nematic liquid crystalline compound 5CB (4-cyano-4-n-pentylbiphenyl) is given. The "nonlinear" relaxation is related to the strong pretransitional rise in the lifetime of prenematic fluctuations. The "linear" relaxation has a clear non-Debye and non-Arrhenius form. In the immediate vicinity of the nematic clearing point it shows a weak pretransitional anomaly. Results obtained coincide with the complex liquid relaxation pattern found in transient grating optical Kerr effect studies [A. Sengupta and M. D. Fayer, J. Chem. Phys. 102, 4193 (1995); R. Torre et al., Philos. Mag. A 77, 645 (1997)]. The striking similarity to the behavior found in critical, binary mixtures suggests the extension of the "fluidlike" hypothesis for the isotropic-nematic transition to dynamic phenomena in the isotropic phase [P. K. Mukherjee. J. Phys.: Condens. Matter 10, 9191 (1998)]. The presence of both glassy and fluidlike features in isotropic 5CB coincides with the recent results of simulation analysis for the system of hard ellipsoids by Latz et al. [Phys. Rev. E 62, 5173 (2000)] and with the novel general picture for liquid-liquid transitions proposed by Tanaka [Phys. Rev. E 62, 6968 (2000)].

Quasicritical behavior of the low-frequency dielectric permittivity in the isotropic phase of liquid crystalline materials

Results presented give evidence of the existence of quasicritical, fluidlike behavior in the isotropic phase of 4-cyano-4-pentyl-biphenyl (5CB) for frequencies ranging from the static to the ionic-dominated [low-frequency (LF)] region. Despite the boost of dielectric permittivity on lowering the frequency below 1 kHz, values of the isotropic-nematic transition discontinuity (approximately 1.1 K) and the critical exponent alpha (approximately 0.5) remain constant. It is shown that the contribution from residual ionic impurities is a linear function of temperature in the critical, prenematic fluctuation-dominated region. The validity of the fluidlike and critical behavior for LF dielectric permittivity confirmed results of a derivative analysis of the experimental data: d(epsilon)/dT proportional to (T-T*)(-alpha), originally proposed for critical mixtures. Results of a preliminary test in the isotropic phase of 4-decyl-4'-isothiocyanatobiphenyl (10BT), on approaching the smectic-E phase, may indicate a general validity of results obtained.