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Journal ArticleDOI

Landau Theory of the Direct Smectic-A to Isotropic Phase Transition

10 Aug 1997-International Journal of Modern Physics B (World Scientific Publishing Company)-Vol. 11, Iss: 20, pp 2425-2432
Abstract: Direct smectic-A to isotropic phase transition is studied by means of Landau theory. The theory predicts the first order behaviour of the smectic-A to isotropic phase transition. The temperature difference TAI-T* is also calculated from the scaling equation of state and compared with the nematic to isotropic phase transition
Topics: Landau theory (64%), Quantum phase transition (63%), Phase transition (60%), Quantum critical point (60%), Isotropy (57%)
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Journal ArticleDOI
01 May 2000-Physical Review E
TL;DR: Despite significant differences between N, Sm-A, and Sm-E phases the same pretransitional behavior of epsilon and NDE in the isotropic phase, described by critical exponents gamma = 1 and alpha = 0.5, was found.
Abstract: Results of studies of static dielectric permittivity (epsilon) and nonlinear dielectric effect (NDE) in the isotropic phase of 4-n-4'-isothiocyanatobiphenyl (nBT) homologous series from n = 2 to n = 10 exhibiting the isotropic-smectic E (I-SmE) transition, are presented. They are compared with results of similar studies in 4-cyano-4-n-alkylbiphenyls (nCB) from n = 4 to n = 12. In this homologous series isotropic-nematic (I-N) and isotropic-smectic-A (I-Sm-A) transitions take place. Despite significant differences between N, Sm-A, and Sm-E phases the same pretransitional behavior of epsilon and NDE in the isotropic phase, described by critical exponents gamma = 1 and alpha = 0.5, was found. It has been shown that when the length of the alkyl chain of a compound increases the discontinuity of the transition drops in nBT and rises in nCB. The influence of pressure on the discontinuity is also discussed.

55 citations


Journal ArticleDOI
01 May 2000-Physical Review E
TL;DR: It was found that pressure first decreases the discontinuity of the transition, and that next a gradual rise appears, unlike the one observed for the isotropic-nematic transition, where only an increase of delta T with rising pressure was observed.
Abstract: Results are presented of temperature and pressure studies of static dielectric permittivity (epsilon) and the nonlinear dielectric effect (NDE) in the isotropic phase of smectogenic n-cyanobiphenyls: 9CB, 10CB, and 12CB (4-cyano-4'-n-alkylbiphenyl, n = 9, 10, and 12). For the mentioned properties, pretransitional effects can be well portrayed by applying the relations used for the isotropic phase of nematogens, where evidence of the quasicritical, fluidlike behavior with exponents alpha approximately 0.5 and gamma = 1 exists. This kind of behavior one can also observe on approaching the isotropic-smectic A transition. NDE studies in 10CB and 12CB made it possible to determine the pressure evolution of the discontinuity (delta T) of the I-SmA transition. It was found that pressure first decreases the discontinuity of the transition, and that next a gradual rise appears. This behavior is unlike the one observed for the isotropic-nematic transition, where only an increase of delta T with rising pressure was observed.

51 citations


Journal ArticleDOI
Abstract: The present review represents an attempt towards a unified picture of the direct transition from the isotropic to smectic-A phase. The isotropic to smectic-A phase transition is described in detail, including structure and nature revealed by experiments, molecular models and Landau-de Gennes theory. The wide variety of phenomena including effects of nonmesogenic impurities and electric field, hydrodynamics, fluctuations, and elastic theory of liquid crystalline elastomers is discussed. A brief review on the general information about the isotropic to lamellar phase transition in lyotropic liquid crystals has been given. The various predictions are compared with the available experimental results. Open questions and possible approaches to finding answers are discussed.

19 citations


Journal ArticleDOI
Abstract: Results are presented of studies of linear and nonlinear dielectric measurements in the isotropic, smectic E phase in 4,4-alkyl-4´-isothicyanatobiphenyls (4BT). The mean-field behaviour was valid up to the smectic E phase transition. In the smectic E phase the Vogel-Fulcher law describes temperature and pressure dependences of the relaxation time. An analysis of the higher temperature relaxation time data led to predictions of the mode coupling theory with the power law universal function = A (T - TC ) , where = -3.3, TC = 288 ± 15 K. Increase of pressure brings the ´´ curve closer to the Debye shape.

10 citations


Journal ArticleDOI
Abstract: This Letter presents a proton fast field cycling Nuclear Magnetic Resonance study of the molecular dynamics in the region above the smectic A-isotropic phase transition in thermotropic liquid crystals. The proton spin-lattice relaxation time was measured as a function of temperature at different Larmor frequencies and the experimental results were interpreted in term of the Landau De Gennes theory. Mechanisms provided by both individual reorientational fluctuations and fluctuations of the local order parameter can be distinguished at the different Larmor frequency scales.

9 citations


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