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1.
A theory is developed of the anomalous magnetic and electric birefringence in the isotropic phase of nematic liquid crystals
taking into account orientational correlations between neighbouring molecules. Use is made of a modification of Bethe’s method
due to Krieger and James, and the properties of the system are derived in terms of a single parameter, viz., the two-particle
interaction constant. The expressions for the magnetic and electric birefringence are similar in form to those given by the
phenomenological model of de Gennes. Theoretical curves forp-azoxyanisole reproduce the trends in the observed data. A calculation of the nematic-isotropic transition point confirms
that this treatment is an improvement over the mean field approximation in describing pre-transition phenomena in the isotropic
phase. 相似文献
2.
We report the experimental high electric field phase diagram of a nematic
liquid crystal which exhibits a large negative dielectric
anisotropy. We measure simultaneously
the birefringence (Δn) and the dielectric constant
(epsilon⊥) at various applied fields as
functions of the
local temperature of an aligned sample. We also measure the higher harmonics of
the electrical response of the
medium. The following experimental results are noted: (i) enhancement
of orientational order parameter
S in the nematic phase due to both
the Kerr effect and quenching of director fluctuations; (ii) enhancement in the paranematic to
nematic transition temperature (TPN) with field; (iii) divergence of the order parameter
susceptibility beyond the tricritical point as measured
by third harmonic electrical signal; (iv) a small second harmonic
electrical signal which also diverges
near TPN, indicating the presence of
polarised domains. Our measurements show that ΔTPN(= TPN(E)-TNI(0))
varies linearly with |E|
whereas the Landau de Gennes theory predicts a dependence on E2. It is argued that the quenching of director fluctuations by the field makes the dominant contribution to all the observations,
including the thermodynamics of the transition. 相似文献
3.
M. A. Aliev 《Molecular physics》2013,111(10):1583-1590
The isotropic-to-nematic phase transition in a melt of semi-flexible homopolymers with length polydispersity have been considered within the Landau–de Gennes approach. The number of monomer units in chain is assumed to be a random variable distributed by the Schulz–Zimm distribution; the stiffness of macromolecules has been taken into account within discrete worm-like chain model. It was found that increase of polydispersity yields the increase of the temperature of the isotropic–nematic transition. 相似文献
4.
High pressure experimental studies show that in certain mesogenic materials, the nematic-smectic A (N-Sm A) transition temperature
TAN exhibits nonlinear pressure dependence. As a consequence, the material
shows reentrant phenomena that is a phase sequence nematic — smectic A — reentrant nematic appears. The
characteristic features of this phenomenon have been addressed here within
the framework of Landau-de-Gennes theory, where the coupling between nematic
and smectic A order parameters (γ, λeff) plays an important role. The cubic coupling γ is chosen to be negative in order to form Sm A phase whereas the biquadratic
coupling λeff is made large and positive to obtain reentrant behaviour. In the present work, we incorporate the pressure dependence in
the theory through γ and λeff which justifies the experimental pressure dependence in the reentrant transition temperature
. The pressure dependence of γ and λeff are employed in the calculation of excess specific heat capacity near the reentrant transition. The computed heat capacity
shows strong pressure dependence near the reentrant transition which
can be confirmed from high pressure measurement. 相似文献
5.
6.
We propose Landau phenomenology for the phase transition from the conventional nematic into the conical helical orientationally non-uniform structure recently identified in liquid crystals formed by “banana”-shaped molecules. The mean field predictions are mostly in agreement with experimental data. Based on the analogy with de Gennes model, we argue that fluctuations of the order parameter turn the transition to the first order phase transition rather than continuous one predicted by the mean-field theory. This conclusion is in agreement with experimental observations. We discuss the new Goldstone mode to be observed in the low-temperature phase. 相似文献
7.
8.
Arkady I. Leonov 《Mathematical Physics, Analysis and Geometry》2008,11(1):87-116
This paper consists of two parts. The first one develops algebraic theory of linear anisotropic nematic “N-operators” build
up on the additive group of traceless second rank 3D tensors. These operators have been implicitly used in continual theories
of nematic liquid crystals and weakly elastic nematic elastomers. It is shown that there exists a non-commutative, multiplicative
group N6 of N-operators build up on a manifold in 6D space of parameters. Positive N-operators, which in physical applications hold
thermodynamic stability constraints, do not generally form a subgroup of group N6. A three-parametric, commutative transversal-isotropic subgroup of positive symmetric nematic operators is also briefly discussed. The special case of singular, non-negative symmetric N-operators
reveals the algebraic structure of nematic soft deformation modes. The second part of the paper develops a theory of linear
viscoelastic nematodynamics applicable to liquid crystalline polymer. The viscous and elastic nematic components in theory
are described by using the Leslie–Ericksen–Parodi (LEP) approach for viscous nematics and de Gennes free energy for weakly
elastic nematic elastomers. The case of applied external magnetic field exemplifies the occurrence of non-symmetric stresses.
In spite of multi-(10) parametric character of the theory, the use of nematic operators presents it in a transparent form.
When the magnetic field is absent, the theory is simplified for symmetric case with six parameters, and takes an extremely
simple, two-parametric form for viscoelastic nematodynamics with possible soft deformation modes. It is shown that the linear
nematodynamics is always reducible to the LEP-like equations where the coefficients are changed for linear memory functionals
whose parameters are calculated from original viscosities and moduli.
相似文献
9.
Neupane K Kang SW Sharma S Carney D Meyer T Mehl GH Allender DW Kumar S Sprunt S 《Physical review letters》2006,97(20):207802
Dynamic light scattering from orientational order fluctuations in a liquid crystalline tetrapode reveals successive, weakly first-order isotropic to uniaxial and uniaxial to biaxial nematic phase transitions. The order parameter relaxation rates exhibit temperature dependences consistent with Landau-de Gennes mean field theory. Combined with previous evidence of a second-order uniaxial-biaxial transition in a closely related tetrapode, the present study supports the existence of a nematic-nematic tricritical point in thermotropic liquid crystals. 相似文献
10.
Static Kerr effect and the pre-transition temperature were investigated for the 6CHBT, 6BOBT and the nematic mixtures, M1 (40% 6CHBT: 60% 6BOBT), M2 (30% 6CHBT: 70% 6BOBT) and M3 (20% 6CHBT: 80% 6BOBT) at temperatures above the nematic-isotropic transition temperature. The linear dependence of (T-T*)−1 on the Kerr constant is found to be in good agreement with the predications of the Landau-De Gennes model. The hypothetical
second order phase transition temperatures and susceptibility values were determined for these compounds. 相似文献
11.
Nematic liquid crystals at rough and fluctuating interfaces are analyzed within
the Frank elastic theory and the Landau–de Gennes theory. We study specifically
interfaces that locally favor planar anchoring. In the first part we reconsider the
phenomenon of Berreman anchoring on fixed rough surfaces, and derive new simple
expressions for the corresponding azimuthal anchoring energy. Surprisingly, we find
that for strongly aligning surfaces, it depends only on the geometrical surface
anisotropy and the bulk elastic constants, and not on the precise values
of the chemical surface parameters.
In the second part, we calculate the capillary waves at nematic-isotropic
interfaces. If one neglects elastic interactions, the capillary wave spectrum
is characterized by an anisotropic interfacial tension. With elastic interactions,
the interfacial tension, i.e., the coefficient of the leading q2 term of
the capillary wave spectrum, becomes isotropic. However, the elastic interactions
introduce a strongly anisotropic cubic q3 term. The amplitudes of
capillary waves are largest in the direction perpendicular to the director.
These results are in agreement with previous molecular dynamics simulations. 相似文献
12.
《Physics letters. A》1997,235(6):621-628
A sandwich cell made of two flat glass substrates covered with obliquely evaporated SiOx and filled with a common thermotropic nematic liquid crystal is considered. The two glasses are assembled with their evaporation directions antiparallel to each other in order to provide a uniformly pretilted nematic layer, without any local distortion. By means of a Landau-de Gennes approach, based on the general symmetry properties of the flat glass surface, including the anisotropy induced by the SiOx, and of the nematic tensor order parameter, a temperature variation of the director azimuthal and tilt angle in the nematic phase is predicted. Observations via polarizing microscopy were performed and the fits, made according to the proposed model, compare well with the experimental data presented here. 相似文献
13.
We present a theory of orientational order in nematic liquid crystals which interpolates between several distinct approaches based on the director field (Oseen and Frank), order parameter tensor (Landau and de Gennes), and orientation probability density function (Onsager). As in density-functional theories, the suggested free energy is a functional of spatially-dependent orientation distribution, however, the nonlocal effects are taken into account via phenomenological elastic terms rather than by means of a direct pair-correlation function. In illustration of this approach we consider a simplified model with orientation parameter on a circle and reveal its relation to the complex Ginzburg-Landau theory. 相似文献
15.
G. Albertini M. Corinaldesi S. Mazkedian S. Melone M.G. Ponzi-Bossi F. Rustichelli 《Solid State Communications》1977,24(6):433-437
We report an investigation by X-ray diffraction of the cybotactic cluster formation near the nematic-smectic A phase transition of TBBA liquid crystal. The results are discussed on the basis of the Landau theory developed by de Gennes and McMillan. The dimensions of the cybotactic clusters are evaluated as a function of the temperature. 相似文献
16.
A quasiparticle pattern advanced in Landau’s first article on Fermi-liquid theory is adapted to elucidate the properties of
a class of strongly correlated Fermi systems characterized by a Lifshitz phase diagram featuring a quantum critical point
(QCP) where the density of states diverges. The necessary condition for stability of the Landau Fermi-Liquid state is shown
to break down in such systems, triggering a cascade of topological phase transitions that lead, without symmetry violation,
to states with multi-connected Fermi surfaces. The end point of this evolution is found to be an exceptional state whose spectrum
of single-particle excitations exhibits a completely flat portion at zero temperature. Analysis of the evolution of the temperature
dependence of the single-particle spectrum yields results that provide a natural explanation of classical behavior of this
class of Fermi systems in the QCP region. 相似文献
17.
Exact formulae for the elastic moduli of the nematic elastomers are obtained by the implicit function method based on somewhat
general energy functions. The formulae indicate that both the moduli parallel and perpendicular to the director of the nematic
elastomers are smaller than the modulus of the classical elastomers because of the mechanical-nematic coupling. Moreover,
the moduli are generally anisotropic due to the biaxiality induced by stretching the nematic elastomers perpendicular to the
director. Then we get the explicit analytical expressions of the parallel and perpendicular moduli by making use of the Landau-de
Gennes free energy and the neo-classical elastic energy. Very different from the classical elastomers, they are both strongly
nonlinear functions of the temperature in the nematic phase. Furthermore, their ratio, the degree of anisotropy, changes with
the temperature as well. The results agree qualitatively with some experiments. Better quantitative agreement is obtained
by some modifications of the constitutive relation of the elastic energy. 相似文献
18.
Summary A mean-field theory for the nematic-isotropic phase transition in semi-rigid polymer solutions is proposed. Phase diagrams
are calculated. New effects due to chain flexibility are found for the transition temperature and the nematic order parameter
of the polymer. An abrupt increase of chain extension in the nematic phase is demonstrated.
Paper presented at the ?Meeting on Lyotropics and Related Fields?, held in Rende, Cosenza, September 13–18, 1982. 相似文献
19.
In this work we show, that with the introduction of the micellar shape anisotropy in the order parameter of a lyotropic liquid crystal, it is possible to understand, in a unified way, the features observed in the phase diagram of these materials. Using the de Gennes connection between microscopic and macroscopic order parameters, such an extended order parameter is constructed from a geometrical conception of the micelles. Our theoretical results will be correlated with refractive index experimental data near the uniaxial-biaxial nematic phase transitions of a lyotropic mixture of potassium laurate, decanol and deuterium oxide. 相似文献
20.
Measurements of X-ray scattering intensity from a ferroelectric liquid crystal are carefully carried out to clarify the mechanism of the ferroelectric phase transition. The temperature dependences of the scattering intensity and the tilt angle of the molecules in the ferroelectric smectic C phase are discussed on the base of Landau's theory. 相似文献