Nematic LC to nematic glass phase transition in pores

The properties of a nematic liquid crystal in a porous matrix are discussed. On scales exceeding a certain characteristic scale the liquid crystal behaves as a nematic glass. Application of a weak magnetic field restores long-range orientational order. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 9, 695–698 (10 May 1997)     

Spin nematic and antinematic states in a spin-3/2 isotropic non-Heisenberg magnet

The ground state phase diagram of a general isotropic spin-3/2 system with nearest-neighbor exchange is shown to contain unconventionally ordered spin nematic and antinematic states, as well as usual ferro- and antiferromagnetic phases. The two nematic phases have spontaneously broken rotational symmetry characterized by the long-range order of the nematic director u, as well as the broken time-reversal symmetry described by the pseudospin vector σ. Nematic phase differs from antinematic one by the type of ordering in σ vectors (uniform versus staggered). The ferromagnet-nematic and antiferromagnet-antinematic phase boundaries exhibit enhanced Sp(4) symmetry and correspond to the recently studied effective theory for spin-3/2 cold gases. We discuss optical properties and topological defects in the nematic phases.     

Nematic order in square lattice frustrated ferromagnets

We present a new scenario for the breakdown of ferromagnetic order in a two-dimensional quantum magnet with competing ferromagnetic and antiferromagnetic interactions. In this, dynamical effects lead to the formation of two-magnon bound states, which undergo Bose-Einstein condensation, giving rise to bond-centered nematic order. This scenario is explored in some detail for an extended Heisenberg model on a square lattice. In particular, we present numerical evidence confirming the existence of a state with d-wave nematic correlations but no long-range magnetic order, lying between the saturated ferromagnetic and collinear antiferromagnetic phases of the ferromagnetic model J1-J2. We argue by continuity of spectra that this phase is also present in a model with 4-spin cyclic exchange.     

Universal nuclear spin relaxation and long-range order in nematics strongly confined in mass fractal silica gels

We show how the low-frequency dependence of the proton spin-lattice relaxation time T1(nu) of octylcyanobiphenyl liquid crystals confined in high-density silica gels evidences a long-range order nematic phase in spite of the strong confinement and random disorder of the gels. The universal value and frequency dependence observed, T1(nu) proportional, variant nu(2/3), is interpreted within a relaxation model due to director fluctuations in nematic liquid crystals confined to mass fractal porous media. The model provides a relation T1(nu) proportional, variant nu(2-d/2), giving a reliable value of the structural fractal dimension d(f)=2.67 for all the host silica gels.     

Long-range order and dynamic structure factor of a nematic under a thermal gradient

We use a fluctuating hydrodynamic approach to calculate the orientation fluctuations correlation functions of a thermotropic nematic liquid crystal in a nonequilibrium state induced by a stationary heat flux. Since in this nonequilibrium stationary state the hydrodynamic fluctuations evolve on three widely separated times scales, we use a time-scale perturbation procedure in order to partially diagonalize the hydrodynamic matrix. The wave number and frequency dependence of these orientation correlation functions is evaluated and their explicit functional form on position is also calculated analytically in and out of equilibrium. We show that for both states these correlations are long-ranged. This result shows that indeed, even in equilibrium there is long-range orientational order in the nematic, consistently with the well known properties of these systems.We also calculate the dynamic structure of the fluid in both states for a geometry consistent with light scattering experiments. We find that as with isotropic simple fluids, the external temperature gradient introduces an asymmetry in the spectrum shifting its maximum by an amount proportional to the magnitude of the gradient. This effect may be of the order of 7 per cent. Also, the width at half height may decrease by a factor of about 10 per cent. Since to our knowledge there are no experimental results available in the literature to compare with, the predictions of our model calculation remains to be assessed.     

Dichroism of absorption by impurity molecules with internal rotation in nematic liquid crystals

Expressions are obtained for the components of the permittivity tensor in the optical region for a nematic liquid crystal with impurity molecules having the internal rotation conformational degree of freedom. The dependence of the intensity and dichroism of the impurity absorption bands in isotropic and nematic phases on the parameters of conformational, orientation, and mixed conformation-orientation orders of molecules is found. The influence of the mutual correlation of conformational and orientation degrees of freedom of molecules on these parameters is studied. The dependence of the oscillator strengths of molecular transitions on the phase state of a medium and the orientation order of the impurity subsystem is analyzed. The self-consistent nature of molecular, structural, and spectral changes upon the nematic-isotropic liquid phase transition is demonstrated.     

Transitions in lyotropic liquid crystals caused by concentration change and γ-radiation

A molecular-statistical theory of phase transitions in lyotropic liquid crystals, which describes the phase transitions between the isotropic (micellar), nematic and lamellar phases was developed. The equations describing the dependence of parameters of orientation and translational long-range order on the concentration were obtained. It was shown that depending on the values of the model microscopic constants, the nematic phase–lamellar phase transition can be both of the first and the second order. The influence of intensive and low intense γ-radiation on the phase transitions mentioned herein was considered. It was shown that the irradiation changes the model constants responsible for the phase transitions. On this basis, it can be assumed that the γ-radiation influences the course of the dependence of the long-range order parameters on concentration as well as it changes the values of the critical concentrations of the phase transitions and even the phase transition order.     

Orbital glass and spin glass states of <Superscript>3</Superscript>He-A in aerogel

Glass states of superfluid A-like phase of ³He in aerogel induced by random orientations of aerogel strands are investigated theoretically and experimentally. In anisotropic aerogel with stretching deformation two glass phases are observed. Both phases represent the anisotropic glass of the orbital ferromagnetic vector Ηthe orbital glass (OG). The phases differ by the spin structure: the spin nematic vector \(\hat d\) can be either in the ordered spin nematic (SN) state or in the disordered spin-glass (SG) state. The first phase (OG-SN) is formed under conventional cooling from normal ³He. The second phase (OG-SG) is metastable, being obtained by cooling through the superfluid transition temperature, when large enough resonant continuous radio-frequency excitation is applied. NMR signature of different phases allows us to measure the parameter of the global anisotropy of the orbital glass induced by deformation.     

Aggregates of amphiphilic molecules in lyotropic liquid crystals

Summary Aamphiphilic molecules are made of two parts with different chemical affinities. They build interfaces which limit the contact between these two parts. We shall focus our attention on the case of soap molecules in the presence of water, in which the interfaces separate the paraffinic chains from the water and delimit aggregates of various shapes. Classically the aggregates can be infinite, with flat or cylindrical interfacial curvatures, and packed with long-range translational order in the well-known lamellar, cubic and hexagonal phases; or they can be finite, quasi-spherical and packed without any long-range order in the micellar phase. However, recent investigations show that a long-range ordering of finite aggregates is possible. There is one example of translational order, in which the micelles build a cubic structure, and much more examples now of orientational order. In the latter case the micelles, oblate or prolate spheroids, are dispersed in the solution with their axes nearly parallel to each other, a nematic phase is formed. We shall discuss the possible factors controlling the shapes of the aggregates and their interactions. Laboratoire associé au CNRS (LA No. 2).     

Entropy-driven enhanced self-diffusion in confined reentrant supernematics

We present a molecular dynamics study of reentrant nematic phases using the Gay-Berne-Kihara model of a liquid crystal in nanoconfinement. At densities above those characteristic of smectic A phases, reentrant nematic phases form that are characterized by a large value of the nematic order parameter S?1. Along the nematic director these "supernematic" phases exhibit a remarkably high self-diffusivity, which exceeds that for ordinary, lower-density nematic phases by an order of magnitude. Enhancement of self-diffusivity is attributed to a decrease of rotational configurational entropy in confinement. Recent developments in the pulsed field gradient NMR technique are shown to provide favorable conditions for an experimental confirmation of our simulations.     

A comparison of orientational order measurements in cholesteric and nematic liquid crystals

We present a quantitative comparison of orientation order parameter measurements at constant pressure and constant density in cholesteric and nematic liquid crystals. The two phases show great similarity. In addition, the volume dependence of the mean-field potential is estimated for this cholesteric, and found to differ from measurements in nematics.     

Refusing to twist: demonstration of a line hexatic phase in DNA liquid crystals

We report conclusive high resolution small angle x-ray scattering evidence that long DNA fragments form an untwisted line hexatic phase between the cholesteric and the crystalline phases. The line hexatic phase is a liquid-crystalline phase with long-range hexagonal bond-orientational order, long-range nematic order, but liquidlike, i.e., short-range, positional order. So far, it has not been seen in any other three dimensional system. By line-shape analysis of x-ray scattering data we found that positional order decreases when the line hexatic phase is compressed. We suggest that such anomalous behavior is a result of the chiral nature of DNA molecules.     

利用单轴压强下的电阻变化研究铁基超导体中的向列涨落

铁基超导体中普遍存在着反铁磁、超导和向列相,因此研究向列相的性质及其与反铁磁、超导的关系对于理解铁基超导体的低能物理及高温超导电性具有非常重要的作用.所谓向列相是指电子态自发破缺了晶格的面内四重旋转对称性而形成的有序态,从而导致样品的某些物理性质出现了两重的各向异性.我们通过自主研发的单轴压强装置,可以在低温下原位改变压强,测量电阻的变化,从而得到向列极化率.本文介绍了我们利用该装置在最近几年研究铁基超导体的向列相和向列涨落所取得的一些成果,包括详细研究了BaFe_(2-x)Ni_xAs_2体系中的向列量子临界点及其量子临界涨落,并提出了基于向列涨落强弱调节的铁基超导体统一相图.这些结果表明,向列相及其涨落与反铁磁和超导均有很强的耦合,对于理解铁基超导体中磁性和超导电性非常关键.     

Interface in nematics

Summary The structures of the interface between an isotropic liquid and a nematic liquid crystal and also between different uniaxial nematic phases are studied. It is shown that, at a planar orientation in the case of the nematic-isotropic liquid interface and also at a mutually orthogonal orientation of the director in the case of two nematic phases, the interface has the structure of a biaxial nematic liquid crystal. Work presented at the second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988.     

Theory of quantum Hall nematics

Transport measurements on two-dimensional electron systems in moderate magnetic fields suggest the existence of a spontaneously orientationally ordered, compressible liquid state. We develop and analyze a microscopic theory of such a "quantum Hall nematic" (QHN) phase, predict the existence of a novel, highly anisotropic q(3) density-director mode, find that the T = 0 long-range orientational order is unstable to weak disorder, and compute the tunneling into such a strongly correlated state. This microscopic approach is supported and complemented by a hydrodynamic model of the QHN, which, in the dissipationless limit, reproduces the modes of the microscopic model.     

Orientational order parameter studies in two symmetric dimeric liquid crystals – an optical study

The optical technique developed by [W. Kuczynski, B. Zywucki, and J. Malecki, Determination of orientational order parameter in various liquid-crystalline phases, Mol. Cryst. Liq. Cryst. 381 (2002), pp. 1–19; B.J. Zywucki and W. Kuczynski, IEEE transactions on optical phenomena – The orientational order in nematic liquid crystals from birefringence measurements, Dielectr. Electr. Insul. 8 (2001), pp. 512–515] is fabricated and used to determine the orientational order parameter in two dimeric liquid crystalline compounds nematic and SmA phases of α,ω-bis(4-alkylanilinebenzylidene-4′-oxy)alkane (m.OnO.m) homologous series. The compounds studied are 5.O8O.5 and 5.O10O.5 which exhibit nematic and SmA, and nematic phases, respectively. The orientational order parameter in both the phases of nematic and SmA phases of the compound one and the nematic phase of the compound two are obtained using the principle of Newton's rings which gives directly the birefringence, δn of the liquid crystal dimer. The merits of the technique used are presented over the conventional techniques for the determination of orientational order parameter. The results for the two compounds are compared with those values estimated from n _e, n _o and density using the two internal field models due to Vuks and Neugebauer applicable to nematic phase.     

Phase transitions in liquid crystals under negative pressures

We report the first measurements of orientational order parameters and phase transition temperatures in nematic and smectic A liquid crystals under negative pressures generated by an isochoric cooling of small droplets embedded in a glass former. Comparison of isobaric and isochoric measurements allows us to estimate the coefficients coupling the order parameter and density of an extended Landau--de Gennes model of the nematic phase.     

Macroscopic behavior of non-polar tetrahedratic nematic liquid crystals

We discuss the symmetry properties and the macroscopic behavior of a nematic liquid crystal phase with D_2d symmetry. Such a phase is a prime candidate for nematic phases made from banana-shaped molecules where the usual quadrupolar order coexists with octupolar (tetrahedratic) order. The resulting nematic phase is nonpolar. While this phase could resemble the classic D _∞h nematic in the polarizing microscope, it has many static as well as reversible and irreversible properties unknown to nonpolar nematics without octupolar order. In particular, there is a linear gradient term in the free energy that selects parity leading to ambidextrously helical ground states when the molecules are achiral. In addition, there are static and irreversible coupling terms of a type only met otherwise in macroscopically chiral liquid crystals, e.g. the ambidextrous analogues of Lehmann-type effects known from cholesteric liquid crystals. We also discuss the role of hydrodynamic rotations about the nematic director. For example, we show how strong external fields could alter the D_2d symmetry, and describe the non-hydrodynamic aspects of the dynamics, if the two order structures, the nematic and the tetrahedratic one, rotate relative to each other. Finally, we discuss certain nonlinear aspects of the dynamics related to the non-commutativity of three-dimensional finite rotations as well as other structural nonlinear hydrodynamic effects.     

Low-dimensional transport of the triplet excitation of chrysene in porous matrices

The transport kinetics of the triplet state of solid chrysene embedded in the channels of various porous matrices is studied by observing its delayed luminescence. As matrices, a porous glass was used, which was obtained by leaching a sodium borosilicate glass and natural chrysotile-asbestos minerals. It is shown that the annihilation kinetics of the triplet states for all the samples studied is inhomogeneous, with the rate described by a power dependence on time. The value of the exponent depends on the system topology. Its values obtained from the analysis of the decay kinetics of delayed luminescence suggest that the geometry of the porous network of a sodium borosilicate glass is close to that of a three-dimensional percolation cluster.