Determination of the orientational order parameter of the homologous series of 4-cyanophenyl 4-alkylbenzoate (n.CN) by different methods

The orientational order parameters of a homologous series of 4-cyanophenyl 4-alkylbenzoates have been determined at different temperatures from (13)C-NMR, x-ray diffraction, optical birefringence, high resolution density and diamagnetic susceptibility anisotropy measurements. To determine the temperature dependence of the orientational order parameter from the (13)C chemical shift values, the two-dimensional polarization inversion spin exchange at the magic angle (PISEMA) method was also used for the measurement of (13)C-(1)H dipolar couplings at different sites in the oriented phase. The nematic order parameters determined from each of these methods have been compared. Apart from a slight shift in their values their trends with temperature are very similar. The differences among the results obtained by these five different methods have been discussed. From the high resolution density data, the values of the critical exponents near the T(N-I) transition are found to lie between the Ising model and tricritical behaviour.     

Strong orientational effect of stretched aerogel on the (3)He order parameter

Deformation of aerogel strongly modifies the orientation of the order parameter of superfluid (3)He confined in aerogel. We used a radial squeezing of aerogel to keep the orbital angular momentum of the (3)He Cooper pairs in the plane perpendicular to the magnetic field. We did not find strong evidence for a polar phase, with a nodal line along the equator of the Fermi surface, predicted to occur at large radial squeezing. Instead we observed (3)He-A with a clear experimental evidence of the destruction of the long-range order by random anisotropy-the Larkin-Imry-Ma effect. In (3)He-B we observed and identified new modes of NMR, which are impossible to obtain in bulk (3)He-B. One of these modes is characterized by a repulsive interaction between magnons, which is suitable for the magnon Bose-Einstein condensation.     

Orientational ordering from molecular polarizabilities and an odd-even effect in nCB homologous series

The molecular polarizabilities of the homologous series of cyano-alkyl-biphenyl (C _n H_2n+1.φφ.CN for n = 5 to 9) have been calculated using two different internal field models. From these values the orientational order parameters have been obtained. The orientational ordering exhibits an odd-even alteration along the series. The variations of different parameters with the increase of alkyl chain length are discussed. The relative stabilities of the alkyl-cyano-biphenyls are also discussed in terms of the molecular parameters.     

Supersymmetric representation of order parameter fluctuations

We consider the representation of homogeneous and heterophase fluctuations based on the supersymmetric theory of fields. It is shown that the problem reduces to a mixture of noninteracting Fermi and Bose gases. The thermodynamic features resulting from each type of fluctuations are determined, as well as the correlation functions of the spatial distributions of the order parameter and interphase boundaries.Suma Physicotechnical Institute. RETO Scientific-Industrial Complex. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 49–53, April, 1994.     

EPR investigations of critical order parameter fluctuations in ferroelectric tris-sarcosine calcium chloride (TSCC)

The EPR spectra of Mn²⁺-doped TSCC show in the immediate vicinity of the ferroelectric transition a critical line broadening. It is angular-dependent and may be related to a critical slowing down of the polarization fluctuations at the phase transition point. The observed line asymmetry points at a strong influence of the real structure of the crystal on the effects investigated.     

Enhancement of the orientational order parameter of nematic liquid crystals in thin cells

We report measurements of birefringence of several nematic liquid crystals having transverse as well as longitudinal dipole moments in thin (1.4 to ) and thick (7 to cells. Rubbed polyimide-coated glass plates are used to get planar alignment of the nematic director in these cells. We find significant enhancement (6 to ) of ( , where S is the orientational order parameter) in thin cells in all compounds with aromatic cores even at temperatures far ( C) below the nematic-isotropic transition point. The enhancement is larger in compounds having several phenyl rings and lower if the number of phenyl rings is reduced. In a compound that does not have an aromatic core no significant enhancement is observed, implying that the strength of the surface potential depends on the aromaticity of the cores. Assuming a perfect orientational order at the surface, calculations based on the Landau-de Gennes theory show that the thickness averaged enhancement of S is sharply reduced as the temperature is lowered in the nematic phase. The measured order parameter S is further enhanced in thin cells because of the stiffening of the elastic constant which reduces the thermal fluctuations of the nematic director. The combined effect is however too small at low temperatures to account for the experimental data.Received: 22 February 2004, Published online: 24 May 2004PACS: 61.30.-v Liquid crystals - 61.30.Pq Microconfined liquid crystals: droplets, cylinders, randomly confined liquid crystals, polymer dispersed liquid crystals, and porous systems - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitionsSurajit Dhara: Present address: Department of Physics, Birla Institute of Technology and Science, Pilani 333 031, India.     

Giant fluctuations of superconducting order parameter in ferromagnet-superconductor single-electron transistors

Spin dependent transport in a ferromagnet-superconductor single-electron transistor is studied theoretically taking into account spin accumulation, spin relaxation, gap suppression, and charging effects. A strong dependence of the gap on the magnetic state of the electrodes is found, which gives rise to a magnetoresistance of up to 100%. We predict that fluctuations of the spin accumulation can play such an important role as to cause the island to fluctuate between the superconducting and normal states. Furthermore, the device exhibits a nearly complete gate-controlled spin-valve effect.     

Spatial fluctuations of the order parameter and the peak effect in type-II superconductors

Near H_c2 even weak crystal inhomogeneities may lead to large relative fluctuations of |ψ|², to large flux line displacements, and to a volume pinning force quadratic in the pinning potential.     

On order parameter fluctuations in Rochelle salt detected by Brillouin scattering

In both of the nonpolar phases of ferroelectric-ferroelastic Rochelle salt frequency and linewidth of the quasilongitudinal acoustic phonon propagating along [011] have been investigated in a high resolution Brillouin- scattering experiment. Softening and damping of this mode can be quantitatively explained by a piezoelectric coupling to the fluctuations of the polarization along the ferroelectric axis, P₁.     

Temperature dependence of the photonic bandgap and the orientational order parameter for a cholesteric photonic crystal

The reflection spectra of a cholesteric photonic crystal have been measured. The experimental spectra are described by a theoretical expression that follows from the analytical solution of the Maxwell equations. The photonic bandgap width Δν has been determined. The photonic bandgap width changes abruptly as the position of the diffraction band changes. The temperature dependence of the relative bandgap width Δν/ν₀ and the order parameter for a photonic crystal are described by Landau’s theory of phase transitions.     

Local orientational order in the stockmayer model

A new method based on the local analysis of the orientational order has been presented for analysis of the phase behavior of the Stockmayer fluid. A quantity has been introduced to quantitatively describe the ordering degree of particles at small distances. It has been used to analyze the phase diagram of the model under consideration.     

Surface-induced weak orientational order and role of isotropic-nematic interface fluctuations in the appearance of an induced nematic film

Recently the nontrivial spatial and temperature dependence of the surface-induced weak planar orientational order parameter Q(z, T) was determined just above the isotropic-nematic (IN) phase transition point (Ji-H. Lee et al., Phys. Rev. Lett. 102, 167801 (2009)). In this paper we present a theoretical explanation of the observed behaviour. We obtain expressions for the short-range and long-range contributions to the interface potential of the induced nematic film and specify the repulsive character of the interaction between the soft IN interface and the external bounding substrate. It is shown that the small value of the IN interfacial tension results in the renormalization of the repulsive interaction potential due to the thermal fluctuations of the soft IN interface. This leads to an increase of the equilibrium thickness of the induced nematic film and the appearance of a step-like orientational order parameter profile. We find that only renormalized short-range and thermal pseudo-Casimir interactions are essential for the appearance of the induced nematic film, which provide the observed thickness, h ?? 30 nm, of this film. The long-range van der Waals interaction is shown to be negligibly small and the dominant role is played by the renormalized short-range repulsion. Fitting of the experimental order parameter profiles (Ji-H. Lee et al. (2009)) with the expressions based on these interactions makes it possible to determine the material parameters of the system, including the amplitudes of the surface interaction, the IN interfacial tension and the interfacial coherence length. The agreement between theory and experiment confirms the importance of the interface fluctuation renormalization of the interface potentials for soft interfaces.      

The phase-transition order parameter, fluctuations, and nanoclusters in benzil crystals

Bragg and diffuse X-ray reflections from the M points of the Brillouin zone boundary of a benzil crystal paraphrase, the integral intensity of which is associated with the order parameter and its fluctuations, are studied. Original data on the temperature behavior of the order parameter and diffuse scattering are obtained. The relevant critical exponents are determined. A conclusion is drawn as to the proximity of the phase transition in these crystals to the tricritical point. A phase transition model is discussed.     

Mesomorphic properties and orientational order parameter of 4-pentyl cyclohexyl-4-(4-propyl cyclohexyl) benzoate from X-ray diffraction studies

The mesogen 4-pentyI cyclohexyl-4-(4-propyl cyclohexyl) benzoate, molecular formula C₂₇H₄₂O₂, has been investigated at different temperatures using X-ray and texture studies. For X-ray studies an arrangement for taking X-ray photographs at different temperatures has been designed and fabricated. In addition to the nematic phase the mesogen exhibits two smectic phases. On the basis of microscopic investigation and X-ray studies the two smectic phases exhibited have been identified as the SmE and SmB phases. From X-ray patterns, the average intermolecular distances and apparent molecular length/layer thickness for the different mesomorphic phases have been determined as a function of temperature. Orientational order parameters (P2) and (P4) of the sample ordered by a magnetic field have been determined in the SmB phase.     

N.M.R. investigation of critical orientational order fluctuations below the nematic-isotropic transition in liquid crystals

The high-resolution infra-red spectrum of SiH₄ in the region 2101 cm^-1 to 2265 cm^-1 has been analysed. Most of the lines observed have been assigned to transitions of the v ₃ and v ₁ bands of the abundant isotopic species ²⁸SiH₄. The v ₁ band is formally forbidden in the infra-red, but a vibration-rotation interaction between v ₁ and v ₃ lends intensity to the v ₁ transitions. The spectrum has been fitted by diagonalizing the v ₃ = 1 and v ₁ = 1 hamiltonians coupled by the vibration-rotation interaction term. 500 transitions have been fitted with an overall standard deviation of 0·007 cm^-1, using only 15 adjustable parameters (ten in the v ₃ = 1 hamiltonian, four in the v ₁ = 1 hamiltonian, and one interaction coefficient). The calculated intensities are also in good agreement with experiment. Transitions of the other isotopic species ²⁹SiH₄ and ³⁰SiH₄ have also been observed, but these spectra have not been analysed in detail.     

Density of states of narrow superconducting channels in the regime of quantum fluctuations of the order parameter

The current–voltage characteristics of superconductor–insulator–semiconductor (S1–I–S2) tunnel junctions, where superconducting electrode S2 is a thin nanowire, are studied experimentally. The observed blurring of the gap singularities is interpreted as a manifestation of the order parameter quantum fluctuations. We propose a model taking into account the broadening of the density of states due to the interaction of electrons with the Mooij–Schön plasmon mode emerging in a quasi-one-dimensional superconducting channel in the regime of quantum fluctuations of the order parameter. The model gives results that are in a reasonable qualitative agreement with the experimental data.