Orientational order in three nO.m liquid crystalline compounds

Refractive index and density measurements have been carried out on three nO.m liquid crystalline compounds, namely, 4O.2, 4O.3 and 1O.10 belonging to the N-(p-n-alkoxybenzylidene)-p-n-alkylaniline series. From the data, the orientational order parameter has been estimated using Vuks and Neugebauer local field models. Furthermore, the orientational order parameter has been calculated directly from refractive index data employing the Vuks scaling factor method, Neugebauer f(B) parameter, effective geometry parameter and a method proposed by Kuczyński et al. It is observed that order parameter values estimated from different methods agree well near the nematic–isotropic transition and diverge as the nematic phase attains equilibrium. The temperature gradient of refractive indices and the nematic crossover temperatures have also been estimated for these compounds. The results obtained are compared and discussed.     

Orientational order parameter of two cyano nematogens by optical spectroscopy

Refractive index and density measurements of two cyano nematogens, namely p-hexyloxybenzylidene p-aminobenzonitrile and p-octyloxybenzylidene p-aminobenzonitrile have been carried out. From these data, orientational order parameter S has been estimated using different methods. Principal polarizabilities have been calculated using the well-known Vuks isotropic and Neugebauer anisotropic models. Polarizability anisotropy has been estimated using Lippincott δ-function, molecular vibration, Haller's extrapolation and Vuks scaling methods. Using these values, the orientational order parameter S has been estimated at different temperatures in their nematic phases. The values obtained have been compared and discussed. Further, using the experimental refractive index data, the nematic crossover temperatures have also been estimated for these compounds.     

An improved reflectometric method to measure the azimuthal anchoring energy of nematic liquid crystals

Some years ago we developed an automatized reflectometric method to measure the surface azimuthal anchoring energy of nematic liquid crystals on an optically isotropic substrate. This method provides a high accuracy and sensitivity but requires the use of wedge glass plates and a sufficiently high anisotropy of the intensity reflectivity coefficients. This latter condition restricts greatly the number of possible substrates that can be investigated with this technique. Here we develop a new reflectometric method which offers comparable or better accuracy and sensitivity but does not require wedge plates and high anisotropy of the reflectivity coefficients. The method is fully automated and provides a direct measurement of the azimuthal director angle. The experimental procedure exploits the dependence of the reflectivity tensor on the surface director orientation. The measurement of the azimuthal angle does not require any knowledge of the optical parameters of the nematic material and of the optically isotropic substrate, and provides an absolute accuracy better than 0.2^° in the whole range 0-360^° and a sensitivity better than 0.1^°. This reflectometric method can be also used with weakly anisotropic substrates as well as thin rubbed polymeric layers. In this latter case, the effective uncertainty in the measurement of the director azimuthal angle depends on the substrate anisotropy. A simple and direct experimental procedure to estimate this uncertainty is proposed. Received 8 January 2002 and Received in final form 27 February 2003 / Published online: 15 April 2003 RID="a" ID="a"e-mail: faetti@df.unipi.it     

EPR study of Dy 3 + ions in DyBa 2 Cu 3 O 6 + x

Concentrated polycrystalline DyBa₂Cu₃O_6+x compounds are studied by X-band EPR spectroscopy. A broad resonance line due to the highly anisotropic EPR spectrum of Dy³⁺ ions is identified on several specimens at low temperatures. Powder simulation of the EPR spectra complies with the ground Kramers doublet predicted by crystal field analysis of Dy³⁺ ions. Calculations of the second and fourth moments of the resonance lines due to the dipole-dipole interactions of Dy³⁺ indicate the presence of substantial exchange narrowing of the dipolar-broadened EPR linewidth. Received 3 June 2001     

Propagation properties of elastic waves in semi-infinite phononic crystals and related waveguides

The transmission and reflection coefficients of two-dimensional semi-infinite solid-solid phononic crystal systems and fluid-fluid phononic waveguide structures have been investigated. The numerical results show that the transmission spectra for longitudinally and transversally polarized incident waves are different, and the spectra of the transmission and reflection coefficients of the semi-infinite system agree well with the band structure. The numerical results show that when a guided wave incident, localized modes are excited, and different polarities have different coupling efficiencies with the incident guided wave. At the same time, far from the cutoff frequency, the guided wave couples out of semi-infinite waveguide highly efficiently.     

Antiferromagnetic order and phase transitions in GdS as studied with X-ray resonance-exchange scattering

We report on X-ray resonance exchange and neutron scattering of metallic GdS. At the L_II and L _III absorption edges of Gd, resonance enhancements of more than two orders of magnitude over the non-resonant magnetic scattering are observed. Polarisation analysis proves that these enhancements are due to dipolar transitions from the 2p to the 5d states. The branching ratio between the L_II and L _III edges of 2.5 suggests a polarisation of the 5d electrons in the ground state. The antiferromagnetic order is of type II in the fcc lattice. Single crystal diffraction of hot neutrons suggests that the spin direction lies within the (111) planes with a value for the sublattice magnetisation of 6.51(3) . The critical exponent for the sublattice magnetisation has a value of β = 0.38(2) in agreement with a pure Heisenberg model. Above T _N, a sharp component persists in the critical diffuse scattering. Lattice distortions give indications for two additional low-temperature phase transitions at about 49 K and 32 K. We argue that these transitions are not connected to spin reorientations and discuss the possible influence of fourth-order exchange interactions. Received 19 November 1999 and Received in final form 12 December 2000     

Phase transition and random-field induced domain wall response of SrTi O 3

The dielectric permittivity ε^′ - i of SrTi ¹⁸O ₃ (STO18) is studied under a dc electric field E as a function of the temperature, T. In ε^′ vs. T, a double-peak is found when 0 < E < 30 KV/m. While the peak at high-T is attributed to the smeared ferroelectric phase transition, the low-T one is induced by domain wall motion. The transverse Ising model including an external homogeneous and quenched random-fields is successfully used to describe both the smeared phase transition and the domain wall response in the low-T domain state. The calculations are in good agreement with the experimental results. Received 4 January 2002 / Received in final form 25 March 2002 Published online 19 July 2002     

Element-specific magnetic long- and short-range order and competing interactions in Gd x Eu 1 - x S

We report on an investigation of the magnetic properties of Gd _x Eu _{1 - x} S mixed crystals with compositions in the range of 0.6 < x < 1. For the two samples Gd _0.8 Eu _0.2 S and Gd _0.73 Eu _0.27 S a long-range antiferromagnetic order was observed at low temperatures. Element-specific measurements exhibited a different temperature dependence of the reduced sublattice magnetisation of the two magnetic species. A model calculation and Monte Carlo simulations revealed that the different temperature dependence is due to frustration effects. These frustration effects lead to a breakdown of the long-range order for higher europium contents. For the Gd _0.67 Eu _0.33 S-sample we were able to observe a short-range antiferromagnetic order with correlation lengths of a few 10? with X-ray resonance exchange scattering. Received 25 April 2001 and Received in final form 3 December 2001     

Note on the impact parameter analysis of high-energy proton-proton collisions

Following a prior analysis of measured pp elastic differential cross-sections, the impact parameter representation in terms of profile functions is calculated from two different parametrizations of single diffractive dissociation data. The derivative of this quantity, with respect to the collision energy squared s, measures the growth rate of the reaction's blackness. Its distribution in impact parameter space allows detailed insight into the growth pattern of the total diffractive cross-section and the approaching unitarity limit. Comparing the results with the elastic case, the different mechanisms of unitarization of two parametrizations are discussed. Received: 19 January 2001 / Accepted: 11 July 2001     

A new scaling analysis of the susceptibility and the specific heat of the non-Fermi liquid system with and 0.5

The usual scaling equations at a phase transition, employed out of their usual validity range, with T_c a negative constant, fit properties observed in correlated electron systems with unmatched accuracy. We illustrate this behavior with our data in Ce(Ru_1-xRh_x)₂Si₂ for x = 0.4 and 0.5 and comment these results and what they imply physically. Received 22 June 1998 and Received in final form 29 September 1998     

Pseudo-gap and possible Spin-Peierls transition in the vanadium oxide VOSb 2 O 4

We report on the magnetic susceptibility and electron spin resonance measurements on polycrystalline samples of the vanadium oxide VOSb₂O₄, a quasi-one-dimensional S = 1/2 Heisenberg system. The susceptibility vanishes at zero temperature, but there is no cusp at the onset of the susceptibility drop, and the ESR linewidth exhibits an increase characteristic of a phase transition at a much lower temperature. We show that this behaviour is consistent with the formation of a pseudo-gap in a spin-Peierls system in the adiabatic limit. Received 7 February 2001 and Received in final form 24 April 2001     

Adatom diffusion on a square lattice. Theoretical and numerical studies

A two-dimensional lattice-gas model with square symmetry is investigated by using the real-space renormalization group (RSRG) approach with blocks of different size and symmetries. It has been shown that the precision of the method depends strongly not only on the number of sites in the block but also on its symmetry. In general, the accuracy of the method increases with the number of sites in the block. The minimal relative error in determining the critical values of the interaction parameters is equal to . Using the RSRG method, we have explored phase diagrams of both a two-dimensional Ising spin model and of a square lattice gas with lateral interactions between adparticles. We also have investigated the influence of the attractive and repulsive interactions on both the thermodynamic properties of the lattice gas and the diffusion of adsorbed particles over surface. We have calculated adsorption isotherms and coverage dependences of the pair correlation function, isothermal susceptibility and the chemical diffusion coefficient. In addition, we have included in our analysis the interaction of the activated particle in the saddle point with its nearest neighbors. We have also used Monte Carlo (MC) technique to calculate these dependences. Despite the fact that both methods constitute very different approaches, the correspondence of the numerical data is surprisingly good. Therefore, we conclude that the RSRG approach can be applied to characterize the thermodynamic and kinetic properties of systems of particles with strong lateral interactions. Received 1st September 1998 and Received in final form 8 March 2000     

DFT study of BaTiO<Subscript>3</Subscript> (001) surface with O and O<Subscript>2</Subscript> adsorption

Progress of scanning tunneling microscopy (STM) allowed to handle various molecules adsorbed on a given surface. New concepts emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale is thus particularly invaluable. In this work, within the framework of density functional theory (DFT), we present an electronic and structural ab initio study of a BaTiO₃ (001) surface (perovskite structure) in its paraelectric phase. As far as we know the atomic and molecular adsorption of oxygen at surface is then analyzed for the first time in the literature. Relaxation is taken into account for several layers. Its analysis for a depth of at least four layers enables us to conclude that a reasonable approximation for a BaTiO₃ (001) surface is provided with a slab made up of nine plans. The relative stability of two possible terminations is considered. By using a kinetic energy cut off of 400 eV, we found that a surface with BaO termination is more stable than with TiO₂ termination. Consequently, a surface with BaO termination was chosen to adsorb either O atom or O₂ molecule and the corresponding calculations were performed with a coverage 1 on a (1×1) cell. A series of cases with O₂ molecule adsorbed in various geometrical configurations are also analyzed. For O₂, the most favorable adsorption is obtained when the molecule is placed horizontally, with its axis, directed along the Ba-Ba axis and with its centre of gravity located above a Ba atom. The corresponding value of the adsorption energy is -9.70 eV per molecule (-4.85 eV per O atom). The molecule is then rather extended since the O–O distance measures 1.829 ?. By comparison, the adsorption energy of an O atom directly located above a Ba atom is only -3.50 eV. Therefore we are allowed to conclude that the O–O interaction stabilizes atomic adsorption. Also the local densities of states (LDOS) corresponding to various situations are discussed in the present paper. Up to now, we are not aware of experimental data to be compared to our calculated results.     

Pressure dependence of the Shubnikov-de Haas oscillation spectrum of \beta'-(BEDT-TTF)4(NH4)[ Cr(C2 O4)3] .DMF

The Shubnikov-de Haas (SdH) oscillation spectra of the -(BEDT-TTF)₄(NH₄)[ Cr(C₂O₄)₃] .DMF organic metal have been studied in pulsed magnetic fields of up to either 36 T at ambient pressure or 50 T under hydrostatic pressures of up to 1 GPa. The ambient pressure SdH oscillation spectra can be accounted for by up to six fundamental frequencies which points to a rather complex Fermi surface (FS). A noticeable pressure-induced modification of the FS topology is evidenced since the number of frequencies observed in the spectra progressively decreases as the pressure increases. Above 0.8 GPa, only three compensated orbits are observed, as it is the case for several other isostructural salts of the same family at ambient pressure. Contrary to other organic metals, of which the FS can be regarded as a network of orbits, no frequency combinations are observed for the studied salt, likely due to high magnetic breakdown gap values or (and) high disorder level evidenced by Dingle temperatures as large as ≃7 K.     

A theoretical study on the excited states of MC3 (M = Sc, V, and Cr)

The ground state structures of MC₃ (M = Sc, V, and Cr) and their anions have been investigated, employing the first-principles DFT at the B3LYP level. The calculations predict that the equilibrium geometries of both neutral MC₃ and their anions are cyclic structures with C_2v symmetry. The Mulliken charge and spin populations of MC₃ and their anions have also been calculated, and it is found the electron charge changes mainly take place on the M atoms from anions to neutral molecules. The low-lying excited states for the clusters are calculated with time-dependent DFT to assign the features of the photoelectron spectra. Our results agree well with the available experimental and theoretical data.     

Charge distribution on annealed polyelectrolytes

We investigate the equilibrium charge distribution along a single annealed polyelectrolyte chain under different conditions. The coupling between the conformation of the chain and the local charge distribution is described for various solvent qualities and salt concentration. In salt free solution, we find a slight charge depletion in the central part of the chain: the charges accumulate at the ends. The effect is less important if salt is added to the solution since the charge inhomogeneity is localized close to the chain ends over a distance of order of the Debye length. In the case of poor solvent conditions we find a different charge per monomer in the beads and in the strings in the framework of the necklace model. This inhomogeneity leads to a charge instability and a first order transition between spherical globules and elongated chains. Received 19 March 1999 and Received in final form 2 August 1999     

Level lifetimes in 3p and 3d configurations of Fe XII

Transitions between the five fine-structure levels in the 3s ²3p ³ ground configuration of Fe XII (P-like) are of interest in astrophysics and terrestrial plasma diagnostics. The decay rates give rise to level lifetimes in the millisecond range, which have been measured recently at a heavy-ion storage ring. While most of the 3s ²3p ²3d levels are short-lived, two of these levels have no E1 decay channels and may also have millisecond lifetimes. We present HFR and MCDF calculations of the E1, M1, E2 and M2 transition rates between the 3s ²3p ³, 3s3p⁴ and 3s ²3p ²3d levels and compare the lifetime results to most recent experimental data as well as to other predictions. Received 2 October 2001 / Received in final form 22 January 2002 Published online 28 June 2002     

Raman light scattering,infrared absorption and DSC studies of the phase transition and vibrational and reorientational dynamics of H2O ligands and ClO4– anions in [Ba(H2O)3](ClO4)2

[Ba(H₂O)₃](ClO₄)₂ between 90 and 300 K possesses two solid phases. One phase transition of the first‐order type at: = 211.3 K (on heating) and = 204.6 K (on cooling) was determined by differential scanning calorimetry. The entropy change value (ΔS ≈ 15 Jmol^–1 K^–1), associated with the observed phase transition, indicates a moderate degree of molecular dynamical disorder. Both, vibrational and reorientational motions of H₂O ligands and ClO₄^– anions, in the high‐temperature and low‐temperature phases, were investigated by Fourier transform far‐infrared and middle‐infrared and Raman light scattering spectroscopies. The temperature dependences of the full‐width at half‐maximum values of the bands associated with ρ_w(H₂O) mode, in both infrared (~570 cm^–1) and Raman light scattering (~535 cm^–1) spectra, suggest that the observed phase transition is not associated with a sudden change of a speed of the H₂O reorientational motions. Ligands reorient fast, with correlation time of the order of several picoseconds, with a mean activation energy value E_a = 5.1 kJ mol^–1 in both high and low temperature phases. On the other hand, measurements of temperature dependences of full‐width at half‐maximum values of the infrared band at ~460 cm^–1, associated with δ_d(OClO)E mode, and Raman band at ~1105 cm^–1, associated with ν_as(ClO)F₂ mode, revealed the existence of a fast ClO₄^– reorientation in phase I and in phase II, with the E_a(I) and E_a(II) values equal to 8.0 and 6.5 kJ mol^–1, respectively. These reorientational motions of ClO₄^– are slightly distorted at the T_C. Fourier transform far‐infrared and middle‐infrared spectra with decreasing of temperature indicated characteristic changes at the vicinity of PT at T_C, which suggested lowering of the crystal structure symmetry. All these experimental facts suggest that the discovered phase transition is associated with small change of H₂O ligands and somewhat major change of ClO₄^– anions reorientational dynamics, and with insignificant change of the crystal structure, too. Copyright © 2012 John Wiley & Sons, Ltd.