Change of the Prenematic Order in the Isotropic Phase Close to the Isotropic–Nematic Phase Transition with Increasing Aerosil Nanoparticles Density

The influence of the hydrophilic and hydrophobic aerosil nanoparticles density on the formation of the prenematic order close to the isotropic – nematic (I–N) phase transition is presented. The lowering of the activation energy, with the increase of aerosil nanoparticles density, suggests a change of the prenematic order in the isotropic phase. The temperature dependence of the static dielectric permittivity for 4–n–pentyl–4′–cyanobiphenyl (5CB) exhibits an anomaly near the I–N phase transition. With the increase of aerosil density, a substantial modification in the anomaly of the static permittivity was observed in the isotropic phase. Indicated changes are different for hydrophilic and hydrophobic aerosil as a result of different surface interaction between nematic molecules and aerosil nanoparticles.     

Quasi-elastic neutron scattering of cyanobiphenyl compounds with different terminal chains

Quasi-elastic neutron scattering measurements were performed for five cyanobiphenyl compounds with different terminal chains (5CB, 5*CB, 8OCB, 8*OCB and 6O2OCB) in their liquid and liquid-crystalline phases to investigate the molecular dynamics. The spectra were successfully decomposed into two components, indicating that two types of molecular motions are involved. For the broad (or fast) component, the temperature dependence as well as the momentum transfer dependence did not differ very much among those compounds. For the narrow (or slow) component, on the other hand, the compounds with a branched alkyl-chain (5*CB and 8*OCB) gave significantly smaller diffusion constants than the others. A molecular dynamics simulation for 5*CB and 5CB suggests that the reorientational motion of the whole molecule around the long molecular axis is responsible for the broad component, whereas a diffusive motion of the terminal chain is responsible for the narrow one.     

Structural,electrochemical and optical properties of 4′-n-alkyl-4-cyanobiphenyl (nCB) – A computational approach

The structures of nCB (n = 6 & 7 where n is the number of carbon atoms in the alkyl chain) have been optimized using the Becke3-Lee-Yang-Parr (B3LYP) hybrid functional with 6–31G+(d) basis set using the crystallographic geometry as input. The electronic structures of the dimer molecules have been computed using the optimized geometries. The spectra of the dimer molecules have been calculated by employing the DFT method. The features of electronic transitions and excited states have been calculated via configuration interaction singles (CIS) with the semiempirical Hamiltonian Zerner intermediate neglect of differential overlap (ZINDO). The photo sensitivity of liquid crystalline alkyl cyanobiphenyl has been presented on ultraviolet (UV) absorption based approach through Density functional theory (DFT) calculations. The structural and electrochemical properties such as HOMO (H), LUMO (L), and energy gap (Eg = E_L – E_H) have been investigated. A comparison of dimers during the different modes of interactions suggests an absorption maxima at longer wavelength for 7CB, indicating the high photo sensitivity. Further, the 6CB dimers exhibit a lower band gap; hence its conductivity is high in comparison with the 7CB dimers.     

Study of Isothermal Crystallization Kinetics of 5CB with Differential Scanning Calorimetry and Broadband Dielectric Spectroscopy

The isothermal crystallization kinetics of 4-n-pentyl-4–cyanobiphenyl (5CB) has been studied with Differential Scanning Calorimetry (DSC) and Dielectric Relaxation Spectroscopy (DRS). 5CB is a well characterized material which makes it ideal for a dielectric and thermal comparative study. The effect that isothermal crystallization exerts on the behavior of relaxation processes α and δ by cooling the isotropic liquid below the nematic phase or through the process of cold crystallization by heating it from the supercooled state is explored by comparing the DRS and DSC. Furthermore, by employing the Avrami equation, we compare the ability to probe the crystallization with each technique.     

Liquid crystalline phase transition studies in N(p-n-butoxybenzylidene) p-n decylaniline, 40.10

The synthesis, characterization and the temperature dependence of density and ultrasonic velocity of N(p-n-Butoxybenzylidene) p-n-decylaniline, 40.10 are presented. The compound exhibits as trivariant liquid crystalline mesomorphism Viz., NAB the Isotropic – Nematic, and Smectic A – Smectic B phase transitions are found to be first order. However, the interesting Nematic Smectic A (NA) transition shows a second order nature. The NA transition results are discussed in the light of the available data on other nO.m compounds. The computed molecular parameters: adiabatic compressibility (β_ad) molar sound velocity (R_n) and molar compressibility (A_w) are presented.     

A new equation for the temperature dependence of density of saturated aqueous solutions of electrolytes

A new empirical three-parameter equation d = d₀ exp[β′(T – T₀)²] (where β′, d₀, and T₀ are constants for a salt) has been proposed to describe the dependence of density d (g/ml³) of saturated aqueous solutions of electrolytes on temperature T (K) in the whole range of temperature in which solubility increases or decreases with an increase in temperature.     

Macroscopic properties and their temperature dependence of the mesogen 4 cyano-4/-n-nonyl biphenyl as a function of temperature

The thermal dependence of optical and dielectric properties of one member namely 4 cyano-4^/-n-nonyl biphenyl (code name 9CB) of the mesogenic homologous series of 4 cyano-4^/-n-alkyl biphenyl (nCBs) is reported. The variation of orientational order parameter with temperature is determined and comparison made with theoretical Maier–Saupe values as well as the order parameter values of the other member of the series. The effect of temperature on the effective molecular dipole moment μ and the angle of inclination β of the dipole axis with the preferred direction are investigated.     

Temperature quenching of photoluminescence of ordered GaInP2 alloy under different excitation densities

The photoluminescence (PL) emission from an epi‐structure containing an atomically ordered GaInP₂ layer and a GaAs layer was studied under excitation power densities of 0.03 – 3 W/cm²at temperatures of 10 to 300 K. The quenching of the integrated PL intensity from both: the GaInP₂and the GaAs layers is stronger under low excitation, than under high excitation density. The temperature dependence, however, have different shapes being the PL decay observed for the GaInP₂ layer stronger than that for the GaAs layer. Comparing the temperature dependence of the PL intensity from the ordered GaInP₂ and the GaAs layers under different excitation densities and analyzing them together, we conclude that the inhomogeneity of the ordered layer is responsible for the different temperature behavior of the GaInP₂ alloy PL emission. To explain the experimentally observed PL intensity temperature dependence an additional nonradiative recombination mechanism due to a thermally activated escape of the carriers from its confinement within regions of lower bandgap has to be taken into account. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal conductivity of glassy primary mono-hydroxylalcohols

Thermal conductivity κ(T) data for homologous series of primary mono-hydric alcohols – methanol, ethanol, 1-propanol, 1-butanol – within their molecular glass states are analyzed within the temperature region stretching from 2 K up to the glass transition T_g. Below 7 K κ(T) increases with the number of carbon atoms in the molecule. Furthermore, its temperature dependence exhibits a behavior akin to that predicted by the phenomenological model of soft potentials (SPM). The data are then rationalized in terms of the SPM parameters such as characteristic energy W and the parameter ¯С which measures the strength of the coupling of acoustic modes with two-level systems. The dependence of such parameters upon the molecular mass of the alcohol seems to agree with the SPM predictions. The parameters ¯С and W were used to calculate the boson peak energies. The values obtained agree well with literature data.     

Annealing Effects on Photoconductivity Spectra of CuInSe2 Single Crystals

Photoconductivity spectra of as-grown n-type and p-type CuInSe₂ single crystals and of crystals annealed under maximum and minimum selenium pressure are measured in the photon energy range hv = 0.75 – 3.0 eV and as a function of temperature in the range T = 80 – 320 K. A model with trapping of minority carriers is used to explain the temperature dependence of the photoconductivity. The trap energies present in the samples depend on their preparation conditions.     

New experimental results on nucleation of Au on NaCl: Dependence on deposition rate and time

Systematic experiments have shown that saturation density, nucleation rate, and condensation coefficient are considerably smaller than previously reported. The low condensation coefficient and the linear deposition rate dependence of the nucleation rate show that the model usually applied for the theoretical explanation of the nucleation process needs some revision. From measured condensation coefficients we have estimated the diffusion length of adatoms, x_s = (14.4 ± 2.4) nm, and the difference between desorption and diffusion energy, E_a – E_sd = (0.31 ± 0.01) eV. Possible reasons for the deviation of the new results from the earlier ones are discussed.     

Density studies across smectic-smectic transitions in 70.6 and 80.10

The temperature dependence of density in N(p-n-heptyloxy benzylidene) p-n-hexylaniline (70.6) and in N(p-n-octyloxy benezylidene) p-n-decylaniline (80.10) is carried out to study the nature of the phase transitions associated density jumps, to estimate the pressure dependence of transition temperatures and pretransitional effects. The compounds 70.6 exhibits a phase variant ACFG whereas 80.10 exhibits a new phase sequence variant ABF. The isotropic-smectic A, semctic A-smectic B and semctic C-smectic F phase transitions are found to be first order. The calculated thermal expansion coefficient also supported the density results. The density jumps, associated enthalpies and estimated pressure dependence of transition temperatures at the isotropic-smectic A phase transformation in other compounds are presented.     

Linear optical properties and Raman spectroscopy of natural fluorapatite

Precision refractive indices and their dispersion in the visible and near IR range (430 – 2400 nm) and thermal expansion (including its anisotropy and temperature dependence between 144 K and 673 K) were determined for gem‐quality crystals of fluorapatite from Durango, Mexico. In addition, results of a polarized Raman spectroscopy study on these crystals are given. Fluorapatite crystals show moderate values of thermal expansion with small anisotropy. The linear optical properties of the crystals allow no phase matching for third harmonic generation, but signalize, together with the Raman characteristics, a potential suitability of fluorapatite as nonlinear optical material for χ⁽³⁾‐based stimulated Raman scattering.     

On the analysis of the vibrational Boson peak and low-energy excitations in glasses

Implications of reduction procedures applied to the low energy part of the vibrational density of states in glasses and supercooled liquids are considered by advancing a detailed comparison between the excess – over the Debye limit – vibrational density of states g(ω) and the frequency-reduced representation g(ω)/ω² usually referred to as the Boson peak. Analyzing representative experimental data from inelastic neutron and Raman scattering, we show that reduction procedures distort to a great extent the otherwise symmetric excess density of states. The frequency of the maximum and the intensity of the excess experience dramatic changes; the former is reduced while the latter increases. The frequency and the intensity of the Boson peak are also sensitive to the distribution of the excess. In the light of the critical appraisal between the two forms of the density of states (i.e. the excess and the frequency-reduced one) we discuss changes of the Boson peak spectral features that are induced under the presence of external stimuli such as temperature (quenching rate, annealing), pressure, and irradiation. The majority of the Boson peak changes induced by the presence of those stimuli can be reasonably traced back to simple and expected modifications of the excess density of states and can be quite satisfactorily accounted for by the Euclidean random matrix theory. Parallels to the heat capacity ‘Boson peak’ are also briefly discussed.     

Strahlende Rekombination aus Nichtgleichgewichtszuständen in katodenstrahlangeregtem GaN

The radiative recombination of electron-beam excited hot carriers in GaN is investigated at liquid neon temperature. An emission peak at 3.495 eV beyond the free exciton energy level has been found which seems to arise from a band-to-band transition of free carriers. When high concentrations of carriers are created a thermal quasiequilibrium in the free carrier system is established with a temperature T_e higher than that of the lattice and growing with increasing excitation density. – Analyzing the dependence of T_e on excess-carrier generation rate, we find that the polaroptical scattering is the dominant energy loss mechanism.     

Density Studies in Terephthalylidene-bis-p-n-dodecylaniline

The temperature dependence of density in Terephthalylidene-bis-p-n-dodecylaniline (TBAA12) is carried out to investigate the phase transitions, associated volume jumps, order of the transitions and to estimate the pressure dependence of transition temperatures and pretransitional effects. The compound exhibits smectic C, smectic I, smectic F and smectic G phases. The isotropic-smectic C and smectic C-smectic I transitions are found to be first order from the density jumps and the calculated thermal expansion coefficient data also supported the density results. The smectic I-smectic F transition is detectable by a change of slope in density studies, which is a second order transition inferred from thermal expansion coefficient variation with temperature.     

Photoconductivity Spectra of n-type CuInSe2 Single Crystals

Photoconductivity spectra of n-type CuInSe₂ single crystals are measured in the photon energy range hv = 0.75 – 3.1 eV and as a function of temperature in the range T = 80 – 320 K. It is found that the photoconductivity of as-grown crystals is a nearly pure surface effect, while sensitization of the crystal volume is observed only after sufficiently long annealing in the presence of powdered material. To explain the temperature dependence of the photoconductivity carrier trapping processes must be taken into account.     

The effect of temperature on the raman spectra of strontium formate dihydrate single crystals

The temperature dependence of the polarized Raman Spectra (3000 cm⁻¹ – 10 cm⁻¹, between 293 K and 10 K) of Sr(HCOO)₂ · 2H₂O(2D₂O) and partially deuterated samples is reported. From the analysis of the internal and external modes associated with water molecules and formate ions a phase transition is deduced. This is interpreted on the basis of the positional disorder of the hydrogen atoms, situated in non-equivalent lattice positions at low temperature.     

Phase transition studies in mesogenic dimers

Schiff base liquid crystal dimers, both symmetric and non‐symmetric, in which two anisotropic groups are linked by a flexible spacer, exhibit a rich variety of smectic mesomorphism. The interest in this class of mesogens stems not only from their ability to act as model compounds for semi‐flexible mainchain liquid crystal polymers but also from their quite different properties compared to conventional low molar mass liquid crystals (monomers). We report here the phase transition studies on two examples of these schiff base liquid crystal dimers using the Differential Scanning Calorimetry and density measurements as a function of temperature. The symmetric liquid crystal dimer, α,ω‐bis (4‐n‐dodecylaniline benzylidene‐4′‐oxy)decane (10.O12O.10) exhibits a very rare Isotropic to G transition. Where as, the non‐symmetric dimer, α‐(4‐cyano biphenyl‐4′‐yloxy)‐ω‐(4‐n‐decylanilinebenzylidene‐4′‐oxy)decane (CB.O10O.10) exhibits a rare nematic to intercalated smectic A phase transition. The transitions studied, isotropic to nematic and isotropic to G exhibit a large density jump at the transition confirming their first order nature. Nematic to intercalated smectic A transition is found to be a second order transition whose behavior is similar to nematic to smectic A transition. Calculated values of pressure dependence of transition temperature and thermal expansion coefficient are also reported.     

Dielectric relaxation of 4-cyano-4-n-pentylbiphenyl (5CB) thin layer adsorbed on carbon nanotube – MD simulation

The ultra-thin layer of mesogenous molecules 5CB adsorbed on a single walled carbon nanotube (SWNT) has been studied by molecular dynamics (MD) simulation method. The focus was on the temperature dependence (200 K < T < 350 K) and time evolution of the total dipole moment autocorrelation function. The frequency dependence of the dielectric loss of ultra-thin mesogene layer covering carbon nanotube has been also studied. The layer reveals a non-Debye dielectric relaxation. The shape of dielectric loss spectrum significantly depends on the temperature of the system studied.