Mechanical properties of graphene under molecular hydrogen physisorption: An ab initio study

This paper investigates the mechanical properties of graphene subjected to adsorption of molecular hydrogen through an ab initio approach. First, using density functional theory (DFT) with both generalized gradient and local density approximation functionals, the most stable configuration for physisorption of molecular hydrogen on the graphene is determined. All possible adsorption sites are considered, and it is revealed that the most stable state happens above the center of a hexagon with the equilibrium distance of 2.7 Å when the axis of the hydrogen molecule is parallel to the graphene surface. Thereafter, DFT calculations are performed to obtain the in-plane stiffness and Poisson’s ratio of graphene under the above-mentioned adsorption position. It is found that the effect of hydrogen physisorption on the mechanical properties of graphene is not very significant.     

QENS study of molecular motions in partially deuterated liquid crystal 4BT

Molecular rotational dynamic in the crystalline and smectic E phases of selectively deuterated (in the outer phenyl ring) 4-n-butyl-isothiocyanatobiphenyl was investigated by means of quasielastic neutron scattering. The 120° reorientation of the methyl group in the crystal phase could be identified at 125 K. Additionally, ring flips were seen at 200 K. The structure of crystal and smectic E phases was obtained by X-ray measurements and the data give the identification of type of solid phases.     

Surface transitions in nematic liquid crystals oriented with Langmuir-Blodgett films

In this paper two types of surface transitions in nematic liquid crystals are theoretically analyzed with a simple model in which the anchoring energy is the result of both steric and electric interactions. The physical model is proper to explain both the transition temperature experimentally observed in homeotropically aligned nematic liquid crystals, and the change in the mean molecular direction due to the variation of the number of surfactant films, piled on the boundary surfaces with the Langmuir-Blodgett technique.Partially supported by the TEMPUS program     

Nuclear magnetic resonance spectra of deuterated methanes and 2,2-dimethylpropane in nematic liquid crystals

A simple variational approach is used to study the expectation values <r ₁₂ ^-1> and <r ₁₂ ²> for the two-electron 1s2p ¹ P and 1s2p ³ P. Several ‘anomalous’ phenomena are clearly displayed by the results, which provide some grounds for criticism of traditional arguments concerning spatial correlation between parallel spins.     

Liquid-vapor interface in liquid binary alloys: an ab initio molecular dynamics study

We report the results of an ab initio molecular dynamics simulation of the liquid-vapor interface of two binary liquid alloys, Na(0.3)K(0.7) and Li(0.4)Na(0.6), whose bulk behavior exhibits rather differing ordering tendencies. The study has been performed using samples of 2000 and 3000 particles, respectively, in a slab geometry with periodic boundary conditions. In both cases, the total ionic density distributions along the normal to the interface display some layering with a virtually pure monolayer of the lower surface tension component located outermost at the interface. However, the two systems behave very differently below the interface which can be accounted for by their different ordering tendencies in the bulk.     

Surface structure of liquid Li and Na: an ab initio molecular dynamics study

Molecular dynamics simulations of the liquid-vapor interfaces of liquid metals have been performed using first principles methods. Results are presented for liquid lithium and sodium near their respective triple points, for samples of 2000 particles in a slab geometry. The atomic density profiles show a pronounced stratification extending several atomic diameters into the bulk, which is similar to that already experimentally observed in liquid K, Ga, In, and Hg.     

Optical Kerr effect in nematic liquid crystals: a molecular dynamics simulation study

Molecular dynamics simulations of the Gay-Berne model have been undertaken and optical response properties calculated for the isotropic and nematic phases. The components of the optical response were calculated for the coordinate system with respect to the director. Furthermore, the response for the components was separated into the orientational and collision induced contributions in order to analyse the mechanism of the optical response. It was observed, in particular, that one of the depolarized components of the response function does not vanish after long times for the nematic phase, unlike in the isotropic phase. This means that the orientation of the director can be permanently changed by instant irradiation with polarized light. The results give a microscopic picture of the optical Kerr effect in nematic liquid crystals.     

Thermodynamic functions of molecular hydrogen from ab initio energy levels

A tabulation is presented of the thermodynamic properties of molecular hydrogen (H₂, D₂ and HD), calculated using energy levels which had been obtained via a solution of the Schrödinger equation. The present results should be more accurate than prior computations based on spectroscopic extrapolation formulae. They provide the first example of an accurate determination of a bulk, macroscopic property directly from first principles (input: masses and Coulomb's law).

The results are tabulated for ortho, para, the equilibrium and the frozen (or ‘normal’) mixture (3 : 1 for H₂, 1 : 2 for D₂) over the temperature range 5–300 K and for the frozen mixture up to 6000 K.

The importance of anharmonicity and of quasi-bound levels at high temperatures is discussed.     

Random lasing from dye-doped chiral nematic liquid crystals in oriented and non-oriented cells

These are the experimental results describing random lasing in dye-doped chiral nematic liquid crystals. A novel random lasing emission is studied in this article based on the helical domains of dye-doped chiral nematic liquid crystals in oriented and non-oriented cells. Under frequency doubled 532 nm Nd:YAG (yttrium aluminum garnet) laser-pumped optical excitation, we carefully observed and analysed random lasing from dye-doped chiral nematic liquid crystals with wavelength ranges from 600 nm to 620 nm. In addition, the line-width of multi-mode peaks is less than 0.2 nm. The difference between the two random lasing behaviours in the oriented and non-oriented cells arises from the fact that random lasing appearing in the oriented cell results from stronger multiple scattering of light generated by the spiral domains of the liquid crystal molecules. Furthermore, chiral nematic liquid crystal micro-domains with different orientations can induce variation of the diffusion constant, thereby resulting in a decrease or increase in the lasing intensity of the random lasers, and an increase or decrease in their energy thresholds. In addition, a detailed comparison of the two experimental results is also presented in the article, showing the dependence of the lasing threshold and the number of lasing modes on the transport mean free path, the excited area, and the sample size. This process allows us to obtain a random laser by changing the structure of the sample, realising tunable random lasers at low cost.     

An equation of state for nematic liquid crystals

A derivation of an equation of state for nematic liquid crystals is given based upon slightly non-spherical molecules interacting via anisotropic attractive forces. The results are in reasonable agreement with the experimental data for p-azoxyanisole.     

Multi-centre bonding in the carboranes: An ab initio and semi-empirical molecular orbital study

The electronic structures of the closo-carboranes 1,5-C₂B₃H₅, 1,2-C₂B₄H₆, 1,6-C₂B₄H₆ and 2,4-C₂B₅H₇ are discussed using the results of ab initio calculations in both minimal and extended bases of contracted gaussian type functions fitted to Slater type orbitals. A comparison is made with the results from semi-empirical INDO calculations, and the bonding in 1,7-C₂B₆H₈, 1,6-C₂B₇H₉, 5,9-C₂B₇H₁₃, 1,6-C₂B₈H₁₀ and the 1,2-, 1,7- and 1,12-isomers of C₂B₁₀H₁₂ studied using the latter method.

The delocalized molecular orbitals from both the ab initio and semi-empirical calculations are transformed to localized orbitals to provide an analysis of the multi-centre bonding in these electron-deficient molecules, which requires no prior assumptions about the nature of the bonding.     

Threshold effects in homeotropically oriented nematic liquid crystals in an external electric field

The electric field-induced orientational transition in a homeotropically oriented nematic liquid crystal cell is investigated. The interaction with the field as a result of anisotropy of the permittivity and flexoelectric polarization is taken into account. For an arbitrary energy of interaction of the nematic with the substrate simple relations are derived for determining the threshold characteristics of the phase transition. It is shown that, in contrast with planar orientation, in fields above a critical value a periodic structure can occur only by virtue of the flexoelectric effect. The resulting dependences for the threshold parameters in the given geometry are exceptionally useful for determining experimentally the surface energy and the difference in the flexoelectric coefficients. Zh. éksp. Teor. Fiz. 116, 543–550 (August 1999)     

Phenylazoimidazole as a possible optical molecular switch: An ab initio study

By applying nonequilibrium Green's function formalism combined first-principles density functional theory, we investigate the electronic transport properties of the phenylazoimidazole optical molecular switch. The molecule that comprises the switch can convert between the cis and the trans forms upon photoexcitation. The influence of HOMO–LUMO gaps and the spatial distributions of molecular orbitals on the electronic transport through the molecular device are discussed in detail. Theoretical results show that the current through the trans form is significantly larger than through the cis form, which suggests this system has attractive potential application in future molecular switch technology.     

Multiple quantum and high-resolution NMR, molecular structure, and order parameters of partially oriented ortho and meta dimethyl-, dichloro-, and chloromethylbenzenes codissolved in nematic liquid crystals

We develop a strategy for analyzing complex nuclear magnetic resonance (NMR) spectra of several solutes codissolved in liquid-crystal phases. Spectral parameters of solutes m- or o-xylene were estimated by analyzing 2D multiple-quantum NMR spectra using a modified version of a least-squares fitting routine which adjusts chemical shifts, order parameters, structural parameters, and/or dipolar couplings independently. These estimates were used to facilitate analysis of the high-resolution spectra which contain resonances from many solutes. Calculated spectra of m- or o-xylene were subtracted from the experimental high-resolution spectra leaving resonances from the other solutes readily visible. Accurate spectral parameters of all codissolved solutes were determined from the high-resolution spectra. Order parameters and structural parameters (including vibrationally corrected parameters) of m- and o-xylene, m- and o-chlorotoluene, and m- and o-dichlorobenzene were calculated from the dipolar couplings.     

An ab initio molecular dynamics study of iron phases at high pressure and temperature

The crystal structure of iron, the major component of the Earth's inner core (IC), is unknown for the IC high pressure (P; 3.3-3.6 Mbar) and temperature (T; 5000-7000 K). There is mounting evidence that the hexagonal close-packed (hcp) phase of iron, stable at the high P of the IC and a low T, might be unstable under the IC conditions due to the impact of high T and impurities. Experiments at the IC P and T are difficult and do not provide a conclusive answer as regards the iron stability at the pressure of the IC and temperatures close to the iron melting curve. Recent theory provides contradictory results regarding the nature of the stable Fe phase. We investigated the possibility of body-centered cubic (bcc) phase stabilization at the P and T in the vicinity of the Fe melting curve by using ab initio molecular dynamics. Thermodynamic calculations, relying on the model of uncorrelated harmonic oscillators, provide nearly identical free energies within the error bars of our calculations. However, direct simulation of iron crystallization demonstrates that liquid iron freezes in the bcc structure at the P of the IC and T = 6000 K. All attempts to grow the hcp phase from the liquid failed. The mechanism of bcc stabilization is explained. This resolves most of the earlier confusion.     

X-ray study on the phase transitions in triammonium hydrogen disulfate crystals and deuterated crystals

The structural phase transitions in triammonium hydrogen disulfate crystals and deuterated crystals below room temperature have been studied by X-ray diffraction. Three phases are observed in the temperature range from 25°C down to — 160°C. The space groups in three different phases are identified as C2/c, P2/n (or Pn), and C2 for (NH₄)₃H(SO₄)₂ and (ND₄)₃D(SO₄)₂ crystals. No isotope effect on the structural phase transitions in these crystals could be detected by these studies. The occurrence of structural phase transitions caused by the reorientation of SO₄ groups and/or the shift of oxygen atoms from the sulfate atom in the SO₄ group are suggested from the diffraction photographs.