Polyamorphic transformation of silicon in first-principles molecular-dynamics simulation

Isothermal–isobaric first-principles molecular-dynamics simulations were performed to investigate polyamorphic transformations of silicon. By pressurizing a low-density amorphous form of silicon (normal amorphous silicon with tetrahedral coordination), a new high-density amorphous form that has a strong resemblance to high-density amorphous water is obtained. It is found that the high-density amorphous form is reversely transformed to the low-density amorphous form by decompression and heating. The present simulations have revealed remarkable similarities between silicon and water, which is definitely of use to understand phase relations of polyamorphs of tetrahedrally coordinated substances.     

Theory of Systems of Rodlike Particles: I. Athermal systems

An improved treatment of the lattice model proposed in 1956 is presented for a system of hard rods, with axial ratio x, dispersed in a diluent. Interparticle forces, apart from repulsions on contact, are deliberately ignored within the scope of the present paper. The equilibrium distribution of orientations of the rodlike particles in a phase with nematic order is derived and explicitly taken into account. Thermodynamic properties in the asymptotic limit of high degree of order reduce approximately. to those obtained previously. Calculations carried out for equilibrium between anistropic (nematic) and isotropic phases yield ν′_x/ν_x = 1.4653 for the ratio of volume fractions in the respective phases in the limit x → ∞. The product xν_x, expressing the combined covolume of solute species in the isotropic phase at coexistence is 7.89 in the same limit. The calculated critical value of the axial ratio for coexistence of the two phases in the neat liquid is x_crit, = 6.417. Results are compared with those of other theories. Those that rely on the virial expansion are subject to errors implicit in treating the solvent as a continuum.     

Local microstructure of liquid and amorphous Al2O3

We present a local microstructure study of liquid and amorphous alumina by means of a molecular dynamic method. The simulation was performed on systems prepared at five different densities and at temperatures from 400 to 800 K for amorphous glass and at 3000 K for liquid. The local microstructure has been analyzed through the pair radial distribution function, bond-angle distribution and the characteristics of voids and void aggregations. We focused on two kinds of void aggregations: void clusters and void tubes. The results showed that many voids clustered nearby and created a very large cavity with volume five times bigger than volume of the aluminum atom. In addition, we found a void tube in low-density systems, which consisted of 75% of all voids and spread over the entire system. The diffusion mechanism in low-density and high-density systems has also been discussed and presented here.     

Computer simulation of GeO2 liquid

The local structure and dynamic of liquid GeO₂ have been investigated by mean of molecular dynamics simulation. We have prepared 13 systems upon pressure ranging from −0.41 to 48.9 GPa. The local structure was analyzed through the pair radial distribution function, coordination number and bond-angle statistics, the characteristics of voids and void aggregations. Two kinds of void aggregations have been examined: void clusters and void tubes. It was found a large number of vacancy-like voids and microscopic cavities in low-density system. Furthermore, there is a large void tube consisting of 94% oxygen vacancy-like voids and it spreads over whole system. The considerable changes in dynamic and local structure between high-density and low-density systems indicated the possibility of the liquid–liquid phase transition.     

单分子膜系统中二维生长过程的研究

当处于气液界面的类脂类化合物的单分子膜被压缩时,随着分子间距的缩小,单分子膜将经历一系列相变过程.通过荧光显微术可以观测到新相的成核和生长过程.由于单分子膜的二维特性,该系统中的实验观测对于检验和发展二维界面生长理论尤为重要.本文总结了近年来本课题组与相关单位合作,在单分子膜系统中发现的实验现象以及对其生长机制的系列研究.内容包括对单分子膜系统中的成核、界面稳定性、枝晶生长、形态演变等的观测和分析.     

Anisotropic Surface Order in the Isotropic Phase of Nematic Liquid Crystals

A spinning-drop technique was used to study the thermal variation of surface tension of two nematic liquid crystals at the interface with an isotropic fluid. A sharp increase in interfacial tension was observed in the vicinity of the nematic-isotropic transition. The interfacial tension-temperature characteristics, unlike monotonically decreasing dependence found in most isotropic fluids, showed regions of positive slope, both in the isotropic and anisotropic phases. A positive slope in the isotropic region of the liquid crystal is interesting since it implies that excess order has developed at the surface while the buk interior remains isotropic.     

Direct density determination of low- and high-density glassy polyamorphs following a liquid–liquid phase transition in Y2O3–Al2O3 supercooled liquids

Evidence has been presented for a density-driven phase transition occurring between supercooled liquids in the system Y₂O₃–Al₂O₃. The high- and low-density liquids were quenched to metastably coexisting glasses. Chemical analysis showed the compositions of the two glasses to be identical, and it was inferred that they differed in their densities and entropies. The entropy difference has been verified by calorimetry. Here, we confirm that the chemical compositions of the glassy materials derived from the high- and low-temperature liquids are identical. We present a direct density determination of the two glasses using sink-float techniques. The measured densities are 3.72(3) g/cm³ for the glass derived from the high-temperature liquid (i.e., the high-density amorphous or HDA polyamorph), and 3.58(1) g/cm³ for the low-temperature (low-density, LDA) polyamorph.     

Water in nanoconfined and biological environments : (Plenary Talk, Ngai-Ruocco 2009 IDMRCS Conf.)

We discuss some recent progress in understanding the anomalous behavior of liquid water, by combining information provided by recent experiments and simulations on water in bulk, nanoconfined, and biological environments. We interpret evidence from recent experiments designed to test the hypothesis that liquid water may display “polymorphism” in that it can exist in two different phases — and discuss recent work on water's transport anomalies as well as the unusual behavior of water in biological environments. Finally, we will discuss how the general concept of liquid polymorphism may prove useful in understanding anomalies in other liquids, such as silicon, silica, and carbon, as well as metallic glasses which have in common that they are characterized by two characteristic length scales in their interactions.     

Anisotropic properties of two nematogenic compounds

The diamagnetic susceptibilities and optical polarizabilities of two materials that exhibit nematic liquid crystralline phases in the vicinity of room temperature are reported using a molecular vibration approach and a modified Lippincott δ function potential model. The diamagnetic susceptibilities calculated from polarizabilities are also reported.     

Structural properties of silicon nanoparticles formed by pulsed laser ablation in liquid media

Silicon nanoparticles have been formed as a result of the irradiation of single-crystal silicon targets in distilled water and liquid nitrogen, by, respectively, picosecond and femtosecond laser pulses. The main structural properties of these nanoparticles have been investigated by atomic force microscopy, transmission electron microscopy, electron diffraction, Raman scattering, and photoluminescence spectroscopy. These particles are found to be mainly spherical. The presence of crystalline and amorphous silicon phases under picosecond ablation in water is established experimentally. Irradiation by femtosecond pulses in liquid nitrogen can yield nanoparticles smaller than 5 nm in size, which are quantum dots with a characteristic photoluminescence peak near 750 nm.     

EAM calculations of the thermodynamics of amorphous copper

The structure and thermodynamics of pure amorphous Cu is studied with molecular dynamics, Monte Carlo and quasi-harmonic calculations using the embedded atom method. The pair distribution function, atomic volume, thermal expansion, enthalpy, entropy, and Gibbs free energy are calculated for the cyrstalline, liquid and amorphous phases. The glass transition temperature of the amorphous phase is found to be approximately 400 K ( 0.3 T_m). The free energy difference between the amorphous and supercooled liquid phases was determined to be at most 0.01 eV per atom. The free energy of the liquid and crystalline phases in our calculation is found to agree within 0.4% of experiment over the temperature range 400–2200 K. The free energy of the glass is found to be fairly well described by the simple Turnbull model. This is the first atomic-level calculation of the free energy of a metallic glass.     

Growth and structure of superconducting FeSe crystals

Superconducting crystals of iron selenide FeSe are grown and their phase and elemental compositions are studied. The structural analysis of individual FeSe_{1 ? x} crystals showed the coexistence of two phases in this compound. The resistivities and magnetic susceptibilities of the crystals are investigated.     

Influence of impurity atmosphere on the deformation of silicon crystals

The Alexander–Haasen theory, which describes the deformation kinetics of silicon crystals, has been generalized for impurity crystals. The deformation kinetics of an impurity sample is calculated in a wide range of parameters, including the cases of partial and complete entrainment of impurities by moving dislocations. The developed model, despite its simplicity, adequately describes the qualitative transformation of the stress–strain curves of impurity silicon crystals in dependence of the impurity concentration and other material parameters. The manifestation of negative velocity dependence of the yield stress, observed in natural experiments, is analyzed.     

Bowlic and Polar Liquid Crystal Polymers

Bowlic liquid crystals are made up of polar molecules. Both main-chain and side-chain bowlic polymers are possible. Exactly solvable discrete models describing the phases and conformations of these bowlic and other polar liquid crystal polymers are presented. For the ideal one-dimensional case the model is equivalent to the 1D Ising model. Susceptibility and other properties are calculated. Wave propagation and solitons in these polar polymers are discussed. Possible highly conducting and high T_c superconducting liquid crystal polymers are proposed.     

Electrical Conductivity of Liquid Crystal Mixtures with Induced Smectic Phases

The electrical conductivity of the two liquid crystal mixtures N-(4-ethoxybenzylidene)-4′-amino benzonitrile (PEBAB)/4,4′-di-n-hexyloxy-azoxybenze (HEXOAB) and 4-n-heptyloxy-4′-cyanobiphenyl (7 CBP)/HEXOAB, which exhibit induced smectic A phases is investigated. In the smectic phases, the conductivity anisotropy of the PEBAB/HEXOAB mixtures is negative at the lowest PEBAB concentrations; this behaviour is usually expected for a smectic layer structure. With increasing PEBAB concentration the anisotropy increases and becomes positive. Possibly, this is an indication for a growing double layer structure, which was observed in polar smectic phases. In the nematic phases the conductivity anisotropy of the pure HEXOAB is considerably reduced by adding a relatively small amount of the polar component. An addition of 10 mole% 7 CBP reduces the anisotropy ratio of the electrical conductivity to V = K_∥/K ₁ ≈ 0.2, which probably is the lowest value observed in a nematic phase so far. Besides the negative conductivity anisotropy, these mixtures also exhibit a positive anisotropy of the dielectric constant. They thus fulfil the conditions for inverse dynamic scattering.     

Tetrahedral structures with icosahedral order and their relation to quasicrystals

The possible existence of quasicrystals in tetrahedral phases is considered. It is shown that one of the well-known crystalline silicon phases (the BC8 phase or silicon III) is characterized by the icosahedral local order with three-quarters of the interatomic bonds being directed along the fivefold axes of an icosahedron. This crystal is considered as an approximant of an icosahedral quasicrystal. Higher order approximants and other tetrahedral structures related to quasicrystals are also constructed. It is shown that in these structures, the formation of the intrinsic phason disorder with the preservation of the energetically favorable coordination number four is possible. The ab initio quantum-mechanical calculations for carbon and silicon show that, although all the considered phases are metastable, their energies only slightly differ from the energies of the corresponding stable phases.     

Chiral phases in a system of biaxial molecules

Chiral phases in a system of biaxial molecules are investigated based on a theory of biaxial liquid crystals, which we have presented in past work. Four chiral phases are identified, corresponding to a uniaxial nematic phase, two biaxial nematic phases, and a discotic phase, respectively. We find that in the chiral uniaxial nematic phase, the pitch of the chiral structures does not depend on temperature. In contrast, in the two chiral biaxial nematic phases, as well as in the chiral discotic phase, the pitch is temperature-dependent.     

Merck's Success in the Japanese Liquid Crystal Market

Abstract

Liquid crystal displays are a key component of the information society. They are found in many places in cars, measurement equipment, portable computers, and portable communication equipment. A typical state-of-the-art active matrix colour liquid crystal display today has a diagonal of 10 in, 550 × 650 pixels, and costs about US$ 1200 each in industrial quantities. In many products the liquid crystal display is the dominant component in terms of value. Each pixel has it's own transistor control circuit which at the moment uses amorphous silicon thin film transistor (TFT) technology. Very soon, however, polycrystalline silicon will be used instead of amorphous silicon. Since polycrystalline silicon is the same as that used in ordinary silicon VLSI, all other electronic circuits including the CPU can then be integrated onto the substrate of the liquid crystal display. Therefore, very soon, the business of all those companies will be threatened, who at present purchase liquid crystal displays externally, and build a product around it (see also: IEEE-Spectrum, May 1995, pages 62–69).     

Density Studies in Polymorphic Liquid Crystal N- (p-n-Heptyloxybenzylidene) p-n-Pentylaniline

Density studies in polymorphic liquid crystal, N-(p-n-heptyloxybenzylidene) p-n-pentyl-aniline is carried out in isotropic liquid, nematic, smectic A, smectic C, smectic B and smectic G phases. The temperature variation of density in all the mesophases confirms that isotropic liquid to nematic, nematic to smectic A, smectic C to smectic B, smectic B to smectic G transitions are of first order while smectic A to smectic C transition is of second order. The density variation across smectic C to smectic B is greater than that of any other transition. Thermal expansion coefficients are also calculated.