Phase diagram of two-dimensional binary Yukawa mixtures

We have used Ramakrishnan–Yussouff (RY) density functional theory (DFT) to explore the topology of the phase diagram of two-component charge stabilised colloidal suspensions confined to a two-dimensional plane. The particles of the system interact via purely repulsive soft core Yukawa potential. Pair correlation functions (PCFs) used as input informations in DFT were calculated by solving both the hypernetted chain (HNC) and Percus–Yevick (PY) integral equation theories. To test the relative performance of the HNC and PY theories in the context of phase transitions, we have also studied the corresponding one-component systems. We found that RY DFT with HNC PCFs does not stabilise solid in both the one- and two-component cases, whereas the PY theory does. By considering the freezing into the substitutionally disordered triangular solid, we found that the temperature-composition phase diagrams of the binary mixture are narrow spindles whose thickness depends on the symmetry of the mixture components and the value of the screening constant of the Yukawa potential. Although the phase diagram obtained by RY DFT with structural inputs calculated by the PY theory is found to be shifted to higher temperature region in the temperature-composition plane, however, it captures qualitatively all the essential features of the phase diagram. Our results are in principle verifiable through computer simulations and experiments.     

Fluid–triangular solid phase transitions in a system of two-dimensional nematic quadrupoles

Density functional theory of freezing is used to study the phase transitions in a system of spherical colloidal particles dispersed in nematic host confined to two dimensions. We have considered both the one-component and two-component systems of the colloidal dispersions. Particles are assumed to interact via director distortion-mediated purely repulsive potential which scales as the fifth power of the inverse interparticle separation. The pair correlation functions needed as input information in the density functional theory are calculated by solving Roger–Young integral equation theory. In one-component system, a triangular crystalline phase is found to be stable. On the other hand, considering the freezing of the fluid phase of the binary mixture into a substitutionally disordered triangular solid, the temperature–composition phase diagram is found to have spindle shape for the ratio of quadrupole moment of the particles of the components being 0.9 and 0.8. The phase diagram changes to an azeotrope at a ratio 0.7. The results are verifiable in real-space experiments on nematic quadrupoles confined to a two-dimensional plane.     

Freezing of Classical Fluids in a Quenched Random Potential

The phase diagram of a hard-sphere fluid in the presence of a random pinning potential is studied analytically and numerically. In the analytic work, replicas are introduced for averaging over the quenched disorder, and the hypernetted chain approximation is used to calculate density correlations in the replicated liquid. The freezing transition of the liquid into a nearly crystalline state is studied using a density-functional approach, and the liquid to glass transition is studied using a phenomenological replica symmetry breaking approach. In the numerical work, local minima of a discretized version of the Ramakrishnan-Yussouff free-energy functional are located and the phase diagram in the density-disorder plane is obtained from an analysis of the relative stability of these minima. Both approaches lead to similar results for the phase diagram. The first-order liquid to crystalline solid transition is found to change to a continuous liquid to glass transition as the strength of the disorder is increased above a threshold value.     

Effect of shape anisotropy on the phase diagram of the Gay-Berne fluid

We have used the density functional theory to study the effect of molecular elongation on the isotropic-nematic, isotropic-smectic A and nematic-smectic A phase transitions of a fluid of molecules interacting via the Gay-Berne intermolecular potential. We have considered a range of length-to-width parameter 3.0 ⩽ x₀ ⩽ 4.0 in steps of 0.2 at different densities and temperatures. Pair correlation functions needed as input information in density functional theory are calculated using the Percus-Yevick integral equation theory. Within the small range of elongation, the phase diagram shows significant changes. The fluid at low temperature is found to freeze directly from isotropic to smectic A phase for all the values of x₀ considered by us on increasing the density while the nematic phase stabilizes in between isotropic and smectic A phases only at high temperatures and densities. Both isotropic-nematic and nematic-smectic A transition density and pressure are found to decrease as we increase x₀. The phase diagram obtained is compared with computer simulation result of the same model potential and is found to be in good qualitative agreement.     

Phase behavior of thermally responsive microgel colloids

The phase behavior of poly-N-isopropylacrylamide (PNIPAM) nanoparticles dispersed in water is investigated using a thermodynamic perturbation theory combined with light-scattering and spectrometer measurements. It is shown how the volume transition of PNIPAM particles affects the interaction potential and determines a novel phase diagram that has not been observed in conventional colloids. Because both particle size and attractive potential depend on temperature, PNIPAM aqueous dispersion exhibits phase transitions at a fixed particle number density by either increasing or decreasing temperature.     

Rigorous results in the mean-field theory of freezing

The mean-field theory of freezing, of which the Kirkwood-Monroe theory is a special case, is formulated in terms of a variational principle which can be derived from a statistical model. New upper and lower bounds on the free energy are found, and used to prove the existence of and to locate a freezing transition. The Kirkwood-Monroe theory is shown to obey a scaling law which implies that the freezing transition has no critical point and that the pressure-temperature phase diagram is a parabola. For a suitable choice of interaction potential, the free energy and density distribution of a crystalline state are calculated with very small error. The variational principle yields an integral equation found previously by van Kampen. This equation is derived rigorously, and shown to admit solutions of any chosen periodicity. The equation is transformed into a nonlinear integral equation of the Hammerstein type, and some standard uniqueness theorems applied. The equation is also transformed into a set of linear equations closely resembling the Kirkwood-Salsburg equations. Many unsolved problems are raised.     

Phase diagram and electrical conductivity of high energy-density water from density functional theory

The electrical conductivity and structure of water between 2000-70,000 K and 0.1-3.7 g/cm3 is studied by finite temperature density functional theory (DFT). Proton conduction is investigated quantitatively by analyzing diffusion, the pair-correlation function, and Wannier center locations, while the electronic conduction is calculated in the Kubo-Greenwood formalism. The conductivity formulation is valid across three phase transitions (molecular liquid, ionic liquid, superionic, electronic liquid). Above 100 GPa the superionic phase directly borders an electronically conducting fluid, not an insulating ionic fluid, as previously concluded. For simulations of high energy-density systems to be quantitative, we conclude that finite temperature DFT should be employed.     

Density functional theory of freezing of a system of highly elongated ellipsoidal oligomer solutions

We have used the density functional theory of freezing to study the liquid crystalline phase behavior of a system of highly elongated ellipsoidal conjugated oligomers dispersed in three different solvents namely chloroform, toluene and their equimolar mixture. The molecules are assumed to interact via solvent-implicit coarse-grained Gay–Berne potential. Pair correlation functions needed as input in the density functional theory have been calculated using the Percus–Yevick (PY) integral equation theory. Considering the isotropic and nematic phases, we have calculated the isotropic–nematic phase transition parameters and presented the temperature–density and pressure–temperature phase diagrams. Different solvent conditions are found not only to affect the transition parameters but also determine the capability of oligomers to form nematic phase in various thermodynamic conditions. In principle, our results are verifiable through computer simulations.     

Decay of bcc-structure and of bond-orientational order in a fluid

The decay of an initially prepared bcc structure and the bond-orientational order (anisotropy of the first coordination shell) are studied in a non-equilibrium molecular dynamics simulation for a fluid of 1024 particles interacting with a repulsive r^-12 potential. Data are presented for pair-correlation functions and order parameters associated with an angle dependence described by cubic harmonics of the ranks 4, 6 and 8. These cubic pair-correlation functions and cubic order parameters are defined by the expansion of the pair-correlation function and of the bond orientational distribution function with respect to Cartesian tensors. The relaxation of the local anisotropy shows a pretransformational slowing down for densities approaching the freezing point.     

A simulation study of an asymmetric exclusion model with open and periodic boundaries for parallel dynamics

The effect of jumping rate probability on the phase diagram of an asymmetric exclusion model is studied by numerical simulations. Density, current and velocity of particles are calculated for parallel dynamics. In the open boundaries case for one species of particles (particles 1), a passage from first to second order transition occurs by decreasing the jumping rate. In the periodic boundaries case, by introducing another species of particle (particle 2) which plays the role of obstacle for particles 1, the average velocity of particles 1 increases with increasing the jumping rate for small density. While the average velocity of particle 2 decreases for small and intermediate densities. Received: 21 October 1997     

Equilibrium adsorption of water-like molecules on single nanospheres

Water-like gas adsorption on spherical nano-aerosol particles has been studied for particles of different sizes at different temperatures with density functional theory. The water-like fluid was modelled as Lennard–Jones (LJ) spheres with four hydrogen-bonding sites with parameters adjusted to the phase diagram of water. For the single sphere case, both the adsorption excesses and density profiles approach those of the plane cases as the spherical substrate sizes increase; at each temperature studied, the size dependence of the transition from thin-film adsorption to thick-film adsorption has been observed. What we found here support earlier suggestions (J. X. Fang, W. H. Marlow, J. X. Lu, and R. R. Lucchese, J. Chem. Phys. 107, 5212 (1997)) that not only the interaction energies between the water molecule and the spherical substrate are sensitive to the size of the spherical substrate, but also the wetting behaviour. In calculations of the excess adsorptions of particles of radii R?=?20σ, 30σ, and 50σ, these substrates show the expected transition from the single molecule to the macroscopic aerosol particles.     

Equilibrium glassy phase in a polydisperse hard-sphere system

The phase diagram of a polydisperse hard-sphere system is examined by numerical minimization of a discretized form of the Ramakrishnan-Yussouff free-energy functional. Crystalline and glassy local minima of the free energy are located and the phase diagram in the density-polydispersity plane is mapped out by comparing the free energies of different local minima. The crystalline phase disappears and the glass becomes the equilibrium phase beyond a "terminal" value of the polydispersity. A crystal-to-glass transition is also observed as the density is increased at high polydispersity. The phase diagram obtained in our study is qualitatively similar to that of hard spheres in a quenched random potential.     

First principles study of structural,electronic and thermoelectric properties of skutterudite GdFe4As12 compounds

The full potential linearized augmented plane wave (FP-LAPW) method has been used to investigate structural, electronic and thermoelectric properties of Skutterudite GdFe₄As₁₂ compounds in the framework of the density functional theory (DFT) within the generalized gradient approximation (GGA) and (GGA+U). The ground-state properties are determined in the cubic structure (Im-3, space group 204). It is found that the most stable phase structure of GdFe4As12 compounds is the ferromagnetic phase and it shows a semi-metallic behavior with narrow gap. The calculation of the density of states near the Fermi level shows the compound to be suitable for the effective thermoelectric application. In addition, the high Seebeck coefficient value is obtained in the n-type region than p-type, indicating the prominence of n-type doping in filled skutterudite GdFe₄As₁₂.     

Integral equation study of the square-well fluid for varying attraction range

In the present work, the properties of the square-well fluid (SW) for varying attraction range are investigated in the framework of the integral equation method. The pair-correlation functions are calculated for a wide range of the potential attraction including the colloidal regime, and the liquid–vapour phase diagram. The critical parameters are determined for attraction ranges spanning from the van der Waals limit down to the metastable region. An overall good agreement is found compared to available simulation data and several theoretical approaches.     

高压下ZnO的结构、弹性性质和吸收光谱的第一性原理研究

 运用基于密度泛函理论（DFT）的平面波赝势方法（PWP）,结合局域密度近似（LDA）以及广义梯度近似（GGA）,系统地研究了ZnO的纤锌矿结构（B4结构）,NaCl结构（B1结构）和CsCl结构（B2结构）在不同压强下的几何结构、弹性性质和吸收光谱。详细研究了ZnO发生的两次相变（B4→B1及B1→B2相变）,得到了不同近似下的相变压强,以及两次相变过程中其弹性常数随压强的变化,并分析了这种变化与相变的关系。发现在高压作用下,ZnO的吸收光谱发生蓝移。通过计算结果和实验结果的比较可以看出,LDA近似下的计算结果更加符合实验结果。     

Investigation of vapour--liquid nucleation properties for spherical and chain-like fluids by density functional theory

The excess Helmholtz free energy functional for nonpolar chain-like molecules is formulated in terms of a weighted density approximation （WDA） for short-range interactions and a Weaks Chandler Andersen （WCA） approximation and a Barker Henderson （BH） theory for long-range attraction. Within the framework of density functional theory （DFT）, vapour liquid interracial properties including density profile and surface tension, and vapour-liquid nucleation properties including density profile, work of formation and number of particles are investigated for spherical and chain- like molecules. The obtained vapour liquid surface tension and the number of particles in critical nucleus for Lennard- Jones （L J） fluids are consistent with the simulation results. The influences of supersaturation, temperature and chain length on vapour liquid nucleation properties are discussed.[第一段]