A classical density functional approach to depletion interaction of Lennard-Jones binary mixtures

In this article, we apply classical density functional theory to investigate the characteristics of depletion interaction in Lennard-Jones (LJ) binary fluid mixtures. First, to confirm the validity of our adopted density functional formalism, we calculate the radial distribution functions using a theoretical approach and compare them with results obtained by molecular dynamics simulation. Then, this approach is applied to two colloids immersed in LJ solvent systems. We investigate the variation of depletion interaction with respect to the distance of two colloids in LJ binary systems. We find that depletion interaction may be attractive or repulsive, mostly depending on the bulk density of the solvent and the temperature of the binary system. For high bulk densities, the repulsive barrier of depletion force is remarkable when the total excluded volume of colloids touches each other and reaches a maximum. The height of the repulsive barrier is related to the parameters of the LJ potential and bulk density. Moreover, the depletion force may exhibit attractive wells if the bulk density of the solvent is low. The attractive well tends to appear when the surface–surface distance of colloids is half of the size of the polymer and deepens with temperature lowering in a fixed bulk density. In contrast with the hard-sphere system, no oscillation of depletion potential around zero is observed.     

Density profiles and solvation force for a liquid in a slit

Computer simulations and density functional theory results are reported for a Lennard-Jones liquid in a slit or pore formed by two parallel hard walls. Both density profiles and solvation forces are computed. Two classes of calculation are performed. In the first class, a high bulk density is selected and, starting from a high temperature, the temperature is reduced until the temperature corresponding to bulk liquid—vapour coexistence is reached. For small slit widths or exceedingly large widths, the density in the slit decreases continuously until the slit is virtually empty or ‘dry’. When the slit width is somewhat larger than a molecular diameter, but still finite, the density in the slit decreases continuously as the temperature is decreased until there is an abrupt change in the density in the slit. Below this temperature, the density is smaller. Further decreases in the temperature, result in a continuous decrease in the slit density until the slit is virtually empty. In the second class, the density and temperature for bulk coexistence are chosen and the bulk density is increased. At the temperature and bulk density for bulk coexistence, the slit is virtually empty and remains so for all widths that we consider. As the bulk density is increased at constant temperature, the slit remains empty as the width is increased until some specific width is reached and then starts to fill abruptly. The agreement of the density functional and simulation results is qualitative but good.     

Behavior of a wetting phase near a solid boundary: vapor near a weakly attractive surface

Density profiles of a LJ vapor near a weakly attractive surface with long-range fluid wall potential was studied along the pore coexistence curve. There are two localized density maxima near the pore wall: the first one is caused by localization of the molecules in the minimum of the fluid-wall potential, and the second one reflects adsorption of molecules at the first layer at higher densities. In addition, a third, weak density maximum is observed close to the critical temperature due to the competition between the long-range attractive tail of the fluid-wall potential and the effect of missing neighbors. This maximum separates the region of a gradual density depletion toward the surface due to the missing neighbor effect and the adsorption region further from the surface, where the density gradually increases toward the surface due to the attractive fluid-wall potential. When approaching the bulk critical temperature, this maximum moves away from the surface due to the divergence of the bulk correlation length. Applicability of various equations to describe the vapor density profiles is examined. Excess adsorption of vapor at low temperatures turns into excess depletion at higher temperatures. The crossover temperature increases with increasing pore size and strengthening fluid-wall interaction. The problems of the theory of the surface critical behavior of Ising models in a case of a non vanishing surface field and its mapping on a fluid is discussed.     

Bulk viscosity,interaction and the viability of phantom solutions

We study the dynamics of a bulk viscosity model in the Eckart approach for a spatially flat Friedmann–Robertson–Walker (FRW) Universe. We have included radiation and dark energy, assumed as perfect fluids, and dark matter treated as an imperfect fluid having bulk viscosity. We also introduce an interaction term between the dark matter and dark energy components. Considering that the bulk viscosity is proportional to the dark matter energy density and imposing a complete cosmological dynamics, we find bounds on the bulk viscosity in order to reproduce a matter-dominated era (MDE). This constraint is independent of the interaction term. Some late time phantom solutions are mathematically possible. However, the constraint imposed by a MDE restricts the interaction parameter, in the phantom solutions, to a region consistent with a null value, eliminating the possibility of late time stable solutions with \(w<-1\). From the different cases that we study, the only possible scenario, with bulk viscosity and interaction term, belongs to the quintessence region. In the latter case, we find bounds on the interaction parameter compatible with latest observational data.     

Softening of magnetorotons in a semi-infinite Fibonacci superlattice

Calculations of interlayer and intralayer screening of the Coulomb interaction on the softening of bulk and surface magnetoroton modes are presented for density and position modulations of the two-dimensional (2D) electron gas (EG) layers of a semi-infinite quasiperiodic superlattice. It is shown that the softening of these modes is due to an increase in the screening by all other layers of the effective intralayer Coulomb interaction. Numerical results are obtained for variable thickness of a 2DEG layer, the separation between layers and the distance between the surface layer and the top metal gate. The critical values of the structure parameters, determining the interlayer and intralayer screening of the Coulomb interaction, are obtained and used in constructing the phase diagrams showing the separation between the quantum fluid and charge-density wave phases.     

Generalized Poisson-Boltzmann Equation Taking into Account Ionic Interaction and Steric Effects

Generalized Poisson-Boltzmann equation which takes into account both ionic interaction in bulk solution and steric effects of adsorbed ions has been suggested.We found that,for inorganic cations adsorption on negatively charged surface,the steric effect is not significant for surface charge density 0.0032 Cdm 2,while the ionic interaction is an important effect for electrolyte concentration 0.15 moll in bulk solution.We conclude that for most actual cases the original PB equation can give reliable result in describing inorganic cation adsorption.     

Generalized Poisson-Boltzmann Equation Taking into Account Ionic Interaction and Steric Effects

Generalized Poisson-Boltzmann equation which takes into account both ionic interaction in bulk solution and steric effects of adsorbed ions has been suggested. We found that, for inorganic cations adsorption on negatively charged surface, the steric effect is not significant for surface charge density <0.0032 C/dm², while the ionic interaction is an important effect for electrolyte concentration >0.15 mol/l in bulk solution. We conclude that for most actual cases the original PB equation can give reliable result in describing inorganic cation adsorption.     

极化子效应对量子盘中线性和非线性光吸收系数的影响

利用密度矩阵的方法,得出了考虑极化子效应的量子盘的线性和非线性光吸收系数的解析表达式,并以GaAs为例讨论了光吸收系数与不同的入射光子能量和量子盘的厚度之间的关系.结果表明,极化子效应对吸收系数有相当的影响 关键词： 量子盘 光学吸收系数 极化子效应     

Structure and phase transitions in a network-forming associating Lennard-Jones fluid in a slit-like pore: a density functional approach

A study is reported of adsorption of an associating Lennard-Jones fluid with four associative sites per molecule in a slit-like pore. The density distribution of particles in the pore and thermodynamics properties are evaluated by using a density functional method. It is found that at low temperatures the fluid exhibits a set of layering transitions, followed by capillary condensation. Transitions are localized by analysing the grand canonical potential. The density profiles of particles and the distribution of unbound and differently bonded particles demonstrate changes in the structure of the fluid in the pore along the phase coexistence. The critical temperature is lower for a confined fluid, compared with the bulk counterpart. However, an increase in the energy of association increases the critical temperature. The envelope of the capillary condensation is narrower than the bulk liquid-vapour phase diagram. The dependence of the solvation force on the energy of association and on the bulk density is discussed.     

计算平均力势的理论方法

提出了一个用于计算平均力势的普适性的理论框架,方法克服了以前的方法的缺陷,仅仅需要溶剂粒子在单个溶质粒子附近的密度分布作为输入.计算了两个大尺寸溶质粒子浸在小尺寸硬球溶剂浴中的平均力势,理论预言与可能的模拟数据符合.调查了溶剂-溶质相互作用势、溶剂密度、溶质粒子尺寸对过量平均力势的影响.结论是:溶剂粒子在单个溶质粒子附近的减少导致吸引的过量平均力势,而溶剂粒子在单个溶质粒子附近的聚集导致排斥的过量平均力势,高溶剂密度与大溶质粒子尺寸能强化这种趋势.讨论了这种空耗吸引-聚集排斥与生物学中的疏水吸引-水化排斥的联系.     

带电聚合电介质表面吸附理论研究

摘要：应用聚合电介质吸附的定标理论,根据介质和表面电介质常数的比率,考虑多化合价吸附电介质之间强相关性作用,我们提出一种表面排斥电荷的近似定标理论方法,根据这种方法把电介质表面吸附层的相图分为本质上不同的两大类。从相图可知：当表面电荷密度低（或体带相反电荷离子密度高）,这时表面和体带相反电荷离子密度几乎相同;一旦表面电荷密度足够高,就使带相反电荷的离子在表面上浓缩。据此,可确定在这个区域内,低化合价聚合电介质形成一个相关的多链状态,当化合价足够高时,由于近邻链之间的更强排斥增强,使状态转变成单链。     

带电聚合电介质表面吸附理论研究

应用聚合电介质吸附的定标理论,根据介质和表面电介质常数的比率,考虑多化合价吸附电介质之间强相关性作用,我们提出一种表面排斥电荷的近似定标理论方法,根据这种方法把电介质表面吸附层的相图分为本质上不同的两大类。从相图可知：当表面电荷密度低（或体带相反电荷离子密度高）,这时表面和体带相反电荷离子密度几乎相同;一旦表面电荷密度足够高,就使带相反电荷的离子在表面上浓缩。据此,可确定在这个区域内,低化合价聚合电介质形成一个相关的多链状态,当化合价足够高时,由于近邻链之间的更强排斥增强,使状态转变成单链。     

Water dynamics as affected by interaction with biomolecules and change of thermodynamic state: a neutron scattering study

The dynamics of water as subtly perturbed by both the interaction with biomolecules and the variation of temperature and pressure has been investigated via neutron scattering spectroscopy. A measurement of inelastic neutron scattering devoted to the study of the coherent THz dynamics of water in a water-rich mixture with DNA (hydration level of 1 g DNA/15 g D(2)O) at room temperature is reported. The DNA hydration water coherent dynamics is characterised by the presence of collective modes, whose dispersion relations are similar to those observed in bulk water. These dispersion relations are well described by the interaction model developed in the case of bulk water, and the existence of a fast sound is experimentally demonstrated. The behaviour of the collective water dynamics was complemented by studying the single-particle dynamics of bulk water along the isotherm T = 298 K in the pressure range 0.1-350 MPa by means of incoherent scattering. This experiment is an attempt to simulate the change of the water molecular arrangement due to the interaction with DNA, by increasing the pressure as the presence of the biomolecule produces an increase in the density. An anomaly is found in the behaviour of the relaxation time derived from the quasi-elastic scattering signal, which can be related to the hypothetical second critical point in water. This anomaly and the transition from slow to fast sound take place in the same Q range, thus suggesting that the two phenomena could be related at some microscopic level.     

Theory of the interface between a classical plasma and a hard wall

The interfacial density profile of a classical one-component plasma confined by a hard wall is studied in planar and spherical geometries. The approach adapts to interfacial problems a modified hypernetted-chain approximation developed by Lado and by Rosenfeld and Ashcroft for the bulk structure of simple liquids. The specific new aim is to embody self-consistently into the theory a “contact theorem”, fixing the plasma density at the wall through an equilibrium condition which involves the electrical potential drop across the interface and the bulk pressure. The theory is brought into fully quantitative contact with computer simulation data for a plasma confined in a spherical cavity of large but finite radius. It is also shown that the interfacial potential at the point of zero charge is accurately reproduced by suitably combining the contact theorem with relevant bulk properties in a simple, approximate representation of the interfacial charge density profile.     

Electronic structure of a cobalt monolayer on Cu(100)

Angular resolved photoemission spectra using synchrotron radiation have been measured for different amounts of cobalt evaporated on Cu(100). At room temperature cobalt grows layer-bylayer forming well-ordered layers in registry with the substrate, as judged by AES, LEED and UPS measurements. The energy position and linewidth of the Cu peaks remain unchanged when cobalt is deposited onto the surface, suggesting a rather weak interaction between the d-bands of Co and Cu. The two-dimensional band structure of the monolayer of cobalt has been determined. We have obtained a value for the magnetic exchange splitting of ΔE_exch = 0.80 ± 0.15 eV, which is nearly identical to the bulk value. A shift in the energy positions of the critical points for the monolayer versus bulk of cobalt is interpreted in terms of a narrower 2D density of states in the monolayer as compared to the bulk. A resonant valence-band two-electron satellite has been found. The correlation energy and screening effects of the two d-holes are very similar to the corresponding bulk values, while the decreased intensity of the satellite at resonance compared to the one for Co(0001) suggests that there are more d-states relative to s-states in the monolayer than in a bulk cobalt single crystal, in agreement with recent models of the valence band electronic structure at surfaces.     

Dynamics and stability of a linear cluster of spherical magnetic nanoparticles

Magnetic particles freely moving in a fluid may organize themselves into dense phases, bulk clusters, or linear chains. The dynamics of particles forming a chain is analyzed theoretically taking into account the magnetic dipole interaction as well as the Van der Waals molecular interaction. The vibrational spectrum has two branches (the magnetic branch associated with the rotation of the magnetic moment of a particle and the elastic branch associated with the displacement of particles). In the case of particles with constant mass density and magnetic moment, which is interesting for applications, these two modes are in fact independent; i.e., the effects of mode hybridization are weak. However, these effects can be manifested for hollow particles. From analysis of the vibrational spectrum, the criterion for the chain stability to a transition to a denser phase is established.     

Propagation of azimuthal waves along the surface of a metal current-carrying cylinder immersed into a magnetized plasma

It is shown that a metal current-carrying cylinder immersed into a cold magnetized plasma is a waveguide structure, in which coupled bulk ordinarily polarized and surface extraordinarily polarized waves can propagate along the azimuthal direction. Their interaction, stipulated by the fact that besides the longitudinal component, the external magnetic field has also a weak azimuthal component, is studied. Analytical expressions for the corrections to the eigenfrequency of these waves, stipulated by the effect of a constant azimuthal magnetic field, are obtained for the case of a uniform plasma density. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 2, pp. 122–135, February 2008.     

Dynamically generated Mott gap from holography

In the fermionic sector of top-down approaches to holographic systems, one generically finds that the fermions are coupled to gravity and gauge fields in a variety of ways, beyond minimal coupling. In this Letter, we take one such interaction-a Pauli, or dipole, interaction-and study its effects on fermion correlators. We find that this interaction modifies the fermion spectral density in a remarkable way. As we change the strength of the interaction, we find that spectral weight is transferred between bands, and beyond a critical value, a gap emerges in the fermion density of states. A possible interpretation of this bulk interaction then is that it drives the dynamical formation of a (Mott) gap, in the absence of continuous symmetry breaking.     

Ultra-low Young's modulus and high super-exchange interactions in monolayer CrN: A promising candidate for flexible spintronic applications

Monolayer CrN has been predicted to be half-metallic ferromagnet with high Curie temperature.Due to bulk CrN's biocompatibility,the monolayer is a promising candidate for bio-related devices.Here,using first-principles calculations based on density functional theory,we find that the formation energy of the bulk CrN stacking from layers with square lattice is only 68 meV/atom above the convex hull,suggesting a great potential to fabricate the monolayer CrN in a square lattice by using molecular beam epitaxy method.The monolayer CrN is then proved to be a soft material with an ultra-low Young's modulus and can sustain very large strains.Moreover,the analysis of the projected density of states demonstrates that the ferromagnetic half-metallicity originates from the splitting of Cr-d orbitals in the CrN square crystal field,the bonding interaction between Cr-N,and that between Cr-Cr atoms.It is worth noting that the super-exchange interaction is much larger than the direct-exchange interaction and contributes to the ultra-high Curie temperature,which is obtained from Monte Carlo simulations based on Heisenberg model.Our findings suggest that the monolayer CrN can be an indispensable candidate for nanoscale flexible spintronic applications with good biocompatibility and is considerable appealing to be realized in experiment.     

The magnetic phase transition in Cu-doped ZnO: From bulk to nanocluster

Using the first-principles calculations based on the density functional theory, we have investigated the magnetic properties of Cu-doped ZnO both in bulk and nanocluster. The calculated results show that the substitutional Cu ions are spin polarized and have a tendency to assemble. It is found that the ground state has shown a change from ferromagnetic phase to antiferromagnetic phase as the size for the doping system decreases from bulk to nanocluster. In bulk ZnO, the ferromagnetism is attributed to the strong hybridization between Cu 3d and O 2p states. In ZnO nanocluster, however, the antiferromagnetic exchange interaction is dominant because of the very close Cu–Cu distance.