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.     

Bulk Viscosity and Decaying Vacuum Density in Friedmann Universe

We present an isotropic and homogeneous flat cosmological model for bulk viscous fluid distribution. We consider the vacuum density proportional to Hubble expansion parameter and time dependent bulk viscosity related to the velocity and acceleration of universe. The behaviour of resulting solutions are in accordance with recent astronomical observations. The model obtained evolves with a decelerating expansion followed by late time acceleration. Cosmological term Λ being very large at initial epoch relaxes to a genuine cosmological constant asymptotically. Presence of bulk viscosity prevents the matter density to vanish asymptotically and the matter density continues to be of the order of vacuum density after a finite time. Thus, we obtain a universe having the possibility of cosmic coincidence.     

Asymptotic density profile of a classical fluid against a hard wall

The asymptotic density profile of classical simple fluids in contact with a hard wall is studied using the hypernetted chain approximation for inhomogeneous systems. It is shown that the one-particle distribution function tends very slowly to the density of the bulk, also in absence of a long-range wall-particle interaction, when the pair interaction between particles in the bulk varies as an inverse power at large distances.     

粉末颗粒气力加注特性实验研究

粉末发动机是以粉末颗粒为燃料的新型发动机,具有多次起动和推力调节的功能.粉末加注是粉末发动机实验组织过程中的重要环节.本研究通过搭建粉末供应系统开展粉末气力加注实验,研究对比了集粉箱加注位置、流化气量对粉末气力加注特性的影响.考虑了供粉过程中储箱内粉末堆积密度的动态变化,并建立了相应的计算方法,同时还采用控制系统理论揭示了储箱内粉末堆积密度的变化规律.结果表明:在相同条件下,较大的流化气量有利于加注过程稳定,但集粉箱加注率较低;气力加注方式下集粉箱内的粉末堆积密度大于储箱内初始堆积密度;采用较小的流化气量与集粉箱壁面切向加注方式有利于提高粉末粒径分布均匀性;集粉箱壁面切向加注方式下,流化气量较小时储箱内粉末的堆积密度是先增大后减小,且堆积密度最终值小于初始值,而流化气量较大时,储箱内粉末的堆积密度是先增大后减小再增大后减小,且堆积密度最终值大于初始值;储箱内粉末堆积密度的动态变化过程类似于欠阻尼二阶系统,流化气量较小时系统阻尼系数较小,而流化气量较大时系统阻尼系数较大,且是一个变阻尼过程.     

Magnetism of linear chains

Accurate self-consistent local spin density electronic structure calculations for linear chains of Ni and Fe atoms are presented which realistically treat three-dimensional nature. The one-dimensional character of the bands manifests itself in high density of states arising from van Hove singularities. Both transition metals are “strong” Stoner ferromagnets with large magnetic moments (3.3 and 1.1μ_B for Fe and Ni, respectively) and have large s- and d-exchange splittings. From fits of our results to simple tight-binding models, we find that the d-d effective exchange interaction is similar to that in bulk. However, the use of standard bulk tight-binding parameters is found to be inappropriate since they qualitatively change the results by, for example, misplacing the Fermi level with respect to the band edges. Moreover, in contrast to the bulk, the linear chains also show a rather large s-exchange interaction and hence a large and positive valence contribution to the contract hyperfine field; for Fe this results in a positive contact hyperfine field of the same magnitude as in the bulk. The unique signatures of these linear systems should make their characterization experimentally feasible.     

Freezing and glass transition of hard spheres in cavities

Dynamical and static properties of N=13-4000 hard spheres in spherical cavities with smooth and rough walls have been calculated by molecular-dynamics computer simulations. We use a dynamical criterion to distinguish between fluidlike and solidlike states. The associated crossover densities show a strong dependence both on the system size and on the surface roughness. For large N, these crossover densities tend to the bulk glass transition density for rough walls and to the bulk crystallization density for smooth walls. The crossover densities for finite N are found to be significantly smaller than the corresponding bulk densities. A detailed examination of the layer-resolved radial- and tangential mean-square displacements reveals qualitatively different dynamics for smooth and rough cavities.     

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.     

Boundary Slip and Surface Interaction： A Lattice Boltzmann Simulation

The factors affecting slip length in Couette geometry flows are analysed by means of a two-phase mesoscopic lattice Boltzmann model including non-ideal fluid-fluid and fluid-wall interactions. The main factors influencing the boundary slip are the strength of interactions between fluid-fluid and fluid-wall particles. Other factors, such as fluid viscosity, bulk pressure may also change the slip length. We find that boundary slip only occurs under a certain density （bulk pressure）. If the density is large enough, the slip length will tend to zero. In our simulations, a low density layer near the wall does not need to be postulated a priori but emerges naturally from the underlying non-ideal mesoscopic dynamics. It is the low density layer that induces the boundary slip. The results may be helpful to understand recent experimental observations on the slippage of micro flows.     

Pion condensation in a relativistic field theory consistent with bulk properties of nuclear matter

Pion condensation has not previously been investigated in a theory that accounts for the known bulk properties of nuclear matter, its saturation energy and density and compressibility. We have formulated and solved self-consistently, in the mean field approximation, a relativistic field theory that possesses a condensate solution and reproduces the correct bulk properties of nuclear matter. The theory is solved in its relativistically covariant form for a general class of space-time dependent pion condensates. Self-consistency and compatibility with bulk properties of nuclear matter turn out to be very stringent conditions on the existence and energy of the condensate, but they do allow a weak condensate energy to develop. The spin-isospin density oscillations, on the other hand, can be large. It is encouraging, as concerns the possible existence of new phases of nuclear matter, that this is so, unlike the Lee-Wick density isomer, that appears to be incompatible with nuclear matter properties.     

Localization and delocalization errors in density functional theory and implications for band-gap prediction

The band-gap problem and other systematic failures of approximate exchange-correlation functionals are explained from an analysis of total energy for fractional charges. The deviation from the correct intrinsic linear behavior in finite systems leads to delocalization and localization errors in large and bulk systems. Functionals whose energy is convex for fractional charges such as the local density approximation display an incorrect apparent linearity in the bulk limit, due to the delocalization error. Concave functionals also have an incorrect apparent linearity in the bulk calculation, due to the localization error and imposed symmetry. This resolves an apparent paradox and identifies the physical nature of the error to be addressed to obtain accurate band gaps from density functional theory.     

A comparison study on the electronic structures,lattice dynamics and thermoelectric properties of bulk silicon and silicon nanotubes

In order to investigate the mechanism of the electron and phonon transport in a silicon nanotube(SiNT),the electronic structures,the lattice dynamics,and the thermoelectric properties of bulk silicon(bulk Si)and a SiNT have been calculated in this work using density functional theory and Boltzmann transport theory.Our results suggest that the thermal conductivity of a SiNT is reduced by a factor of 1,while its electrical conductivity is improved significantly,although the Seebeck coefficient is increased slightly as compared to those of the bulk Si.As a consequence,the figure of merit(ZT)of a SiNT at 1200 K is enhanced by 12 times from 0.08 for bulk Si to 1.10.The large enhancement in electrical conductivity originates from the largely increased density of states at the Fermi energy level and the obviously narrowed band gap.The significant reduction in thermal conductivity is ascribed to the remarkably suppressed phonon thermal conductivity caused by a weakened covalent bonding,a decreased phonon density of states,a reduced phonon vibration frequency,as well as a shortened mean free path of phonons.The other factors influencing the thermoelectric properties have also been studied from the perspective of electronic structures and lattice dynamics.     

Gate-voltage control of chemical potential and weak antilocalization in Bi₂Se₃

We report that Bi?Se? thin films can be epitaxially grown on SrTiO? substrates, which allow for very large tunablity in carrier density with a back gate. The observed low field magnetoconductivity due to weak antilocalization (WAL) has a very weak gate-voltage dependence unless the electron density is reduced to very low values. Such a transition in WAL is correlated with unusual changes in longitudinal and Hall resistivities. Our results suggest a much suppressed bulk conductivity at large negative gate voltages and a possible role of surface states in the WAL phenomena.     

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.     

Surviving structure in colloidal suspensions squeezed from 3D to 2D

Combining colloidal-probe experiments and computer simulations, we analyze the solvation forces F of charged silica colloids confined in films of various thicknesses h. We show that the oscillations characterizing F(h), for sufficiently large h, are determined by the dominant wavelength of the bulk radial distribution function. As a consequence, both quantities display the same power-law density dependence. This is the first direct evidence, in a system treatable both by experiment and by simulation, that the structural wavelength in bulk and confinement coincide, in agreement with predictions from density functional theory. Moreover, theoretical and experimental data are in excellent quantitative agreement.     

Viscosity of hadron matter within relativistic mean-field-based model with scaled hadron masses and couplings

The shear (η) and bulk (ζ) viscosities are calculated in a quasiparticle relaxation-time approximation for a hadron matter described within the relativistic mean-field-based model with scaled hadron masses and couplings. Comparison with results of other models is presented. We demonstrate that a small value of the shear viscosity to entropy density ratio required for explaining a large elliptic flow observed at RHIC may be reached in the hadron phase. Relatively large values of the bulk viscosity are noted in the case of a baryon-enriched matter.     

SiGe superlattice nanocrystal pure and doped with substitutional phosphorus single atom: Density functional theory study

Ab initio density functional theory is used to simulate electronic structure of hydrogenated SiGe nanocrystal superlattice pure and doped with substitutional P single atom. The results of electronic structure calculations are compared to the same size silicon and germanium nanocrystals. The comparison reveals that the energy gap of the three kinds of nanocrystals is nearly the same in non-relativistic and relativistic limits. Because of large width of gap in the present small nanocrystals the relativistic corrections are not as much important as in the case of bulk crystals. The doping of SiGe nanocrystal with P single atom introduced an impurity level at 4 eV below original conduction band edge. This result is much larger than comparable silicon bulk and nanocrystal doping with P atoms. Results also show that the deep internal angles and bonds in SiGe nanocrystals reach approximately the angles and structure of bulk crystals after nearly three surface layers. A double positively charged layer is located at the Ge terminated surface of SiGe nanocrystal. This layer is enhanced and is accompanied with a large increase of the dipole moment of the nanocrystal in the case of P doped nanocrystal. Due to oscillatory lattice potential in SiGe superlattice, density of states show that bands are broken up to sub-bands in comparison with silicon nanocrystal density of states especially at the conduction band.     

Far-infrared absorption of large electron-hole drops in stressed Ge

The far-infrared attenuation spectrum due to a large electron-hole drop in inhomogeneously stressed Ge has been measured and compared to the attenuation by small drops in unstressed Ge. The spectrum is analyzed using the full Mie theory for the absorption due to a large sphere; the experimental results are interpreted as bulk plasma absorption in a drop with pair density considerably lowered by the strain.     

Ultrastiff cubic TiO2 identified via first-principles calculations

The crystal structures and compressibilities of fluorite- and pyrite-structured TiO2 under varying hydrostatic pressures are calculated using gradient-corrected density functional as well as hybrid density functional-Hartree-Fock formulations. The results suggest that fluorite TiO2 is a highly incompressible solid with a large bulk modulus value (K(0) approximately 395 GPa), approaching that of ultrahard cotunnite TiO2 (K(0)=431 GPa). The bulk modulus obtained for pyrite TiO2 is considerably smaller (K(0) approximately 220-260 GPa), nonetheless larger than the value determined experimentally for cubic TiO2. Calculated shear modulus values indicate that fluorite TiO2 has the potential to be an ultrahard material, if it could be stabilized under ambient conditions.     

Assembly of body-centered cubic crystals in hard spheres

We investigate the crystallization of monodisperse hard spheres confined by two square patterned substrates (possessing the basic character of the body-centered cubic (bcc) crystal structure) at varying substrate separations via molecular dynamics simulation. Through slowly increasing the density of the system, we find that crystallization under the influence of square patterned substrates can set in at lower densities compared with the homogeneous crystallization. As the substrate separation decreases, the density, where crystallization occurs (i.e., pressure drops), becomes small. Moreover, two distinct regimes are identified in the plane of bcc particle fraction and density for the separation range investigated. For large substrate separations, the bcc particle fraction displays a local maximum as the density is increased, and the resulting formed crystals have a polycrystalline structure. However, and more importantly, another situation emerges for small substrate separations: the capillary effects (stemming from the presence of two substrates) overwhelm the bulk driving forces (stemming from the spontaneous thermal fluctuations in the bulk) during the densification, eventually resulting in the formation of a defect-free bcc crystal (unstable with respect to the bulk hard-sphere crystals) by using two square patterned substrates.     

Transport Analysis of Low Density Discharges on JET

Energy and particle balance of plasma in discharges with low density on JET is investigated. Self-consistent transport analysis accounting bulk and edge plasma phenomena is carried out. It is shown that large difference between the ion edge temperature and the electron one could be the result of the influence of the secondary electron emission from the limiter in low density discharges.