Fluids of clusters in attractive colloids

We show that colloidal particles with attractive interactions induced by a nonadsorbing polymer exhibit a stable phase consisting of a fluid of clusters of particles. This phase persists even in the absence of any long-range repulsion due to charge, contrary to expectations based on simulation and theory. Cluster morphology depends strongly on the range of the interparticle attraction: With a shorter range, clusters are tenuous and branched; with a longer range, they are more compact.     

Attraction of nonlocal dark optical solitons

We study the formation and interaction of spatial dark optical solitons in materials with a nonlocal nonlinear response. We show that unlike in local materials, where dark solitons typically repel, the nonlocal nonlinearity leads to a long-range attraction and formation of stable bound states of dark solitons.     

Strong attraction between charged spheres due to metastable ionized states

We report a mechanism which can lead to long-range attractions between like-charged spherical macroions, stemming from the existence of metastable ionized states. We show that the ground state of a single highly charged colloid plus a few excess counterions is overcharged. For the case of two highly charged macroions in their neutralizing divalent counterion solution we demonstrate that, in the regime of strong Coulomb coupling, the counterion clouds are very likely to be unevenly distributed, leading to one overcharged and one undercharged macroion. This long-living metastable configuration in turn leads to a long-range Coulomb attraction.     

BKT phase transition in a 2D system with long-range dipole-dipole interaction

We consider phase transitions in 2D XY-like systems with long-range dipole-dipole interactions and demonstrate that BKT-type phase transition always occurs separating the ordered (ferroelectric) and the disordered (paraelectric) phases. The low-temperature phase corresponds to a thermal state with bound vortex-antivortex pairs characterized by linear attraction at large distances. Using the Maier-Schwabl topological charge model, we show that bound vortex pairs polarize and screen the vortex-antivortex interaction, leaving only the logarithmic attraction at sufficiently large separations between the vortices. At higher temperatures the pairs dissociate and the phase transition similar to BKT occurs, though at a larger temperature than in a system without the dipole-dipole interaction.     

Attractive and repulsive dispersion forces between biomolecules in aqueous solution

It has been shown before [1] that two large molecular systems having low-lying giant dipole vibrations may interact through strong long-range forces. This paper deals with the influence of a polar medium with Debye dispersion on this interaction between the molecules. It is shown that besides the normal van der Waals attraction also repulsion can occur, but this only in a limited range of the molecular eigenfrequencies just above the characteristics relaxation frequency τ^?1 of the medium.     

Observation of attraction between dark solitons

We demonstrate a dramatic change in the interaction forces between dark solitons in nonlocal nonlinear media. We present what we believe is the first experimental evidence of attraction of dark solitons. Our results indicate that attraction should be observable in other nonlocal systems, such as Bose-Einstein condensates with repulsive long-range interparticle interaction.     

Sensitivity improvement and new acquisition scheme of heteronuclear active-coupling-pattern-tilting spectroscopy

A simplified phase-cycling scheme for heteronuclear active-coupling-pattern tilting (ACT) spectroscopy is presented. It is demonstrated that the theoretically expected twofold sensitivity gain over earlier implementations can be experimentally realized. A further intensity increase by a factor of 2 is obtained with standard sensitivity-enhancement pulse-sequence elements. The HSQC-HECADE sequence presented is designed for an accurate determination of heteronuclear one-bond and, with subsequent I-spin isotropic mixing, long-range coupling constants. As an exemplary application, the determination of the (3)J(N,Hbeta) coupling constants in a peptide at natural isotope abundance is demonstrated. Additionally, a new polarization-transfer step for the long-range HSQC-HECADE experiment is proposed which avoids a fixed delay tuned to a specific coupling-constant value. Thus, the long-range correlation experiment does not require prior knowledge of the coupling constants to be measured and yields more uniform cross-peak intensity for a broad range of active coupling constants.     

Giant SALR cluster reproduction,with implications for their chemical evolution

ABSTRACT

Particles with SALR (short-range attraction and long-range repulsion) interactions are common to many physical systems, especially biological and soft matter, yet their behaviour is still not completely understood. Using Monte Carlo simulations and a thermodynamic model, it is shown here that giant SALR clusters can grow and reproduce in these fluids. Giant cluster growth and reproduction should therefore be common to a wide range of natural and synthetic systems under suitable conditions. If, in addition, cluster fitness selection occurs then chemical evolution of giant SALR cluster might be observed in suitable systems.     

Long-range order in quasi-one-dimensional conductors

We study the formation of the charge-density wave long-range order in a system of repulsive 1D electrons coupled to 3D phonons. We show that the charge-density wave can be stabilized by interaction with phonons in quasi-1D crystals and semiconducting nanowires. In the case of metallic atomic chains, interaction with phonons of a 3D substrate is not enough, and violation of the translational invariance by commensurable perturbation or disorder is needed. The possibility of stabilization of superconductivity in 1D electrons with attraction by means of tunnel coupling to a 3D metal is considered.     

Pattern formation and glassy phase in the phi4 theory with a screened electrostatic repulsion

We study analytically the structural properties of a system with a short-range attraction and a competing long-range screened repulsion. This model contains the essential features of the effective interaction potential among charged colloids in polymeric solutions and provides novel insights on the equilibrium phase diagram of these systems. Within the self-consistent Hartree approximation and by using a replica approach, we show that varying the parameters of the repulsive potential and the temperature yields a phase coexistence, a lamellar, and a glassy phase. Our results strongly suggest that the cluster phase observed in charged colloids might be the signature of an underlying equilibrium lamellar phase, hidden on experimental time scales.     

Diffraction from surfaces with interacting steps

Diffraction techniques are potentially powerful probes of atomic steps on crystalline surfaces. Correlation among the surface steps yields information on the interactions among steps. We consider surfaces with a cut-off geometric distribution of terrace widths which result from strong, short-range step-step interactions. We calculate the diffraction angular profile analytically and compare this profile with the two extreme cases of negligible and long-range interactions. For a particular case of mean terrace width of seven atoms, we show that a strong interaction with range greater than only two atoms produces measurable splitting in the diffracted intensity. The reciprocal space characteristics of this model surface are expounded.     

Molecular simulation study of triangle-well fluids confined in slit pores

Grand canonical Monte Carlo simulations are used to study the behaviour of triangle-well (TW) fluids with variable well widths confined inside slit pores. The effect of individual factors influencing the properties of confined fluids such as fluid–fluid interactions, pore size and pore wall–fluid interactions are obtained using simulations as it is difficult to experimentally determine the same. An interesting observation of this study is that inside the narrow pore of slit height h* = 5 at the high-pressure condition of P* = 0.8, for the TW fluid with long-range attraction or for the fluid at a low temperature for even a short-range attraction, the density profiles show layering such that there is a sticking tendency of the particles at centre, while there is a depletion of particles near the wall (as the layers at the centre have higher density peak heights than near the walls).     

原子势对阈上电离平台的影响

本文从理论上分别研究了长程和短程原子势对阈上电离光电子谱平台结构的影响. 发现在相当大的激光参数范围内, 长程势的阈上电离谱总是呈现出清晰的双平台结构; 对于短程势, 阈上电离谱双平台的界限不再清晰, 随着入射激光强度的减小, 逐渐从双平台过渡到单平台. 基于经典分析和量子力学数值模拟, 阐明了在不同模型势下, 电离速率的差别和再散射电子弹性碰撞截面的不同导致了上述平台结构的差异．此外, 还讨论了激光脉冲空间强度分布对这一现象的影响. 关键词： 阈上电离 离子势影响 中红外激光脉冲     

Long-range interactions

A long-range potential is one whose range, the distance of effective influence, is unbounded or infinite. In this paper we show, using a definition of the range of a potential and certain other theoretical considerations, that the only long-range potential isV(r)=c/r, wherec is a constant.Supported in part by NASA Grant NSG-8035.     

强场非次序双电离中再碰撞动力学的强度依赖

利用三维经典系综模型系统地研究了不同强度线偏振激光脉冲驱动下He原子的非次序双电离.结果表明在非次序双电离中回碰电子的返回次数、两电子的碰撞距离和电子对的关联特性都强烈地依赖于激光强度.对于750 nm,随着激光强度的增加,单次返回诱导的非次序双电离事件逐渐减少,而多次返回事件的比例显著增加.对于1500 nm,随着激光强度的增加,前三次返回诱导的非次序双电离事件都会减少,返回次数大于3的轨道对非次序双电离的贡献逐渐增加.这是因为在高强度下每次返回过程中母核的库仑吸引对返回电子横向偏离的补偿较弱,所以需要更多次的返回来补偿电子的横向偏离以实现再碰撞.轨道分析表明非次序双电离中两电子的碰撞距离随激光波长和强度的增加而逐渐减小.最后讨论了非次序双电离中电子对的关联特性对返回次数的依赖.     

基于舷侧阵声纳的鱼雷远程自导性能研究

针对鱼雷的远程自导问题,研究了恶劣水文条件下浅海波导中鱼雷舷侧阵声纳对主动声纳信号的探测能力和测向精度,计算结果表明:在潜艇反舰作战中,通过利用水面舰艇发射的主动声纳信号,可实现潜射鱼雷对目标的远程被动探测,其被动测向精度一般可满足自导精度要求。     

Effect of an electric field on dewetting transition of nitrogen-water system

We investigate the influence of an external electric field on the dewetting behavior of nitrogen-water systems between two hydrophobic plates using molecular dynamics simulations. It is found that the critical distance of dewetting increases obviously with the electric field strength, indicating that the effective range of hydrophobic attraction is extended. The mechanism behind this interesting phenomenon is related to the rearrangement of hydrogen bond networks between water molecules induced by the external electric field. Changes in the hydrogen bond networks and in the dipole orientation of the water molecules result in the redistribution of the neutral nitrogen molecules, especially in the region close to the hydrophobic plates. Our findings may be helpful for understanding the effects of the electric field on the long-range hydrophobic interactions.     

强激光场中长程势与短程势原子产生高次谐波与电离特性研究

用数值方法求解含时薛定谔方程，研究了具有长程势和短程势的一维原子在强激光场中的高次谐波和电离特性. 在强激光场中，长程势和短程势原子产生的高次谐波具有相似的特性，对应的平台和截止位置相同，但是短程势原子没有低阶的高次谐波，而长程势和短程势原子在激光场中的电离概率明显不同. 研究结果表明，原子的激发态结构对低阶的高次谐波和原子的电离概率有重要影响. 关键词： 强激光场 高次谐波 电离概率     

Exact solution of the mean spherical model for fluids of non-spherical molecules

The mean spherical model is solved in closed form for a fluid of molecules represented by hard spheres together with short-range angle-dependent repulsion and long-range multipole attraction, with no restriction placed on the parity of the angular functions. This constitutes an extension of a previous report which presented a solution for the same model but with the restriction that the angular terms had even parity.     

From realistic to simple models of associating fluids. II. Primitive models of ammonia,ethanol and models of water revisited

Recently developed methodology to construct primitive models of associating fluids directly from realistic intermolecular potential functions is applied to ammonia, ethanol and several models of water. Hard cores of the molecules are pictured as fused hard-sphere bodies defined by composite short-range repulsions, and the Coulombic repulsions and attractions are approximated by hard-sphere and square-well potentials, respectively. Hard-sphere diameters are determined directly from the parent potential using a theoretical route and the range of the square-well attraction is adjusted using constraints imposed on hydrogen bonding. It is shown that the developed primitive models, despite their simplicity and lack of any long-range interactions, are able to reproduce the structural properties (the set of the site–site correlation functions) of the parent realistic models and may thus serve well as a reference in the perturbation theory.