Non-local effects in the macroscopic theory of Van der Waals and adhesive forces

The role of deviations from perfect screening in the Van der Waals interaction between two metal half-spaces a distance d apart is analyzed for a simple model. The interaction energy which is finite for d → 0 includes asymptotic corrections to the well-known d^?2 law. These corrections lead to a reduction of the order of 30 to 40% of the interaction in the vicinity of equilibrium distances for particle adhesion.     

Van der Waals interaction between spherical voids

The problem of calculating the non-retarded Van der Waals type of interaction between two spherical voids in an electron gas is presented from a new approach. The formulation is based on an integral equation for the self-induced density oscillations derived from a semi-classical treatment of the density-density response theory for inhomogeneous electron systems. The interaction energy between two identical voids is found to obey d^-6 law for large separation d between the voids, the law being determined by the dipolar plasma oscillations alone.     

Casimir-Polder forces in the presence of the cosmic photon heat bath

We study the effect of a photon background at finite temperature T on the Van der Waals interactions among neutral bodies. It turns out that the long-range Casimir-Polder force is unaffected for distances much less than T⁻¹ and strongly enhanced for distances much above T⁻¹.     

The boundary element approach to Van der Waals interactions

We develop a boundary element method to calculate Van der Waals interactions for systems composed of domains of spatially constant dielectric response of a general boundary shape. We achieve this by rewriting the interaction energy expression presented in Phys. Rev. B, 62 (2000) 6997 exclusively in terms of surface integrals of surface operators. We validate this approach in the Lifshitz case and give numerical results for the interaction of two spheres as well as the van der Waals self-interaction of a uniaxial ellipsoid. Our method is simple to implement and is particularly suitable for a full, non-perturbative numerical evaluation of non-retarded van der Waals interactions between objects of a completely general shape.     

Proposal to observe the “Van der Waals force” in hadron-hadron scattering

Effect of the strong “Van der Waals force” on the scattering amplitudes of collision of hadrons, which are regarded as magnetically neutral composites of dyons, is investigated. Similarity between the high energy limit of the amplitude due to the Van der Waals interaction and the Pomeron exchange term is pointed out. Low energy experiments to detect the$\text{(?t)}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ singularity which is characteristic of the Van der Waals interaction are proposed.     

The Van der Waals interaction between molecule and metal surface

A formula for the Van der Waals interaction between a molecule and a metal surface is derived treating the polarization of the metal surface in linear response theory and regarding the metallic system as a semi-infinite electron gas. The formula has a simple form, and can be reduced to a form similar to that given by Bardeen.     

Dispersion dipole between rare-gas atoms

Accurate calculations of the dispersion dipole coefficient D ₇ and its components are described for the systems HeH and HeHe, and used to check an approximate formula for D ₇ developed in a previous paper [1]. By means of a further approximation for a new atomic property, values of D ₇ are obtained for the rare-gas diatoms, based on semi-empirical Van der Waals C ₆ coefficients, Hartree-Fock expectation values and sum-rule values for the atoms. The dispersion contributions to the dipole are compared with the Hartree-Fock contributions [35], and at the conventional collision diameters are found to be of comparable magnitude but usually of opposite sign.     

Weak interactions between tetraphenylporphyrin dimers: A Wannier orbitals study

We investigate the weak interactions taking place in the tetraphenylporphyrin dimers by using Wannier orbitals and DFT. The Van der Waals interaction energies for various geometric configurations of the dimers are then compared to the differences between the total DFT energies of dimers and those of sums over the energies of monomers. The comparison allows us to estimate the weight of the Van der Waals contribution to the total interaction energy since the DFT values include all contributions such as weak chemical interaction or Pauli repulsion to the binding energy. We show that the interaction energy computed with vdW-DF-cx exchange-correlation functional is almost identical with the one computed using Wannier orbitals. By analyzing the properties of Wannier orbitals in monomer and dimer structures, we check the fact the Van der Waals energy can be accurately calculated by using only the orbitals corresponding to the single molecule, which is useful from a practical perspective.     

Lifetimes of Excited Alkalilike Atoms Colliding by Van der Waals Forces with Inert Gas Atoms in the Groundstate

The influence of Van der Waals forces on the lifetime of coherently excited hyperfine-states |F_im_Fi〉 and |F_im_Fi〉 of alkalilike atoms i by collissions with inert gas atoms j in the groundstate |F_jm_Fi〉 has been examined. To do this the timedependent Schrödinger equation has been solved on taking into account a static magnetic field, a radiation field and a Van der Waals force. Introducing an impact parameter model for the colliding atoms with small scattering angles which form a quasimolecule |μ〉 = |F_im_FiF_jm_Fi 〉 an expression for the cross section σ_μμ has been derived which determines the halfwidth Γ _μμ of the coherently excited states. On making use of the irreducible tensor representation the calculation of the cross-section has been reduced to the calculation of multipole transitions. The result has been applied e. g. to the coherently excited 3d¹⁰4p²P_3/2-states of Cu I colliding by dipole-dipole interaction with inert gas atoms He I, and Ar I in the ground state. The values agree well with data which have been derived from zero-level-crossing experiments.     

Einfluss adsorbierter wasserfilme auf die Van der Waals kraft zwischen quarzglasoberflächen

A method to determine Van der Waals forces at low distances is described. Results of experiments carried out with adsorbed water films are in excellent agreement with the theory of Dzyaloshinski. The thickness of adsorbed water films has been varied by changing the vapour pressure of the surrounding atmosphere. The influence of thickness of adsorbed water films has been studied. At high relative humidities the attractive force can be calculated with the help of Dzyaloshinskiis theory by using the dielectric properties of water. The experimentally determined Van der Waals force is discussed in relation to the surface energy of the interface.     

瓜环与喹啉及其衍生物的相互作用

利用荧光光谱分析方法考察六、七及八元瓜环与喹啉、异喹啉、7-甲基喹啉相互作用的结构特征以及光学特性。与核磁共振技术比较,荧光光谱分析方法具有高灵敏性的特点,不仅可观察到喹啉及其衍生物与七元瓜环相互作用而引起的荧光性质变化;还能观察到它们与六元瓜环或八元瓜环相互作用所产生的变化,这是通常的NMR方法所不能实现的。实验表明喹啉、异喹啉、7-甲基喹啉与六、七及八元瓜环有包结作用,形成多种不同包结比的稳定包结配合物。实验也证实,瓜环与客体之间相互作用受到3种效应的协同影响:(1)疏水性的笼体效应(瓜环以其特殊的疏水性笼状结构,可与多种有机物发生疏水性的笼体作用);(2)亲水性的端口效应(瓜环端口环绕的羰基氧通过离子-偶极相互作用来键合金属离子或有机分子的带电部分);(3)相互作用的客体分子或瓜环笼体空间大小,即尺寸效应。     

低密度氢气的分子间平均相互作用势理论和计算

本文提出了分子单中心模型,并通过对构型的平均,两个定理的提出与证明,色散能修正的证明,从数学上论证了这个模型的合理性。由此模型可以直接求出两个氢分子间的平均相互作用势,从而改变了长期以来,总是先算有限几种构型然后再作平均的传统作法。通过数字计算,得到了迄今为止最好的全程平均相互作用势曲线。势阱深度及平衡分子间距分别为-2.91meV和3.44(实验值:-2.85～-3.00meV,3.43～3.49)。     

Self-diffusion in Kr compared with Van der Waals model

The predictions of the Van der Waals model for the self-diffusion in Kr are compared with the experimental results at 220 K and for 0.7ρ_c ? ρ ? 2.1ρ_c. The Van der Waals model fails to represent the Kr data while at the same temperature in the case of CH₄ this model works quite well for ρ_c < ρ ? 3ρ_c.     

The nuclear force and a magnetic monopole model of hadrons

The two-pion exchange potential of the nuclear force is constructed from the phase shifts of the low energy πN and ππ scattering. The large difference between the potentials thus constructed from the phenomenological potentials of Hamada-Johnston and of Bryan is pointed out. It is found that this difference has the form of the attractive Van der Waals potential plus an inner repulsive core. The existence of the Van der Waals force is discussed in connection with the magnetic monopole model of hadrons.     

金刚石对顶砧中触点位置误差对样品电阻率测量精度的影响

利用有限元分析方法,研究了金刚石对顶砧中电极与样品接触点位置变化对范德堡法测量样品电阻率精度的影响.结果表明:当电极中心与样品边缘的间距b≤d/9(d为样品直径)时能得到精确的电阻率测量结果;当电极位置远离样品边缘而逐渐接近样品中心时,其位置变化对电阻率测量精度的影响迅速增大;相同的电极位置变化对具有较大电阻率的半导体样品电阻率测量精度的影响更明显. 关键词： 电阻率 有限元方法 金刚石对顶砧     

Polyimides Containing Azobenzene Side Units: Properties and Correlation with Conformational Parameters

Two series of aromatic polyimides differing by the bridging group between imide rings, polymers a, b, and c, having C?O linkages and polymers d, e, and f, having C(CF₃)₂ groups, have been obtained by polycondensation reaction of aromatic dianhydrides with aromatic diamines containing substituted azobenzene groups. These polymers with good solubility in polar amidic solvents, were able to form transparent flexible films and showed high thermal stability. Conformational parameters of the polymers were calculated by the Monte Carlo method with allowance for hindered rotation. Several physical properties of these polymers, such as solubility, glass transition temperature (T_g), and initial decomposition temperature (T_onset) were measured and discussed in relation to the volume factor of the macromolecular chains. The volume factor is equal to ratio of the Van der Waals volume of the pendent group to the Van der Waals volume of the repeating unit of the main chain.     

Pairing of off-center polarons due to dipole-dipole interaction: Possible clue to the high-temperature superconductivity

An alternative type of small polaron associated with an off-center configurational distortion can arise from the additional coupling of the electron to some appropriate odd vibrational mode. The model is exemplified by quantum-mechanical calculations of the vibronic mixing of two different-parity electronic states of the truncated Coulomb potential by an odd-parity vibration of two-neighboring ions: the Pseudo-Jahn-Teller effect. These calculations manifest the feasibility of the model and suggest that off-center polarons may occur in perovskites. Each off-center polaron is inherently polarizable in an electric field. Consequently, neighboring entities can couple through the universal Van der Waals force to form bipolarons. The off-center vibronic polarizability is temperature-dependent and so is the binding which decreases gradually as the temperature is raised. As a result, these Van der Waals bipolarons may cover the range from small-radius bipolarons, exhibiting a specific type of superconductivity, to large-radius species, which are similar to Cooper pairs. The model compares favorably with many specific features of the high-T _c superconductivity.     

Muonium quantum diffusion in cryocrystals

In this paper we review our recent study on muonium (Mu) diffusion in simplest solids — Van der Waals cryocrystals. We give experimental evidence for the quantum nature of Mu diffusion in these matters. The results are compared with the current theory of quantum diffusion in insulators. The first direct observation of theT ⁷ power-law for the Mu hop rate (in solid nitrogen) is taken as confirmation of two-phonon scattering mechanism. In solid xenon the one-phonon interaction is shown to be dominant. Muonium diffusion in solid neon is discussed in terms of band-like motion. Finally, we report a dramatic effect of impurities on Mu quantum diffusion.     

On the relative strength of He and H2 diffraction from Ag(111)

Under equivalent incident conditions, H₂ diffraction beams on Ag(111) have recently been observed to be about one order of magnitude stronger than He beams. We show that this effect can be attributed to details of the interaction, in particular, the exponential increase of the corrugation amplitude towards the surface. By extending a previously developed theory for the interaction between He and a metal surface, we show that the H₂Ag(111) repulsion is roughly 1.5 times larger than for He. However, the Van der Waals attraction is about three times stronger for H₂, so that the classical turning points of low-energy H₂ particles lie closer to the surface. Because of the stronger corrugation at short distances, H₂ diffraction intensities can be up to an order of magnitude larger than for He.