Relaxation near transition metal surfaces

A simple calculation of the relaxation near transition metal surfaces is described. The d electrons are treated in a tight-binding approximation and Born-Mayer type potentials are used to simulate the repulsive part of the energy at short distances. The interplanar spacing is found to decrease at the center of the series and to increase on the wings. For almost empty or filled bands one obtains no relaxation.     

Surface-plasmon-polariton-assisted dipole-dipole interaction near metal surfaces

We investigate the surface plasmon polariton (SPP)-assisted interaction between two dipoles near a metal surface. The radiation energy from a dipole can excite SPPs and transport to another dipole through the channel of the localized SPP modes. This energy transfer can be much more efficient than direct energy transfer via dipole-dipole radiation interaction in free space. A simple analytical model is proposed to describe the underlying physics behind the influence of SPP on the dipole-dipole interaction energy, and it predicts a wide variety of complicated interaction features that agree well with rigorous calculations.     

Magnetostatic field noise near metallic surfaces

We develop an effective low-frequency theory of the electromagnetic field in equilibrium with thermal objects. The aim is to compute thermal magnetic noise spectra close to metallic microstructures. We focus on the limit where the material response is characterised by the electric conductivity. At the boundary between empty space and metallic microstructures, a large jump occurs in the dielectric function which leads to a partial screening of low-frequency magnetic fields generated by thermal current fluctuations. We resolve a discrepancy between two approaches used in the past to compute magnetic field noise spectra close to microstructured materials.     

Electron states near surfaces of solids

The mathematical theory of electron eigenstates near the surfaces of solids is developed in stages. The first stage is the study of the eigenstates of solids which terminate at a surface but are otherwise unperturbed; it is proved that near the surface the three-dimensional band edges are “softened” and that van Hove's singularities in the density of states are eliminated. A set of “vacuum states” lying primarily outside the solid is constructed out of plane waves orthogonalized to the eigenstates of the solid. This set of vacuum states is not orthonormal, and it must be orthonormalized by a procedure different from that of Schmidt (which is unsuitable). Effects of surface perturbations are studied. An exact method is elaborated for obtaining eigenfunctions in closed form, for a variety of perturbing potentials that extend arbitrary distances from the surface and include interband matrix elements. It consists of calculating the effects of one surface layer at a time and cumulating the results. The intrinsic instability of certain surfaces against the formation of bands of surface states is shown to be the consequence of the vanishing of a bulk quantity α_zz, one of the components of the inverse-effective mass tensor, at certain points in the B.Z.     

Self-consistent nonlocal potentials near planar surfaces

A new derivation of the self-consistent dynamically screened nonlocal Coulomb potential near a planar surface is presented, based on the integral equation approach. The exact solution is obtained without restrictions on the dynamical response of the solid (e.g. RPA in metals), the type of surface potential or electronic density profile, and without additional matching or boundary conditions. This makes the present approach particularly useful e.g. in the general discussion of model-independent screening phenomena at metal surfaces. The application to the calculation of matrix elements for electron and ion excitation of collective modes and electron-hole pairs is formulated, and the connection with the previous results discussed.     

Hydrodynamic Synchronisation of Model Microswimmers

We define a model microswimmer with a variable cycle time, thus allowing the possibility of phase locking driven by hydrodynamic interactions between swimmers. We find that, for extensile or contractile swimmers, phase locking does occur, with the relative phase of the two swimmers being, in general, close to 0 or π, depending on their relative position and orientation. We show that, as expected on grounds of symmetry, self T-dual swimmers, which are time-reversal covariant, do not phase-lock. We also discuss the phase behaviour of a line of tethered swimmers, or pumps. These show oscillations in their relative phases reminiscent of the metachronal waves of cilia.     

Thermal expansion near plane and stepped surfaces

A theory of the thermal expansion of the plane and (11n) stepped surfaces of fcc crystals is presented. The temperature dependent relaxations arise from cubic anharmonic terms in the crystal potential energy. We show that the thermal expansion depends on the positions of the atoms with respect to the steps and is greatest for the atom of the upper corner. The knowledge of these new atomic positions at each temperature allows us to calculate new atomic force constants and then new vibrational properties at this temperature. The application is made for a Ni crystal for which we give the corrections, due to the thermal expansion, on the mean square displacements of stepped surface atoms. The variation with temperature of the optical modes due to a light monolayer is also presented.     

Fluctuating coulomb field near ionic dielectric surfaces

Analytical and numerical studies of the energy density spatial distribution of a fluctuating electric field near the model ionic crystal surface as a function of temperature are carried out. It is established that the fluctuating field energy density decreases as h ^−3.3 with increasing distance h from the crystal surface. The fluctuating field energy increases with temperature and differs from zero at −273°C due to quantum zero-point vibrations of ions in the lattice.     

Interactions between charged rods near salty surfaces

Using both theoretical modeling and computer simulations we study a model system for DNA interactions in the vicinity of charged membranes. We focus on the polarization of the mobile charges in the membranes due to the nearby charged rods (DNA) and the resulting screening of their fields and inter-rod interactions. We find, both within a Debye-Hückel model and in Brownian dynamics simulations, that the confinement of the mobile charges to the surface leads to a qualitative reduction in their ability to screen the charged rods to the degree that the fields and resulting interactions are not finite-ranged as in systems including a bulk salt concentration, but rather decay algebraically and the screening effect is more like an effective increase in the multipole moment of the charged rod. Received 28 September 1999     

Bulk plasmon induced ion neutralization near metal surfaces

A novel mechanism is proposed for ion neutralization near metal surfaces, whereby a bulk plasmon is emitted during the electron capture, induced by the presence of the external ion which does not penetrate the metal. In a semiclassical picture of this mechanism the electrons increase their velocity in the field of the ion until they surpass the threshold velocity for collective excitation, emitting the plasmon and getting bound to the ion.Primary evaluations of bulk plasmon transition rates for He⁺ interacting with Al surfaces indicate that very close to the image plane the bulk collective channel might become more efficient than the surface plasmon mode to neutralize the ion.     

Sampling errors in near field measurements on non-planar surfaces

Techniques of predicting the radiated field of a vibrating structure from measurements of its near field are well known. The effects of spatial sampling and measurements errors on the calculated radiated field have been investigated by several authors for planar measurement surfaces. In this paper an extension of these analyses to non-planar surfaces is presented. Examples of the application of the new sampling criteria and error calculations are shown for a cylindrical near field measurement surface.     

Static and dynamic properties of arcs near plane surfaces

The static properties of an arc near a plane surface have been analyzed by extending Maecker's method to bi-cylindrical coordinates. A direct numerical solution was also carried out. The general method of Phillips was then used to predict the transient properties. Experimental data was taken on the static and dynamic properties of arcs burning near dry and wet surfaces. Agreement with theory was much better for dry surfaces, suggesting that humidity in the arc greatly affects the static and dynamic properties.     

Relaxation time and viscosity of water near hydrophilic surfaces

Recently results of microwave spectroscopy have shown that the water relaxation time is lengthened near the surface of aggregates of hydrogen bond-forming molecules, an effect corresponding to an enhanced viscosity [1–3]. In this communication properties of the so-called bound water are treated by a percolation calculation on the water cluster sizes. The values combined with the statistical model of the dielectric response of bulk water by Haggis et al. [4] yield the bound water relaxation time and in this way its viscosity. Fair agreement with the experimental data is obtained. The new analysis represents a keystep for a better understanding of bound water in physico-chemical and biological systems. Dedicated to Professor Karlheinz Seeger on the occasion of his 60th birthday     

Scaling laws of single polymer dynamics near attractive surfaces

We present a molecular dynamics study of a generic model for single polymer diffusion on surfaces, which have variable atomic-scale corrugation but no artificial, impenetrable obstacles. The diffusion coefficient D scales as D is proportional to (-3/2) with the degree of polymerization N for strongly adsorbed, linear polymers on solid substrates in good solvents. Weaker scaling, i.e., D is proportional to (-1), is found if (i) the substrate is a fluid, e.g., a membrane, (ii) the polymer is a ring polymer, and (iii) the polymer is commensurate with the substrate. In poor solvents, diffusion on solids slows exponentially fast with N. Reptation is not observed in any of the simulations presented here.     

Density functional theory for polyelectrolytes near oppositely charged surfaces

We report a nonlocal density functional theory of polyelectrolyte solutions that faithfully accounts for both short- and long-range correlations neglected in a typical mean-field method. It is shown that for systems with strong electrostatic interactions, the long-range correlations are subdued by direct Coulomb attractions, thereby manifesting strong local excluded-volume effects. The theory has also been used to describe the influence of the polyion chain length and small ion valence on charge inversion due to the adsorption of polyelectrolytes at an oppositely charged surface.