Crystal growth and elasticity

The purpose of this paper is to review some elasticity effects in epitaxial growth. We start by a description of the main ingredients needed to describe elasticity effects (elastic interactions, surface stress, bulk and surface elasticity, thermodynamics of stressed solids). Then we describe how bulk and surface elasticity affect growth mode and surface morphology by means of stress-driven instability. At last stress-strain evolution during crystal growth is reported.Comment: 12 page

Electrodynamic coupling of electric dipole emitters to a fluctuating mode density within a nano-cavity

We investigate the impact of rotational diffusion on the electrodynamic coupling of fluorescent dye molecules (oscillating electric dipoles) to a tunable planar metallic nanocavity. Fast rotational diffusion of the molecules leads to a rapidly fluctuating mode density of the electromagnetic field along the molecules' dipole axis, which significantly changes their coupling to the field as compared to the opposite limit of fixed dipole orientation. We derive a theoretical treatment of the problem and present experimental results for rhodamine 6G molecules in cavities filled with low and high viscosity liquids. The derived theory and presented experimental method is a powerful tool for determining absolute quantum yield values of fluorescence.Comment: 5 pages, 3 figures, accepted by Physical Review Letter

D-Branes and their Absorptivity in Born-Infeld Theory

Standard methods of nonlinear dynamics are used to investigate the stability of particles, branes and D-branes of abelian Born-Infeld theory. In particular the equation of small fluctuations about the D-brane is derived and converted into a modified Mathieu equation and - complementing earlier low-energy investigations in the case of the dilaton-axion system - studied in the high-energy domain. Explicit expressions are derived for the S-matrix and absorption and reflection amplitudes of the scalar fluctuation in the presence of the D-brane. The results confirm physical expectations and numerical studies of others. With the derivation and use of the (hitherto practically unknown) high energy expansion of the Floquet exponent our considerations also close a gap in earlier treatments of the Mathieu equation.Comment: latex, 26 pages, 4 figures, one reference added, to appear in Nucl. Phys.

Interplay of Strain Relaxation and Chemically Induced Diffusion Barriers: Nanostructure Formation in 2D Alloys

We study the formation of nanostructures with alternating stripes composed of bulk-immiscible adsorbates during submonolayer heteroepitaxy. We evaluate the influence of two mechanisms considered in the literature: (i) strain relaxation by alternating arrangement of the adsorbate species, and (ii) kinetic segregation due to chemically induced diffusion barriers. A model ternary system of two adsorbates with opposite misfit relative to the substrate, and symmetric binding is investigated by off-lattice as well as lattice kinetic Monte Carlo simulations. We find that neither of the mechanisms (i) or (ii) alone can account for known experimental observations. Rather, a combination of both is needed. We present an off-lattice model which allows for a qualitative reproduction of stripe patterns as well as island ramification in agreement with recent experimental observations for CoAg/Ru(0001) [R. Q. Hwang, Phys. Rev. Lett. 76, 4757 (1996)]. The quantitative dependencies of stripe width and degree of island ramification on the misfit and interaction strength between the two adsorbate types are presented. Attempts to capture essential features in a simplified lattice gas model show that a detailed incorporation of non-local effects is required.Comment: 24 pages, 12 figure

Absorption Cross Section of Scalar Field in Supergravity Background

It has recently been shown that the equation of motion of a massless scalar field in the background of some specific p branes can be reduced to a modified Mathieu equation. In the following the absorption rate of the scalar by a D3 brane in ten dimensions is calculated in terms of modified Mathieu functions of the first kind, using standard Mathieu coefficients. The relation of the latter to Dougall coefficients (used by others) is investigated. The S-matrix obtained in terms of modified Mathieu functions of the first kind is easily evaluated if known rapidly convergent low energy expansions of these in terms of products of Bessel functions are used. Leading order terms, including the interesting logarithmic contributions, can be obtained analytically.Comment: latex, 42 page

The interface between the stellar wind and interstellar medium around R Cassiopeiae revealed by far-infrared imaging

The circumstellar dust shells of intermediate initial-mass (about 1 to 8 solar masses) evolved stars are generated by copious mass loss during the asymptotic giant branch phase. The density structure of their circumstellar shell is the direct evidence of mass loss processes, from which we can investigate the nature of mass loss. We used the AKARI Infrared Astronomy Satellite and the Spitzer Space Telescope to obtain the surface brightness maps of an evolved star R Cas at far-infrared wavelengths, since the temperature of dust decreases as the distance from the star increases and one needs to probe dust at lower temperatures, i.e., at longer wavelengths. The observed shell structure and the star's known proper motion suggest that the structure represents the interface regions between the dusty wind and the interstellar medium. The deconvolved structures are fitted with the analytic bow shock structure to determine the inclination angle of the bow shock cone. Our data show that (1) the bow shock cone of 1 - 5 x 10^-5 solar masses (dust mass) is inclined at 68 degrees with respect to the plane of the sky, and (2) the dust temperature in the bow shock cone is raised to more than 20 K by collisional shock interaction in addition to the ambient interstellar radiation field. By comparison between the apex vector of the bow shock and space motion vector of the star we infer that there is a flow of interstellar medium local to R Cas whose flow velocity is at least 55.6 km/s, consistent with an environment conducive to dust heating by shock interactions.Comment: 7 pages, 2 figures, accepted for publication in Astronomy and Astrophysic

Some thoughts about nonequilibrium temperature

The main objective of this paper is to show that, within the present framework of the kinetic theoretical approach to irreversible thermodynamics, there is no evidence that provides a basis to modify the ordinary Fourier equation relating the heat flux in a non-equilibrium steady state to the gradient of the local equilibrium temperature. This fact is supported, among other arguments, through the kinetic foundations of generalized hydrodynamics. Some attempts have been recently proposed asserting that, in the presence of non-linearities of the state variables, such a temperature should be replaced by the non-equilibrium temperature as defined in Extended Irreversible Thermodynamics. In the approximations used for such a temperature there is so far no evidence that sustains this proposal.Comment: 13 pages, TeX, no figures, to appear in Mol. Phy