Density-functional study of oxygen adsorption on Mo(112)

Atomic oxygen adsorption on the Mo(112) surface has been investigated by means of first-principles total energy calculations. Among the variety of possible adsorption sites it was found that the bridge sites between two Mo atoms of the topmost row are favored for O adsorption at low and medium coverages. At about one monolayer coverage oxygen atoms prefer to adsorb in a quasi-threefold hollow sites coordinated by two first-layer Mo atoms and one second layer atom. The stability of a structural model for an oxygen-induced $p(2\times 3)$ reconstruction of the missing-row type is examined.Comment: 6 pages, 6 postscript figures, RevTe

Role of interactions in the far-infrared spectrum of a lateral quantum dot molecule

We study the effects of electron-electron correlations and confinement potential on the far-infrared spectrum of a lateral two-electron quantum dot molecule by exact diagonalization. The calculated spectra directly reflect the lowered symmetry of the external confinement potential. Surprisingly, we find interactions to drive the spectrum towards that of a high-symmetry parabolic quantum dot. We conclude that far-infrared spectroscopy is suitable for probing effective confinement of the electrons in a quantum dot system, even if interaction effects cannot be resolved in a direct fashion.Comment: 4 pages, 2 figure

Angular momentum of a strongly focussed Gaussian beam

A circularly polarized rotationally symmetric paraxial laser beams carries hbar angular momentum per photon as spin. Focussing the beam with a rotationally symmetric lens cannot change this angular momentum flux, yet the focussed beam must have spin less than hbar per photon. The remainder of the original spin is converted to orbital angular momentum, manifesting itself as a longitudinal optical vortex at the focus. This demonstrates that optical orbital angular momentum can be generated by a rotationally symmetric optical system which preserves the total angular momentum of the beam.Comment: 4 pages, 3 figure

Wigner molecules in polygonal quantum dots: A density functional study

We investigate the properties of many-electron systems in two-dimensional polygonal (triangle, square, pentagon, hexagon) potential wells by using the density functional theory. The development of the ground state electronic structure as a function of the dot size is of particular interest. First we show that in the case of two electrons, the Wigner molecule formation agrees with the previous exact diagonalization studies. Then we present in detail how the spin symmetry breaks in polygonal geometries as the spin density functional theory is applied. In several cases with more than two electrons, we find a transition to the crystallized state, yielding coincidence with the number of density maxima and the electron number. We show that this transition density, which agrees reasonably well with previous estimations, is rather insensitive to both the shape of the dot and the electron number.Comment: 8 pages, 11 figure

Stopping Power for Low-Energy Electrons

Descriptions of inelastic processes for the slowing down of electrons in condensed matter are presented for the energy range between a few eV and a few keV. Attempts at quantitative theories of stopping are summarized, with an emphasis on obtaining useful cross section expressions for Monte Carlo simulations and analytic transport theories. Inelastic scattering with both electrons (conduction and core) and density fluctuations (phonons) are included. The main emphasis in the theories for the former is in the dielectric (self-energy) formulation for the conduction band and in using generalized oscillator strengths or semiclassical excitation functions for the core. Recent applications to specific systems are discussed

Orientation of biological cells using plane-polarized Gaussian beam optical tweezers

Optical tweezers are widely used for the manipulation of cells and their internal structures. However, the degree of manipulation possible is limited by poor control over the orientation of trapped cells. We show that it is possible to controllably align or rotate disc shaped cells - chloroplasts of Spinacia oleracea - in a plane polarised Gaussian beam trap, using optical torques resulting predominantly from circular polarisation induced in the transmitted beam by the non-spherical shape of the cells.Comment: 9 pages, 6 figure