Confinement of two-dimensional excitons in a non-homogeneous magnetic field

The effective Hamiltonian describing the motion of an exciton in an external non-homogeneous magnetic field is derived. The magnetic field plays the role of an effective potential for the exciton motion, results into an increment of the exciton mass and modifies the exciton kinetic energy operator. In contrast to the homogeneous field case, the exciton in a non-homogeneous magnetic field can also be trapped in the low field region and the field gradient increases the exciton confinement. The trapping energy and wave function of the exciton in a GaAs two-dimensional electron gas for specific circular magnetic field configurations are calculated. The results show than excitons can be trapped by non-homogeneous magnetic fields, and that the trapping energy is strongly correlated with the shape and strength of the non-homogeneous magnetic field profile.Comment: 9 pages, 12 figure

Exciton trapping in magnetic wire structures

The lateral magnetic confinement of quasi two-dimensional excitons into wire like structures is studied. Spin effects are take into account and two different magnetic field profiles are considered, which experimentally can be created by the deposition of a ferromagnetic stripe on a semiconductor quantum well with magnetization parallel or perpendicular to the grown direction of the well. We find that it is possible to confine excitons into one-dimensional (1D) traps. We show that the dependence of the confinement energy on the exciton wave vector, which is related to its free direction of motion along the wire direction, is very small. Through the application of a background magnetic field it is possible to move the position of the trapping region towards the edge of the ferromagnetic stripe or even underneath the stripe. The exact position of this 1D exciton channel depends on the strength of the background magnetic field and on the magnetic polarisation direction of the ferromagnetic film.Comment: 10 pages, 7 figures, to be published in J. Phys: Condens. Matte

Conformations of dendrimers in dilute solution

Conformations of isolated homo- dendrimers of G=1-7 generations with D=1-6 spacers have been studied in the good and poor solvents, as well as across the coil-to-globule transition, by means of a version of the Gaussian self-consistent (GSC) method and Monte Carlo (MC) simulation in continuous space based on the same coarse-grained model. The latter includes harmonic springs between connected monomers and the pair-wise Lennard-Jones potential with a hard core repulsion. The scaling law for the dendrimer size, the degrees of bond stretching and steric congestion, as well as the radial density, static structure factor, and asphericity have been analysed. It is also confirmed that while smaller dendrimers have a dense core, larger ones develop a hollow domain at some separation from the centre.Comment: RevTeX, 14 pages, 19 PS figures, Accepted for publication in J. Chem. Phy

Manual de perdas pós-colheita em frutos e hortaliças.

Produção e perdas de frutos e hortaliças no Brasil. Perdas: definição, tipos e causas. Alguns aspectos envolvidos no controle das perdas: pré-colheita; colheita; pós-colheita; embalagem; transporte; armazenamento. Aspectos de mercado.bitstream/item/33932/1/1997-DOC-0027.pd

Optimization of the thyristor valves design for TCR static VAR compensators

Describes a program developed to optimize the design of thyristor valves for Static VAR Compensators (SVC) of the Thysristor Controlled Reactors (TCR) type

Reptilia, Squamata, Polychrotidae, <i>Anolis fuscoauratus</i> D'Orbigny, 1837: Distribution extension for the state of Rio Grande do Norte, Brazil

We provide the first record of Anolis fuscoauratus D’Orbigny, 1837 for the state of Rio Grande do Norte anda distribution map for the species in the Atlantic Rainforest of northeastern Brazil. The record of A. fuscoauratus in themunicipality of Tibau do Sul, state of Rio Grande do Norte, represents a distribution extension of 170 km northwesternfrom the municipality of Cabedelo, state of Paraíba. This is the northernmost record of this species in the Atlantic Rainforest

Timing the millisecond pulsars in 47 Tucanae

In the last 10 years 20 millisecond pulsars have been discovered in the globular cluster 47 Tucanae. Hitherto, only 3 of these had published timing solutions. Here we improve upon these 3 and present 12 new solutions. These measurements can be used to determine a variety of physical properties of the pulsars and of the cluster. The 15 pulsars have positions determined with typical uncertianties of only a few milliarcsec and they are all located within 1.2 arcmin of the cluster centre. We have also measured the proper motions of 5 of the pulsars, which are consistent with the proper motion of 47 Tuc based on Hipparcos data. The period derivatives measured for many of the pulsars are dominated by the dynamical effects of the cluster gravitational field, and are used to constrain the surface mass density of the cluster. All pulsars have characteristic ages T > 170 Myr and magnetic fields B < 2.4e9 Gauss, and the average T > 1 Gyr. We have measured the rate of advance of periastron for the binary pulsar J0024-7204H, implying a total system mass 1.4+-0.8 solar masses.Comment: 17 pages, 11 included figures, accepted for publication in MNRA