2,432 research outputs found
Diagrammatic quantum field formalism for localized electrons
We introduce a diagrammatic quantum field formalism for the evaluation of
normalized expectation values of operators, and suitable for systems with
localized electrons. It is used to develop a convergent series expansion for
the energy in powers of overlap integrals of single-particle orbitals. This
method gives intuitive and practical rules for writing down the expansion to
arbitrary order of overlap, and can be applied to any spin configuration and to
any dimension. Its applicability for systems with well localized electrons has
been illustrated with examples, including the two-dimensional Wigner crystal
and spin-singlets in the low-density electron gas.Comment: 13 pages, 0 figure
Rayleigh superradiance and dynamic Bragg gratings in an end-pumped Bose-Einstein condensate
We study experimentally superradiant Rayleigh scattering from a Bose-Einstein
condensate (BEC) in a new parameter regime where pump depletion and the
exchange of photons between the endfire modes are important. Through
experiments and simulations we show that collective atom light coupling leads
to the self-organized formation of dynamic Bragg gratings within the sample.
These gratings lead to an efficient back-scattering of pump photons and optical
resonator structures within the BEC.Comment: 5 pages, 3 figure
A Femtosecond Neutron Source
The possibility to use the ultrashort ion bunches produced by circularly
polarized laser pulses to drive a source of fusion neutrons with sub-optical
cycle duration is discussed. A two-side irradiation of a thin foil deuterated
target produces two countermoving ion bunches, whose collision leads to an
ultrashort neutron burst. Using particle-in-cell simulations and analytical
modeling, it is evaluated that, for intensities of a few ,
more than neutrons per Joule may be produced within a time shorter than
one femtosecond. Another scheme based on a layered deuterium-tritium target is
outlined.Comment: 15 pages, 3 figure
Sequential superradiant scattering from atomic Bose-Einstein condensates
We theoretically discuss several aspects of sequential superradiant
scattering from atomic Bose-Einstein condensates. Our treatment is based on the
semiclassical description of the process in terms of the Maxwell-Schroedinger
equations for the coupled matter-wave and optical fields. First, we investigate
sequential scattering in the weak-pulse regime and work out the essential
mechanisms responsible for bringing about the characteristic fan-shaped
side-mode distribution patterns. Second, we discuss the transition between the
Kapitza-Dirac and Bragg regimes of sequential scattering in the strong-pulse
regime. Finally, we consider the situation where superradiance is initiated by
coherently populating an atomic side mode through Bragg diffraction, as in
studies of matter-wave amplification, and describe the effect on the sequential
scattering process.Comment: 9 pages, 4 figures. Submitted to Proceedings of LPHYS'06 worksho
Model of optical phantoms thermal response upon irradiation with 975 nm dermatological laser
We have developed a numerical model describing the optical and thermal behavior of optical tissue phantoms upon laser irradiation. According to our previous studies, the phantoms can be used as substitute of real skin from the optical, as well as thermal point of view. However, the thermal parameters are not entirely similar to those of real tissues thus there is a need to develop mathematical model, describing the thermal and optical response of such materials. This will facilitate the correction factors, which would be invaluable in translation between measurements on skin phantom to real tissues, and gave a good representation of a real case application
Continuous-distribution puddle model for conduction in trilayer graphene
An insulator-to-metal transition is observed in trilayer graphene based on
the temperature dependence of the resistance under different applied gate
voltages. At small gate voltages the resistance decreases with increasing
temperature due to the increase in carrier concentration resulting from thermal
excitation of electron-hole pairs. At large gate voltages excitation of
electron-hole pairs is suppressed, and the resistance increases with increasing
temperature because of the enhanced electron-phonon scattering. We find that
the simple model with overlapping conduction and valence bands, each with
quadratic dispersion relations, is unsatisfactory. Instead, we conclude that
impurities in the substrate that create local puddles of higher electron or
hole densities are responsible for the residual conductivity at low
temperatures. The best fit is obtained using a continuous distribution of
puddles. From the fit the average of the electron and hole effective masses can
be determined.Comment: 18 pages, 5 figure
Energetic beams of negative and neutral hydrogen from intense laser plasma interaction
One of the most striking demonstrations of intermolecular forces is the tension at the surface of liquid n-alkanes. The prediction of surface tension is important in the design of distillation towers, extraction units and tower internals such as bubble caps and trays, since it has a considerable influence on the transfer of mass and energy across interfaces. Surface tension data are needed wherever foaming emulsification, droplet formation or wetting are involved. They are also required in a number of equations for two-phase flow calculations and for determining the flow regime. Petroleum engineers are especially interested in the surface tension in the extraction of crude oil to add surfactants to modify the interfacial properties between crude oil and the geological reservoir to improve production and increase oil yields. In this work, a simple computer program using Arrhenius-type asymptotic exponential function, Vandermoned matrix and Matlab technical computing language, is developed for the estimation of surface tension of paraffin hydrocarbons as a function of molecular weight and temperature. The surface tension is calculated for temperatures in the range of 250 to 440 K and paraffin hydrocarbons molecular weights between 30 and 250. The proposed numerical technique is superior owing to its accuracy and clear numerical background, wherein the relevant coefficients can be retuned quickly if more data become available in the future. Estimations are found to be in excellent agreement with the reliable data in the literature with average absolute deviation being less than 2%
Experimental evaluation of the response of micro-channel plate detector to ions with 10s of MeV energies
Ion acceleration in electrostatic field of charged cavity created by ultra-short laser pulses of 1020–1021 W/cm2
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