24 research outputs found
Nonlinear screening in two-dimensional electron gases
We have performed self-consistent calculations of the nonlinear screening of
a point charge Z in a two-dimensional electron gas using a density functional
theory method. We find that the screened potential for a Z=1 charge supports a
bound state even in the high density limit where one might expect perturbation
theory to apply. To explain this behaviour, we prove a theorem to show that the
results of linear response theory are in fact correct even though bound states
exist.Comment: 4 pages, 4 figure
Spontaneous spin polarization in doped semiconductor quantum wells
We calculate the critical density of the zero-temperature, first-order
ferromagnetic phase transition in n-doped GaAs/AlGaAs quantum wells. We find
that the existence of the ferromagnetic transition is dependent upon the choice
of well width. We demonstrate rigorously that this dependence is governed by
the interplay between different components of the exchange interaction and that
there exists an upper limit for the well width beyond which there is no
transition. We predict that some narrow quantum wells could exhibit this
transition at electron densities lower than the ones that have been considered
experimentally thus far. We use a screened Hartree-Fock approximation with a
polarization-dependent effective mass, which is adjusted to match the critical
density predicted by Monte Carlo calculations for the two-dimensional electron
gas.Comment: Submitted to Eur. Phys. Journal
INTERLAYER VORTICES AND EDGE DISLOCATIONS IN HIGH TEMPERATURE SUPERCONDUCTORS
The interaction of an edge dislocation made of half the superconducting plane
with a magnetic interlayer vortex is considered within the framework of the
Lawrence-Doniach model with negative as well as positive Josephson interlayer
coupling. In the first case the binding energy of the vortex and the
dislocation has been calculated by employing a variational procedure. The
current distribution around the bound vortex turns out to be asymmetric. In the
second case the dislocation carries a spontaneous magnetic half-vortex, whose
binding energy with the dislocation turns out to be infinite. The half-vortex
energy has been calculated by the same variational procedure. Implications of
the possible presence of such half-vortices for the properties of high
temperature superconductors are discussed.Comment: 14 Latex pages, 1 figure available upon request
Quadratic electronic response of a two-dimensional electron gas
The electronic response of a two-dimensional (2D) electron system represents
a key quantity in discussing one-electron properties of electrons in
semiconductor heterojunctions, on the surface of liquid helium and in
copper-oxide planes of high-temperature superconductors. We here report an
evaluation of the wave-vector and frequency dependent dynamical quadratic
density-response function of a 2D electron gas (2DEG), within a self-consistent
field approximation. We use this result to find the correction to the
stopping power of a 2DEG for charged particles moving at a fixed distance from
the plane of the 2D sheet, being the projectile charge. We reproduce, in
the high-density limit, previous full nonlinear calculations of the stopping
power of a 2DEG for slow antiprotons, and we go further to calculate the
correction to the stopping power of a 2DEG for a wide range of
projectile velocities. Our results indicate that linear response calculations
are, for all projectile velocities, less reliable in two dimensions than in
three dimensions.Comment: 17 pages, 5 figures, to appear in Phys. Rev.
Wegner Estimate and Disorder Dependence for Alloy-Type Hamiltonians with Bounded Magnetic Potential
We consider non-ergodic magnetic random Sch\"odinger operators with a bounded
magnetic vector potential. We prove an optimal Wegner estimate valid at all
energies. The proof is an adaptation of the arguments from [Kle13], combined
with a recent quantitative unique continuation estimate for eigenfunctions of
elliptic operators from [BTV15]. This generalizes Klein's result to operators
with a bounded magnetic vector potential. Moreover, we study the dependence of
the Wegner-constant on the disorder parameter. In particular, we show that
above the model-dependent threshold , it is
impossible that the Wegner-constant tends to zero if the disorder increases.
This result is new even for the standard (ergodic) Anderson Hamiltonian with
vanishing magnetic field
Analysis of thermoplastic propellants and their ingredients with DSC and tga
The goal of this paper is to study the thermal behavior of thermoplastic rocket propellants. Thermal decomposition of some thermoplastic propellant compositions (based on polyvinyl chloride, ammonium perchlorate, and aluminum and potassium nitrate) and their ingredients has been examined. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) have been applied for this. The results obtained for propellants have been compared with those obtained for their ingredients. All examined propellants underwent thermal decomposition in two exothermic steps, after a slow endothermic reaction in the beginning. The addition of aluminum did not change this trend, while the addition of potassium nitrate had significant influence on it and exothermic reactions were running in three steps. Special attention was devoted to specific heats' determination of propellant compositions and their ingredients, and obtained results were compared with calculated ones
Structure of inhomogeneous polymer networks prepared from telechelic polybutadiene
The structure and dynamics of segmented polyurethane networks prepared from poly(butadiene) diols (PBD) of different molecular weights M-n = 1200, 2160,2650,4690 and 10,300 g mol(-1), 4,4'-diphenylmethane diisocyanate (MDI) and poly(oxypropylene) triol (POPT) (M, = 710 g mol-1) have been investigated by dynamic mechanical analysis, dielectric relaxation spectroscopy, differential scanning calorimetry (DSC), small- and wide-angle X-ray scattering (SAXS and WAXS). Microphase separation on Danometre scale has been confirmed by SAXS in all networks. Except for the highest molecular weight of PBD, the networks consist of microdomains of similar size and composition dispersed in the soft phase of polybutadiene segments. Dielectric spectroscopy revealed the existence of three types of noncrystalline phases, each characterised by a well-developed dynamic glass transition. Absence of crystallinity has been proven by WAXS and DSC. (C) 2002 Elsevier Science Ltd. All rights reserved.status: publishe
Epoxy networks and thermosensitive hydrogels prepared from a,x-diamino terminated polyoxypropylene and polyoxyethylene bis(glycidyl ether)
A series of stoichiometric hydrophilic epoxy networks was prepared by reaction of a,x-diamino
terminated polyoxypropylene of average molar mass 430, 2000 and 4000 g/mol
with polyoxyethylene bis(glycidyl ether) of average molar mass 526 g/mol. The networks
were investigated by dynamic mechanical analysis (DMA) and differential scanning calorimetry
(DSC). Single glass transitions observed in the networks strongly suggest that
the networks present homogeneous single phase, i.e., polyoxypropylene (POP) and polyoxyethylene
(POE) are homogeneously built in the network structure. The homogeneity
of the networks is also confirmed by absence of a crystallization of POE. Hydrogels obtained
by swelling of the networks in water exhibit a strong dependence of swelling degree on
temperature. The temperature sensitivity of the hydrogels increases with growing molar
mass of the POP used in the network preparation. The swelling behavior of the hydrogel
containing POP of highest molar mass (4000 g/mol) is close to a discontinuity expected
at a temperature ca 10 11 ºC.The result was developed within the CENTEM Project, Reg. No. CZ.1.05/2.1.00/03.0088, co-funded by the ERDF as part of the Ministry of Education, Youth and Sports OP RDI programme. The authors appreciate a generous gift of the Jeffamines used in this study, by Huntsman Corporation.Krakovsky, I.; Cortes Cayuela, J.; Sabater I Serra, R.; Salmerón Sánchez, M.; Mohan Dodda, J. (2014). Epoxy networks and thermosensitive hydrogels prepared from a,x-diamino terminated polyoxypropylene and polyoxyethylene bis(glycidyl ether). European Polymer Journal. 55:144-152. https://doi.org/10.1016/j.eurpolymj.2014.03.032S1441525