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 $Z_1^3$ correction to the stopping power of a 2DEG for charged particles moving at a fixed distance from the plane of the 2D sheet, $Z_1$ 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 $Z_1^3$ 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 $E_0(\infty) \in (0, \infty]$, 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