Violation Of Charge Neutrality Research Articles

Синтез, структурные и магнитные свойства NaZnFe-=SUB=-2-=/SUB=-(VO-=SUB=-4-=/SUB=-)-=SUB=-3-=/SUB=-

A new magnetic compound NaZnFe2(VO4)3 obtained by solid-phase synthesis has been studied using X-ray diffraction, Mössbauer spectroscopy, electron paramagnetic resonance, measurement of the temperature dependence of the dielectric permeability, and magnetometry. The crystalline structure of NaZnFe2(VO4)3 is described by a triclinic spatial group of symmetry P1 with the parameters of an elementary crystalline chain: a = 6.74318 (7) Å, b = 8.1729 (1) Å, c = 9.8421 (1) Å, α = 106.2611 (9)º, β = 104.55 (1)º, γ = 102.337 (1)º, V = 479.88 (1) Å3, Z = 2. Magnetic Fe3+ cations in the cell occupy six positions populated together with diamagnetic Zn2+ cations, which leads to states of magnetic inhomogeneity and local violation of charge neutrality. Data from resonance and magnetic studies of NaZnFe2(VO4)3 confirm the main role of high-spin Fe3+ iron cations in the formation of magnetism with competing exchange magnetic interactions and a high value of the frustration index. It is shown that the magnetic subsystem of a sample with a negative asymptotic Neel temperature undergoes a magnetic transition from the paramagnetic state to the magnetic state of the spin glass when the temperature decreases. T.V. Drokina 1, M.S. Molokeev M.C. 1, 2, D.A. Velikanov 1, O.A. Bayukov 1, A.M. Vorotynov 1, A.L. Freidman 1, G.A. Petrakovskii 1 1 L.V. Kirensky Institute of Physics, Federal Research Center KSC Siberian Branch of Russian Academy of Science, Krasnoyarsk, 660036, Russia 2 Siberian Federal University, Krasnoyarsk, 660074, Russia

Simulations of electrolyte between charged metal surfaces.

We present a new method for simulating ungrounded charged metal slabs inside an electrolyte solution. The ions are free to move between the interior and exterior regions of the slab-electrolyte system. This leads to polarization of both sides of each slab, with a distinct surface charge induced on each surface. Our simulation method is based on the exact solution of the Poisson equation using periodic Green functions. To efficiently perform the calculations, we decouple the electrostatic energy due to surface polarization from that of purely Coulomb interaction between the ions. This allows us to combine a fast 3D Ewald summation technique with an equally fast calculation of polarization. As a demonstration of the method, we calculate ionic density profiles inside an electrolyte solution and explore charge neutrality violation in between charged metal slabs.

Charge neutrality breakdown in confined aqueous electrolytes: Theory and simulation.

We study, using Density Functional theory (DFT) and Monte Carlo simulations, aqueous electrolyte solutions between charged infinite planar surfaces, in contact with a bulk salt reservoir. In agreement with recent experimental observations [Z. Luo et al., Nat. Commun. 6, 6358 (2015)], we find that the confined electrolyte lacks local charge neutrality. We show that a DFT based on a bulk-HNC expansion properly accounts for strong electrostatic correlations and allows us to accurately calculate the ionic density profiles between the charged surfaces, even for electrolytes containing trivalent counterions. The DFT allows us to explore the degree of local charge neutrality violation, as a function of plate separation and bulk electrolyte concentration, and to accurately calculate the interaction force between the charged surfaces.

Modulation doping of quantum dot laser active area and its impact on lasing performance

We present a theoretical study of modulation doping of active region in the quantum dot (QD) laser and corresponding issues of QD charge neutrality violation, a band diagram of the laser and charge carriers distribution in the structure. Modulation doping is discussed as a possible technique to control laser output characteristics. It was shown that modulation doping leads to an increase of threshold current of lasing through excited QD optical transition together with power emission from QD ground state.

Constrained-DFT method for accurate energy-level alignment of metal/molecule interfaces

We present a computational scheme for extracting the energy level alignment of a metal/molecule interface, based on constrained density functional theory and local exchange and correlation functionals. The method, applied here to benzene on Li(100), allows us to evaluate charge transfer energies, as well as the spatial distribution of the image charge induced on the metal surface. We systematically study the energies for charge transfer from the molecule to the substrate as function of the molecule-substrate distance, and investigate the effects arising from image charge confinement and local charge neutrality violation. For benzene on Li(100) we find that the image charge plane is located at about 1.8 \AA above the Li surface, and that our calculated charge transfer energies compare perfectly with those obtained with a classical electrostatic model having the image plane located at the same position. The methodology outlined here can be applied to study any metal/organic interface in the weak coupling limit at the computational cost of a total energy calculation. Most importantly, as the scheme is based on total energies and not on correcting the Kohn-Sham quasiparticle spectrum, accurate results can be obtained with local/semi-local exchange and correlation functionals. This enables a systematic approach to convergence.

An efficient iterative grand canonical Monte Carlo algorithm to determine individual ionic chemical potentials in electrolytes

Two iterative procedures have been proposed recently to calculate the chemical potentials corresponding to prescribed concentrations from grand canonical Monte Carlo (GCMC) simulations. Both are based on repeated GCMC simulations with updated excess chemical potentials until the desired concentrations are established. In this paper, we propose combining our robust and fast converging iteration algorithm [Malasics, Gillespie, and Boda, J. Chem. Phys. 128, 124102 (2008)] with the suggestion of Lamperski [Mol. Simul. 33, 1193 (2007)] to average the chemical potentials in the iterations (instead of just using the chemical potentials obtained in the last iteration). We apply the unified method for various electrolyte solutions and show that our algorithm is more efficient if we use the averaging procedure. We discuss the convergence problems arising from violation of charge neutrality when inserting/deleting individual ions instead of neutral groups of ions (salts). We suggest a correction term to the iteration procedure that makes the algorithm efficient to determine the chemical potentials of individual ions too.

Temperature-insensitive semiconductor quantum dot laser

Different approaches to the design of a genuinely temperature-insensitive quantum dot (QD) laser are proposed. Suppression of the parasitic recombination outside the QDs, which is the dominant source of the temperature dependence of the threshold current in the conventional design of a QD laser, is accomplished either by tunneling injection of carriers into the QDs or by band-gap engineering. Elimination of this recombination channel alone enhances the characteristic temperatures T 0 above 1000 K. Remaining sources of temperature dependence (recombination from higher QD levels, inhomogeneous line broadening, and violation of charge neutrality in QDs) are studied. Tunneling-injection structures are shown to offer an additional advantage of suppressed effects of inhomogeneous broadening and neutrality violation.

THEORY OF THRESHOLD CHARACTERISTICS OF QUANTUM DOT LASERS: EFFECT OF QUANTUM DOT PARAMETER DISPERSION

Gain and threshold current of a quantum dot (QD) laser are analyzed theoretically taking into account the inhomogeneous line broadening caused by fluctuations in QD parameters. Two regions of QD filling by carriers, nonequilibrium and equilibrium, are identified, depending on temperature, barrier heights and QD size. Critical tolearable parameters of the QD structure, at which lasing becomes impossible to attain, are shown to exist. The minimum threshold current density and optimum parameters are calculated. Violation of charge neutrality in QDs is revealed, which affects significantly the threshold current and its temperature dependence. The gain-current dependence is calculated. The voltage dependences of the electron and hole level occupancies in QDs, gain and current are obtained. The observed temperature dependence of threshold current (constant at low temperatures and rapid increase above a certain temperature) is predicted and explained. Violation of charge neutrality is shown to give rise to the slight temperature dependence of the current component associated with the recombination in QDs. The characteristic temperature T0 is calculated considering carrier recombination in the optical confinement layer and violation of charge neutrality in QDs. The inclusion of violation of charge neutrality is shown to be critical for the correct calculation of T0 at low T. T0 is shown to fall off profoundly with increasing T. Theoretical estimations confirm the possibility of a significant reduction of the threshold currents and enhancement of T0 of QD lasers as compared with the conventional quantum wall (QW) lasers. Longitudinal spatial hole burning is analyzed. Unlike QW lasers, thermally excited escapes of carrier from QDs, rather than diffusion, are shown to control the smoothing-out of the spatially nonuniform population inversion and multimode generation in QD lasers. A decrease in the QD size dispersion is shown not only to decrease the threshold current but to increase considerably the relative multimode generation threshold as well. Concurrent with the increase of threshold current, an increase of the multimode generation threshold is shown to occur with a rise in temperature. Ways to optimize the QD laser, aimed at maximizing the multimode generation threshold, are outlined.

Charge neutrality violation in quantum-dot lasers

Theory of quantum-dot (QD) lasers is augmented to include, in a self-consistent manner, the QD-layer charge. The electron- and hole-level occupancies in QDs are obtained through the solution of the problem for the electrostatic-field distribution across the junction. They are shown to differ from each other. As a result, the local neutrality is broken down in each QD, i.e., the QD layer is charged. The key dimensionless parameters controlling the difference of the hole- and electron-level occupancies are revealed. The detailed analysis of the gain and spontaneous radiative recombination current density is given, having regard to the fact of violation of the charge neutrality in QDs. The gain-current density dependence is calculated, The voltage dependences of the electron- and hole-level occupancies, gain, and current density are obtained. Particular emphasis is given to the transparency and lasing threshold characteristics. Optimization of the QD-laser structure is carried out. The optimum surface density of QDs, minimizing the threshold current density, is shown to be distinctly higher than that calculated without regard for the lack of the charge neutrality in QDs.