Coulomb Interaction Of Electrons Research Articles

Time-dependent transport of electrons through a photon cavity

We use a non-Markovian master equation to describe the transport of Coulomb-interacting electrons through an electromagnetic cavity with one quantized photon mode. The central system is a finite-parabolic quantum wire that is coupled weakly to external parabolic quasi-one-dimensional leads at $t=0$. With a stepwise introduction of complexity to the description of the system and a corresponding stepwise truncation of the ensuing many-body spaces, we are able to describe the time-dependent transport of Coulomb-interacting electrons through a geometrically complex central system. We take the full electromagnetic interaction of electrons and cavity photons without resorting to the rotating-wave approximation or reduction of the electron states to two levels into account. We observe that the number of initial cavity photons and their polarizations can have important effects on the transport properties of the system. The quasiparticles formed in the central system have lifetimes limited by the coupling to the leads and radiation processes active on a much longer time scale.

Nonlinear diffraction in a quantum-dot system with allowance for the Hubbard interaction

The propagation of monochromatic laser radiation in a volume system of quantum dots (QDs) that are tunnel-coupled along one axis is considered. The electron energy spectrum of the QD system is modeled in the tight-binding approximation with allowance for the Coulomb interaction of electrons in the Hubbard model. The electromagnetic field of laser radiation in a QD system is described quasi-classically by Maxwell equations; as applied to this problem, they are reduced to a non-one-dimensional wave equation for the vector potential. As a result of the analysis of the wave equation in the approximation of varying amplitudes and phases, an effective equation describing the electromagnetic field in a QD system is obtained and numerically solved. The influence of the parameters of the system and the amplitude and frequency of the field of incident laser radiation on the character of its propagation is investigated. Nonmonotonic dependences of the factor characterizing the laser beam diffraction spread on the parameters of the electron energy spectrum of the system are obtained.

Nonlinear diffraction of super-gaussian laser beams in an array of quantum-dot chains with coulomb interaction taken into account

We consider the propagation of super-Gaussian monochromatic laser beams in a three-dimensional array of quantum dots coupled by the tunneling effect along one axis. The electron energy spectrum of the system corresponds to the Hubbard model, where the Coulomb interaction of electrons in quantum dots is taken into account. The field of the laser beam is described by the Maxwell equations, from which a nonhomogeneous wave equation for the vector potential is obtained. In the approximation of slowly varying amplitudes and phases, the wave equation is reduced to a phenomenological equation describing the electromagnetic field in an array of chains of quantum dots. We study the influence of the system parameters and the frequency of the laser-beam field on the propagation in the medium by solving numerically the phenomenological equation. We obtain the dependence of the factor characterizing the diffraction blooming of the beam in an array of chains of quantum dots on the parameters of the system’s electron energy spectrum.

Study of the one-dimensional periodic polaron structures

One-dimensional and quasi one-dimensional electron structures are of applied interest. For example, in one-dimensional (nano-capillary) electroneutral metal–ammonia systems, exotic electron properties are observed, such as a drastic (by several orders of magnitude) drop of the electrical conductivity with decreasing temperature, which resembles the superconductivity transition. In this work, we studied the possibility of one-dimensional filamentary polaron nano-structure in insulating media. It was established that the interpolaron pair potential for large polarons offers attraction properties. It is known that attraction between the particles may alter the collective properties of a many-particle system. We demonstrated that the initially uniform distribution of the particles becomes unstable in one-dimensional systems and may change to the nonuniform structured state under specific conditions imposed on the temperature, particle concentration, and parameters of the pair interpolaron potential. The possibility of existence of a periodic one-dimensional structure of small-amplitude polarons that is imposed on the polaron uniform distribution is estimated in terms of temperature and concentration criteria. A dispersion relation between existence of the one-dimensional polaron structure and translational velocity of the polarons is found. The upper limit of the translational velocity when the periodic contribution to the distribution vanishes is determined. Periodic contribution disappears virtually stepwise as the velocity approaches its critical value. It is shown that this specific polaron–polaron interaction leads to results that are in principal different from those observed for classical Coulomb electron interaction.

Electronic structure and Mott localization of iron-deficient TlFe1.5Se2with superstructures

Electronic structure and magnetic properties for iron deficient TlFe$_{2-x}$Se$_2$ compounds are studied by first-principles calculations. We find that for the case of $x=0.5$ with a Fe-vacancy ordered orthorhombic superstructure, the ground state exhibits a stripe-like antiferromagnetic ordering and opens a sizable band gap if the short-ranged Coulomb interaction of Fe-3d electrons is moderately strong, manifesting a possible Mott insulating state. While increasing Fe-vacancies from the $x=0$ side, where the band structure is similar to that of a heavily electron-doped FeSe system, the Mott localization can be driven by kinetic energy reduction as evidenced by the band narrowing effect. Implications of this scenario in the recent experiments on TlFe$_{2-x}$Se$_2$ are discussed.

Crystal field and magnetic structure of UO2

The properties of UO$_{2}$ result from rich $f$-electron physics, including electronic Coulomb interactions, spin-orbit and crystal field effects, as well as inter-ionic multipolar coupling. We present a comprehensive theoretical study of the electronic structure of UO$_{2}$ using a combined application of self-consistent DFT+$U$ calculations and a model Hamiltonian. The $\Gamma_{5}$ ground state of U$^{4+}$ and the energies of crystal field excitations $\Gamma_{5} \rightarrow \Gamma_{3,4,1}$ are reproduced in very good agreement with experiment. We also investigate competing non-collinear magnetic structures and confirm 3-k as the T=0 K ground state magnetic structure of UO$_2$.

Enhancement of room temperature ferromagnetism of Fe-doped ZnO epitaxial thin films with Al co-doping

Epitaxial films of ZnO doped with magnetic ion Fe and, in some cases, with 1% Al show clear evidence of room temperature ferromagnetic ordering. The Al doped optimized samples with carrier concentration n c∼8.0×10 20 cm −3 show about 3 times enhanced saturation magnetization (0.58 μ B/Fe 2+) than the one with n c∼3.0×10 20 cm −3 (0.18 μ B/Fe 2+). A clear correlation between the magnetization per transition metal ion and the ratio of the number of carriers to the number of donors have been found as is expected for carrier-induced room temperature ferromagnetism. The transport mechanism of the electrons in all the DMS films at low temperature range has been identified with the Efros's variable range hopping due to the electron–electron Coulomb interaction.

Competing chiral and multipolar electric phases in the extended Falicov-Kimball model

We study the effects of interband hybridization within the framework of an extended Falicov-Kimball model with itinerant $c$ and $f$ electrons. An explicit interband hybridization breaks the U(1) symmetry associated with the conservation of the difference between the total number of particles in each band. As a result, the degeneracy between multipolar electric and chiral orderings is lifted. We analyze the weak- and strong-coupling limits of the $c$-$f$ electron Coulomb interaction at zero temperature, and derive the corresponding mean-field quantum phase diagrams at half-filling for a model defined on a square lattice.

Monte Carlo based microscopic description of electron transport in GaAs/Al0.45Ga0.55As quantum-cascade laser structure

Results of multiparticle Monte Carlo simulations of midinfrared quantum cascade lasers structure initially fabricated by Page et al. are presented. The main aim of this paper is to discuss in details how electric current flows through the structure and which subbands are involved in this process. Monte Carlo method allows to predict the electron population inversion between the lasing levels and gives microscopic insight into processes leading to such behavior. Importance of a subband belonging to the laser injector region, with energy slightly below the upper lasing level, is demonstrated. The electron–electron Coulomb interactions influence the shapes of electron distribution functions; the values of average electron energies and effective subbands’ temperatures are calculated.

Pulsed and monochromatic excitation of semiconductor quantum wells under the conditions of quantum confinement

The reflectance and absorbance of light by quantum wells whose width is comparable to the light wavelength have been calculated. The difference in the refractive indices of the materials of the quantum well and the barriers has been taken into account. Pulsed irradiation with an arbitrary shape of the exciting pulse has been considered, and the existence of two closely spaced discrete excitation levels has been assumed. This pair of levels can correspond to two magnetopolaron states in a quantizing magnetic field directed perpendicular to the plane of the quantum well. The ratio between the magnitudes of nonradiative and radiative dampings of electronic excitations is arbitrary. The final results have been obtained without invoking the approximation in which the Coulomb interaction of electrons and holes is negligible.

Tc map and superconductivity of simple metals at high pressure

We calculate Tc map in region of weak electron–phonon coupling based on simple phonon spectrum. By using linear-response method and density functional theory, we calculate phonon spectra and Eliashberg functions of simple metals under pressure. Based on the evolutions of superconducting parameters of simple metals on the Tc map with increasing pressure, we find that there are two different responses to pressure for simple metals: (1) enhancing electron–phonon interaction λ such as for La and Li, (2) increasing phonon frequency such as for Pb, Pt. The λ threshold effect is found, which origins from the competition between electron–phonon interaction and electron–electron Coulomb interaction and is the reason why Tc of most superconductors of simple metals are higher than 0.1K.

Thermoelectric and thermal rectification properties of quantum dot junctions

The electrical conductance, thermal conductance, thermal power and figure of merit (ZT) of semiconductor quantum dots (QDs) embedded into an insulator matrix connected with metallic electrodes are theoretically investigated in the Coulomb blockade regime. The multilevel Anderson model is used to simulate the multiple QDs junction system. The charge and heat currents in the sequential tunneling process are calculated by the Keldysh Green function technique. In the linear response regime the ZT values are still very impressive in the small tunneling rates case, although the effect of electron Coulomb interaction on ZT is significant. In the nonlinear response regime, we have demonstrated that the thermal rectification behavior can be observed for the coupled QDs system, where the very strong asymmetrical coupling between the dots and electrodes, large energy level separation between dots and strong interdot Coulomb interactions are required.

The effect of interband interactions of phonon and charge fluctuation on the superconducting parametersof MgB2

We have investigated the effect of an unconventional pairing mechanism inMgB2 using a two-band model within the framework of Bogoliubov–Valatin formalism. Theapproach incorporates the intraband s-wave interaction in the s- and p-bands, as well asinterband s-wave interaction between them. The analysis assumes that the pairinginteraction matrix comprises of attractive electron–phonon, charge fluctuation andrepulsive electron–electron (Coulomb) interactions to account for superconductivity inMgB2. The model is used to estimate the transition temperature and isotope effect exponent aswell as to elucidate the importance of interband contributions in the superconductivity ofthe system

Thermoelectric Properties of Multiple Quantum Dots Junction System

The heat current through multiple semiconductor quantum dots embedded in an insulator matrix connected with electrodes is investigated in the framework of Green's function technique. The electrical conductance, thermal conductance, thermal power and figure of merit of a system are calculated and analyzed. Although the interdot as well as intradot Coulomb interactions significantly suppress the figure of merit, the effect of electron Coulomb interactions on ZT can be neglected when the electron Coulomb interactions are ten times larger than the thermal temperature. The temperature-dependent Lorenz number shows a violation of the Wiedemann–Franz law.

Experimental demonstration of resonant-tunneling-diode operation beyond quasibound-state-lifetime limit

We show, first, that the charge relaxation (response) time of resonant-tunneling diode (RTD) can be significantly shorter or longer than the resonant-state lifetime, depending on RTD operating point and RTD parameters. Coulomb interaction of electrons is responsible for the effect. Second, we demonstrate that the operating frequencies of RTDs are limited neither by resonant-state lifetime, nor by relaxation time; particularly in the RTDs with heavily doped collector, the differential conductance can stay negative at the frequencies far beyond the limits imposed by both time constants. We provide experimental evidences for both effects.

Magnetic scattering of neutrons by the itinerant electron ferromagnets at high temperatures

The magnetic scattering of neutrons at T> Tc is investigated on the basis of a simple model in which the electron Coulomb interaction is effective only in an atomic cell and the one-particle energy forms a non-degenerate energy band. First, the magnetic scattering is investigated at temperatures T satisfying Tc < T << Tf, Tf being the degeneracy temperature of electrons or holes. Special attention has been paid to the paramagnetic scattering in the 65% Fe–35% Ni alloy for which Tc=490° is small as compared with the accessible temperatures T˜ 1000°. Second, the magnetic scattering at extremely high temperature, T >> Tf or the bandwidth, is investigated. The cross section is independent of both T and the scattering angle. The absolute value is just one half of that for the localized spin model if the number of electrons is equal to the number of atoms (half-filled band). The ω dependence of the cross section is much broader than that in the case of the localized spin model. The cross section oscillates as a function of ω and even diverges at some particular values of ω in some special cases.

Screening of the Coulomb field in a magnetized electron gas of a quantum cylinder

The quantum theory is constructed for screening of the Coulomb field of a point charge in a magnetized electron gas of a quantum cylinder. The asymptotics of the screened potential are calculated for both degenerate and Boltzmann electron gases. It is demonstrated that, in the degenerate case, apart from the known quasi-classical monotonic part, the result contains the quantum oscillating part, which corresponds to Friedel oscillations. The Aharonov-Bohm oscillations of the screened Coulomb interaction of electrons on a cylindrical surface are described analytically. It is shown that the Friedel oscillations can be represented as a superposition of oscillations with different frequencies which are determined by the macroscopic properties of the nanotube.

Optical Absorption Spectra and Dynamic Jahn-Teller Effect of V2+ Ions in ZnSe

The Hamiltonian matrices for 3d3 ions in a cubic crystal field are introduced, based on a molecular orbital model, including the electronic Coulomb and tetrahedral crystal-field interactions and the spin-orbit coupling. The optical absorption spectra of V2+ ions in ZnSe are studied. Moreover, the various additional levels found close to 5680 cm−1 are considered. These levels are assumed to result from the dynamic Jahn-Teller splitting within the excitation levels 2T2 and 2T1 in ZnSe:V2+. The good agreement between the present results and the experimental observations indicates that the contribution of the covalence reduction factors NE and NT2 and of the Racah parameter A to the optical absorption spectra of V2+ ions in ZnSe is important. However, most of the previous theoretical studies of these spectra in ZnSe:V2+ have neglected the Racah parameter A, based on the classical crystalfield model. A significant charge-transfer effect found in recent works is confirmed in ZnSe:V2+.

Prepositioned single quantum dot in a lateral electric field

We examine the effect of a lateral electric field on the optical properties of a single deterministically positioned InAs/InP quantum dot. We show experimentally that the ground-state excitonic Stark shift is significantly reduced in comparison with the single-particle picture and that the lateral electric field introduces a new previously forbidden optical transition. Results of full configuration-interaction calculations show that the Coulomb interactions of electrons and holes are modified by the electric field leading to the compensation of the single-particle Stark shift. The calculations also account for the appearance of the field-activated optical transition as an excitonic recombination event. The comparison of exciton and predicted charged exciton spectra allows us to exclude the presence of charged exciton complexes within the measured emission spectra. The ability to precisely position a single quantum dot and demonstrate control over the electronic properties of such a dot is expected to find application in scalable techniques for quantum information science.

First-Principles Study on the Effects of Zr Dopant on the CO Adsorption on Ceria

With use of the first-principles density functional theory with the inclusion of the on-site Coulomb interaction of the Ce4f electrons (DFT+U) method, the adsorption of CO on the (110) surface of a stoichiometric Ce0.75Zr0.25O2 system is studied systematically. It is found that, (1) in addition to the carbonate-like CO3 structures, there exists a new adsorption species (a bent CO2− structure) on the Ce0.75Zr0.25O2(110) surface, and (2) the Zr-dopant can enhance the adsorption of CO and improve the CO oxidation process on the ceria surface by promoting the desorption of CO2.