Coulomb Screening Effect Research Articles

Superconductivity-induced resonant Raman scattering in multilayer high-Tcsuperconductors

Resonant Raman scattering below Tc has been discovered in several Bi-, Hg-, Tl-based high-Tc superconductors with three or four CuO2-layers. For Bi2Si2Ca2Cu3O10+d, we found an unexpected crossover of the pair-breaking peak in the A1g-spectrum from a broad bump at hw = 6kBTc for Eexc = 2.54eV to a sharp peak at hw = 8kBTc for Eexc = 2.18eV, together with a strong enhancement of the Ca-phonons. Under resonant conditions, the relative positions of the pair breaking peaks in A1g, B1g, and B2g channels are 2Delta(A1g) = 2Delta(B1g) > 2Delta(B2g). This relation implies that the A1g Raman channel is free from the Coulomb screening effect, just as predicted theoretically for a d-wave multi-layer superconductor but have never been observed experimentally thus far. The observed resonance effect is the evidence that the electronic state in the inner CuO2-planes is different from that of the outer CuO2-planes.

Spin-orbit interaction and spin-charge interference in resonant Raman scattering from III-V semiconductor quantum wells

Due to the spin-orbit interaction in ${A}_{3}{B}_{5}$ semiconductor quantum wells, the resonant Raman scattering amplitude from the charge density excitations (CDE) interferes with that from the spin density excitations (SDE). This spin-charge coupling manifests itself in an asymmetry of the non-spin-flip Raman spectrum with respect to directions of circular polarizations of incident and scattered photons. Consequently, the difference spectrum obtained by subtracting the spectra taken at reversed polarizations has a band in the region of single-particle spin conserving transitions. Since CDE are involved, Coulomb screening effects are expected to have strong influence on the intensity of this band. We have calculated the difference spectrum, taking into account the long range Coulomb interaction in the random phase approximation. We have found that this interaction does not destroy the spin-charge coupling. Our calculations suggest that the experimentally observed non-spin-flip band in the Raman difference spectrum of a GaAs/AlGaAs heterostructure serves as an evidence of the CDE-SDE interference.

Time-resolved micro-photoluminescence of epitaxial laterally overgrown GaN

Epitaxial laterally overgrown GaN (ELOG) structures are microscopically characterized using cathodoluminescence (CL), micro-Raman spectroscopy and time-resolved micro-photoluminescence. Two ELOG samples consisting of a 3 μm thick GaN buffer layer on (0 0 0 1) sapphire masked with SiO 2 stripes parallel to 〈1 1 0 0〉 and 〈1 1 2 0〉 direction, respectively, were investigated. Beside their technological relevance ELOG samples are a superior subject of investigations due to internal gradients in strain and free carrier concentration. To study the influence of the different lateral growth mechanisms on the optical properties of the GaN in the coherently grown and in the overgrown region, we correlate the temporal behavior of the near band gap luminescence with the local free carrier concentration as determined by the peak position of the coupled phonon plasmon modes (LPP). The spatial resolution of both of the microscopic methods is about 1 μm. As a result we observe distinct differences in the decay times of the near band gap luminescence for the two different samples as well as for different regions within one sample. The observed behavior is explained by the fact that extrinsic or intrinsic defects give rise to shallow donor levels, causing a gradient in free carrier concentration. Therefore the near-band-gap emission changes from excitonic luminescence to band-to-band recombination depending on the distance from the interface as evidenced by micro-photoluminescence. Beside these drastic changes we also observe a change in decay times in the region of excitonic emission correlated with the donor concentration which leads to a different influence of the coulomb screening effect.

Electronic Raman scattering in single and double CuO 2-layer Tl [sbnd]Ba [sbnd](Ca) [sbnd]Cu [sbnd]O superconductors

Low energy Raman continuum and the redistribution of the continuum to a peak (‘2Δ-peak’) in the superconducting state have been studied in Tl Ba (Ca) Cu O superconductors with single CuO 2 layer (Tl-2201) and double CuO 2 layer (Tl-2212). The Coulomb screening effect is observed to be much stronger in Tl-2201. The change from Tl-2201 to Tl-2212 of the normalized A 1 g 2Δ-peak intensity is identical within experimental error to that of normalized A 1 g continuum intensity.

An improved electron and hole mobility model for general purpose device simulation

A new, comprehensive, physically-based, semiempirical, local model for transverse-field dependent electron and hole mobility in MOS transistors is presented. In order to accurately predict the measured relationship between the effective mobility and effective electric field over a wide range of substrate doping and bias, we account for the dependence of surface roughness limited mobility on the inversion charge density, in addition to including the effect of coulomb screening of impurities by charge carriers in the bulk mobility term. The result is a single mobility model applicable throughout a generalized device structure that gives good agreement with measured mobility data and measured MOS I-V characteristics over a wide range of substrate doping, channel length, transverse electric field, substrate bias, and temperature.

Stability of two-electron bound states in a model quantum wire

The stability of two-electron bound states is examined in a model quantum wire. It is emphasized that the stability crucially depends on the Coulomb screening effect. If the screening length for the electron-electron interaction is of the order of 10 nm or less, two-electron bound states can be easily formed.

Resonant Coulomb screening of charging of a localized state.

I consider the effect of Coulomb screening on the resonant tunneling through a localized nondegenerate state between two quasi-one-dimensional conductors. It occurs when the local value of the electron density varies significantly on the edges of the conductors. Then the screening becomes resonant and drastically modifies the properties of the localized state. In particular, it leads to the appearance of a hidden symmetry in the problem as a result of which the occupation number of the level ${\mathit{n}}_{\mathit{D}}$ is pinned to 1/2. This means that the level should exhibit resonant conductance (${\mathit{e}}^{2}$/h)${\mathrm{sin}}^{2}$\ensuremath{\pi}${\mathit{n}}_{\mathit{D}}$ in a wide range of energies of the order of the Fermi energies of the conductors. In contrast to the Kondo effect, this effect of the resonant screening is much more stable against temperature and voltage fluctuations. This circumstance can be crucial for the experimental observation. The emergence of this effect in the behavior of impurities in metals is also possible and is briefly sketched.

Renormalization effects in the dense electron-hole magnetoplasma of a strained InxGa1-xAs/GaAs single quantum well after picosecond excitation.

The renormalization of the Landau-level transition energies in the dense quasi-two-dimensional magnetoplasma has been determined from time-resolved luminescence spectra in a strained undoped ${\mathrm{In}}_{0.17}$${\mathrm{Ga}}_{0.83}$As/GaAs quantum well. The neutral electron-hole magnetoplasma was created by 1-ps laser pulses in magnetic fields H\ensuremath{\le}7.5 T. The renormalization has been found to depend strongly not only on the carrier density but also on their distribution over Landau levels. The dependence is explained by the interplay of the attractive exchange and repulsive Pauli contributions in the transition energies and Coulomb interaction screening effect.

A Unified 3-D Mobility Model for the Simulation of Submicron MOS Devices

A calibrated and physically based mobility model is developed for three-dimensional simulation of submicron metal-oxide-semiconductor (MOS) devices, in which the inversion layer mobility is emphasized. This inversion layer mobility can be generalized into a local form, i.e., expressed as functions of the local electric field at each grid point, so that it is well suited for device simulation. The resulting 3-D mobility model accurately characterizes the significant physical scattering effects including the Coulomb screening effect, quantum channel broadening effect, surface roughness scattering, structure-induced lateral surface scattering and velocity saturation limited effects. Results show that this new model can be incorporated into device simulators for accurately predicting drain currents of submicron LDD MOS devices. Moreover, the results compare more favorably with the experimental data than do those for other reported models.

Coulomb screening in low-energy nuclear reactions.

Coulomb screening effects are studied in very-low-energy nuclear reactions. The two-potential formalism is shown to be a convenient method for the separation of long-range (molecular) and short-range (nuclear) dynamics. As specific applications, electron screening effects and muon catalysis in nuclear reactions are investigated. In the case of electron screening in very-low-energy nuclear reactions a method is elaborated for calculation of the enhancement of the astrophysical factor. For muon-catalyzed fusion reactions the sudden approximation is shown to be valid even if the reaction is dominated by a long-lived nuclear resonance.

Carrier-induced optical nonlinear effects in semiconductor quantum well wire structure

Carrier-induced optical nonlinearities in a GaAs quantum well wire structure are calculated within the framework of the effective mass approximation to clarify the nonlinear features of the one-dimensional (1D) system. In the calculation, the excitonic nonlinearity due to phase-space-filling, band-gap renormalization, and Coulomb screening effects is estimated on the basis of an effective 1D Coulomb potential. The 1D potential is derived, taking wire cross section into account. The change in the optical absorption for the band-to-band transition is evaluated, considering the band-filling effect and the changes in the Sommerfeld factor caused by the screening effects. The numerical results show that a blue shift of the excitonic absorption peak occurs at the absorption edge because the blue shift caused by the screening effect exceeds the red shift due to the band-gap shrinkage. In the spectral region above the band edge, where the band-to-band transition determines the optical properties, a decrease in optical absorption caused by band filling is compensated by increasing absorption, which stems from recovery of the Sommerfeld factor because of the Coulomb screening. The effect of the Coulomb interaction in a quantum wire laser is also discussed in relation to the gain spectrum in the high excitation regime. These results show that exact inclusion of the Coulomb interaction is crucial in estimating both linear and nonlinear optical properties in the 1D system.

THE EFFECT OF COULOMB SCREENING AND VELOCITY DISTRIBUTION ON FUSION CROSS-SECTIONS AND RATES IN PHYSICAL PROCESSES

The Coulomb screening effect, in conjunction with a particle velocity distribution, is shown to greatly enhance the cross-sections and reaction rates for deuteron-deuteron (D-D) and proton-deuteron (p-D) fusion for kinetic energies E≲50 eV in the center of mass (CM) frame. This may help to explain the observed fusion reaction rates from recent low-energy experiments, and indicates that p-D fusion may actually serve as an important source of internal energy for planetary bodies.

Nonlinear effects on thermonuclear reaction rates

Coulomb screening and nonlinear effects, together with many-body collisions, may enhance nuclear fusion rates by many orders of magnitude at temperatures much lower than those ordinarily required for such processes in hot plasmas.

On the feasibility of cold fusion

This note estimates the fusion rate for deuterons trapped in palladium under the assumption of lattices structure. The effect of Coulomb screening and lattice vibration are shown to be crucial.

Screening effect on the inverse bremsstrahlung in a plasma in the presence of two laser fields.

The effect of Coulomb screening on the inverse bremsstrahlung heating process in a plasma illuminated by two laser fields is discussed. It is shown that, although the screening effect actually lowers the Coulomb interaction, one might accomplish a reduction of the weakening effect and consequently an enhancement of the collisional plasma heating, by illuminating the plasma with the two electromagnetic waves having a difference in frequency close to the plasma frequency.

Radiolytic production and properties of ultrasmall cadmium sulfide particles

The radiolytic production of CdS particles from solutions containing cadmium ions and a thiol (3-mercapto-1,2-propanediol, RSH) is described. The production of the colloids is initiated by the reaction of solvated electron with the thiol to release HS{sup {minus}} ions. The polynuclear complexes of cadmium with the thiolate form of RSH act as moderators to the growth of the particles and allow reproducible production of practically any predetermined size particles. The complexes at the surface of the particles also stabilize the particles for long periods of time. The particles are strongly fluorescent from size of ca. 8 {angstrom} in radius. Excess electrons in these particles lead to bleaching of the exciton band due to Coulomb screening effects. At higher doses, formation of cadmium atoms leads to increased absorption in the visible range.

Electronic structures in cerium monopnictides

Among Ce-monopnictides, CeBi and CeSb show very interesting magnetic properties, competing and coexisting with the dense Kondo state. The p-f mixing model was used successfully to explain all the anomalous magnetic properties as well as the photoemission spectra. For a better understanding of their electronic states, a more crucial measurement of the de Haas-van Alphen effect has been done and compared with the band calculation. LMTO method was used because it is convenient to select a special character in a muffin-tin sphere and it really can fit the experimental results satisfactorily even if it has no adjustable parameters. However, still some controversies remain. It is pointed out that the d-f Coulomb screening effect for a 4f hole is crucially important to enhance the Kondo temperature in semimetals such as Ce-monopnictides.

Atomic effects in low-energy beta decay: The case of tritium.

In a recent experiment, Simpson found an excess intensity in the measured tritium beta spectrum below 1.5 keV. A detailed account of the decay energy and Coulomb screening effects raises the theoretical curve in precisely this energy range, so that little, if any, of the excess remains. This correction is shown to be approximately given by a simple analytical expression.

Femtosecond excitation of nonthermal carrier populations in GaAs quantum wells.

We report the first direct observation of nonthermal photoexcited carrier distributions in GaAs quantum-well structures. We present experimental studies which show that these distributions thermalize within 200 fs. In addition we are able to show that near the band edge the effect of long-range Coulomb screening on the bleaching of the two-dimensional-exciton resonances is much weaker than that of the Pauli exclusion principle.

Theory of transient excitonic optical nonlinearities in semiconductor quantum-well structures.

We present theoretical results for the effects of an exciton gas and an electron-hole plasma on the excitonic optical absorption in a two-dimensional semiconductor and compare these with recent experimental results on absorption saturation in single- and multiple-quantum-well structures. A simple theoretical description of the nonlinear optical properties of these microstructures is developed for the case of low-density optical excitation near and above the band edge. We argue that the effects of Coulomb screening of excitons by the plasma are relatively weak in these structures but that the consequences of phase-space filling and exchange are significant in each case. We are able to explain the recent unexpected experimental result that ``cold'' excitons are more effective than ``hot'' carriers in saturating the excitonic absorption. Good agreement with the experimental data is obtained without adjustable parameters.