Showing papers on "Free electron model published in 1990"

Soliton Lattice Modulation of Incommensurate Spin Density Wave in Two Dimensional Hubbard Model -A Mean Field Study-

Abstract: Spatially modulated magnetic phases are investigated within the mean field theory for an itinerant electron model, i.e. the Hubbard model on a two-dimensional square lattice. By numerically diagonalizing the Hamiltonian for finite-size systems under a periodic boundary condition, we examine relative stability and physical properties of several possible magnetic states. When the electron fillings are nearly half-full, the diagonally or vertically modulated spin density wave (SDW) state is stabilized over the uniform antiferromagnetic state and a crossover from the vertical to the diagonal states appears. The diagonal or vertical stripe state is characterized by the presence of the midgap band due to the soliton lattice formation inside the main SDW gap, being an insulator. The wave length λ SDW is linearly proportional to the excess carrier concentration. Excess carriers are accommodated in the form of the soliton lattice, forming a charge density wave whose wave length is λ SDW /2.

Metallic non-Fermi-liquid fixed point in two and higher dimensions

Abstract: Quasi-one-dimensional electron systems in two and higher dimensions are studied. It is shown that the interchain electron hopping is renormalized to zero in the infrared limit if the intrachain repulsive interaction is strong enough. In this case the higher-dimensional electron system flows to metallic non-Fermi-liquid fixed points---a Luttinger liquid. Some experimental consequences, in particular the transport properties, of this infrared fixed point are studied. It would be interesting to observe this state in quasi-one-dimensional electron systems.

Study of Ag(111) and Au(111) electrodes by optical second‐harmonic generation

Abstract: We have measured the potential dependence of optical second‐harmonic generation from Ag(111) and Au(111) electrodes at wavelengths below and above the free electron regime. In the free electron regime, the cubic nonlinear susceptibility is dominant, leading to similar effects for positive and negative charging. Above the free electron regime, the quadratic nonlinear susceptibility of the d‐electrons is dominant, leading to different effects for positive and negative charging. We also followed the oxidoreduction of Au(111) and found that second‐harmonic generation is very sensitive to the different steps of the reaction.

High-order harmonic generation in xenon: intensity and propagation effects

Abstract: We present experimental results on high-order harmonic generation in xenon, using a Nd:YAG laser (1064 nm). The harmonic generation efficiency is studied as a function of the laser intensity in the 1013 W/cm2 range. The harmonic conversion rates show a significant change of behavior when the medium becomes ionized (∼1013 W/cm2). This effect can be understood as the change of the nonlinear response of the medium from neutral atoms to ions and also as the influence of free electrons. We study the role of propagation effects (phase matching) by performing experiments using three different lenses (75, 200, and 300 mm) to focus the incident laser beam, thus changing the interaction geometry. Although for each harmonic the conversion rate is strongly dependent on the confocal parameter b (as b3), the behavior of the harmonic frequency distribution shows almost no influence of the focusing geometry.

Auroral modeling of the 3371 Å emission rate: Dependence on characteristic electron energy

Abstract: An efficient two-stream auroral electron model is used to study the deposition of auroral energy and the dependence of auroral emission rates on characteristic energy. This model incorporates the concept of average energy loss to reduce the computation time. This simple two-stream model produces integrated emission rates that are in excellent agreement with the much more complex multistream model of Strickland et al. (1983) but disagrees with a recent study by Rees and Lummerzheim (1989) that indicates that the N2 second positive emission rate is a strongly decreasing function of the characteristic energy of the precipitating flux. These calculations reveal that a 10 keV electron will undergo approximately 160 ionizing collisions, with an average energy loss per collision of 62 eV before thermalizing. The secondary electrons are created with an average energy of 42 eV. When all processes including the backscattered escape fluxes are taken into account, the average energy loss per electron-ion pair is 35 eV in good agreement with laboratory results.

Electron Collision Cross-Sections Determined from Beam and Swarm Data by Boltzmann Analysis

Abstract: Whenever free electrons collide with atoms and molecules, a wide variety of kinetic processes may take place. Electron collision cross-sections, of interest in plasma processing of VLSI production, carbonization of surfaces and so on, modeling of gas lasers, physics of gaseous dielectrics, analysis of high current discharge switches, applications for space science and radiation physics and chemistry, have been determined from available electron beam and electron swarm data utilizing the Boltzmann analysis. The goal of electron collision studies is to provide absolute values of cross-sections for all processes involved, from elastic to inelastic scattering, as a function of incident electron energy and scattering angle. In spite of many experimental and theoretical studies, electron impact cross-sections for atoms and molecules have not been systematically quantified.

X-ray photoelectron diffraction from a free-electron-metal valence band: Evidence for hole-state localization.

Abstract: X-ray-photoelectron diffraction has been measured from an Al(001) single crystal for the free-electron-like valence band and the well-localized 2s core level, using Mg Kα radiation. Both show the same high anisotropy of 65% in the azimuthal distribution of the photoemitted electrons at 45 o away from the surface normal. The similarity between intensity patterns provides evidence for a strong degree of hole localization in high-energy valence-band photoemission

Radiative recombination between free electrons and bare carbon ions.

Abstract: Absolute rate coefficients for radiative recombination between fully stripped carbon ions and free electrons have been measured for relative energies between 0 an 1 eV. The experiment was performed with a merged-beam technique. A fast-pulsing technique was used to turn on and off the electron source to allow subtraction of background signals stemming from electron capture in the rest gas. The data are compared with the theory for radiative recombination, as derived by Stobbe in 1930 and by Bethe and Salpeter in 1957. The present data constitute the first direct test of the theory for radiative recombination.

Anisotropy of optical and transport-properties in the a-b plane of the high-temperature superconductor yba2cu4o8

Abstract: The anisotropy within the a-b plane of YBa2Cu4O8 has been studied on single crystal platelets by optical and electron transport measurements. The optical investigations include polarized reflectivity measurements at room temperature for photon energies between 0.5 and 4 eV and a direct measurement of the ratio of the reflectivity coefficients r b r a . The anisotropy of the electrical resistivity has been determined from AC van der Pauw measurements using the Montgomery formalism for anisotropic materials. The optical reflectivity spectra are dominated by Drude-like free electron contributions with plasmon energies of 2.8 eV and 1.67 eV for the electrical field vector along the b- and a-axis, respectively. The electrical resistivity displays an anisotropy of a factor of about 3. The superconducting transition temperature (midpoint of the resistivity drop) is 76 K. Contrary to YBa2Cu3O7, clear deviations from linear temperature dependences of the resistivity are observed in the normal state of YBa2Cu4O8. An extrapolation of the linear (normal) part gives zero resistance at 36 ± 2 K and not 0 K. The observed anisotropies are compared with results of a tight binding calculation.

Electrons in Metals and Semiconductors

Abstract: 1 The free-electron model.- 1.1 The classical Drude theory.- 1.2 Fermi-Dirac statistics.- 1.3 The Sommerfeld model.- 1.4 The density of states.- 2 Properties of free-electron solids.- 2.1 The electronic heat capacity.- 2.2 The magnetic susceptibility.- 2.3 Transport properties.- 2.4 Hall effect and magnetoresistance.- 2.5 Relaxation effects: high-frequency conductivity.- 2.6 Metals and semiconductors.- 3 Crystal structures and the reciprocal lattice.- 3.1 Crystal structures.- 3.2 The reciprocal lattice.- 4 Electrons in a periodic potential.- 4.1 Bloch's theorem.- 4.2 The Brillouin zone.- 4.3 Nearly free electrons.- 5 Electronic band structures.- 5.1 The band structure of real metals.- 5.2 Metals, semiconductors and insulators.- 5.3 Density of states and heat capacity.- 6 The potential V(r) many-body effects.- 6.1 The one-electron approximation and the choice of V(r).- 6.2 Many-body effects.- 7 The dynamics of Bloch electrons.- 7.1 The velocity and the Lorentz force.- 7.2 Orbits in a magnetic field.- 7.3 Orbit quantization.- 7.4 The de Haas-van Alphen effect.- 8 Collisions.- 8.1 Scattering by static defects.- 8.2 Phonon scattering.- 8.3 Relaxation times and mean free paths.- 9 Electrical conductivity of metals.- 9.1 The basic expression for ?.- 9.2 Temperature dependence of ?.- 9.3 Matthiessen's rule.- 9.4 The Kondo effect.- 10 Metals in a temperature gradient.- 10.1 Thermal conductivity.- 10.2 Thermoelectric effects.- 11 Magnetoresistance and Hall effect.- 11.1 Basic ideas: the free-electron model.- 11.2 Real metals.- 12 Radio-frequency, optical and other properties.- 12.1 Radio-frequency properties.- 12.2 Optical and other properties.- 13 Carriers in semiconductors.- 13.1 The number of carriers.- 13.2 Donors and acceptors.- 13.3 Carrier mobilities and positive holes.- 14 Transport properties of semiconductors.- 14.1 Scattering.- 14.2 Simple transport properties.- 14.3 Cyclotron resonance and optical properties.- 14.4 Orbit quantization and the quantum Hall effect.- 15 Other topics.- 15.1 Charge density waves.- 15.2 Disordered alloys.- 15.3 Localization.- Problems.- Answers to problems.- Further reading.

The electronic structure of liquid alkali metals: calculation of photoemission spectra

Abstract: The authors present a first-principles calculation of the atomic structure (pair correlation function, static structure factor), the electronic structure (total and partial densities of states), and of the photoemission intensities of liquid Li and Na. They find that in liquid Li the electronic density of states shows a strong structure-induced 'Brillouin-kink' which leads to a pronounced narrowing of the width of the occupied band compared to the prediction of the free electron model. The DOS of liquid Na on the other hand is close to the free electron parabola. The origin of the difference is in the strong p component in the electron-ion potential in Li, but not in Na. The observed deviation of the photoemission spectrum of Na from a free electron form is explained in terms of the partial photoionisation cross-sections. A detailed comparison with experiment shows that the accuracy of the theoretical predictions is limited only by the local density approximation for the many-electron interactions.

Two-stream instability in free electron lasers

Abstract: The analysis of the interaction between electromagnetic waves and electron beams in semi-infinite wigglers is presented. The beam is assumed to include two cold streams of electrons; hence, its space-charge waves may be unstable (two-stream instability). In the wiggler, this instability is shown to yield much larger growth rates (up to seven times greater) than in the conventional one-stream free electron laser (FEL). Accordingly, the gain per pass is enhanced by orders of magnitude. The enhancement of the two-stream instability is shown to be most effective for short-period wigglers. >

Non-linear intractivity optical devices for free electron lasers

Abstract: The performance of free electron lasers can be greatly enhanced by including a non-linear optical element within the resonator for the oscillator. A system is described to enhance the peak or average power output of the FEL oscillator, or to modify the time structure of the oscillator output through the use of a non-linear element which modulates or diverts a portion of the optical wave circulating in the resonator.

Consequences of Short Electron-Beam Pulses in the Felix Project

Abstract: We discuss the consequences of short micropulses on the output of infrared and far-infrared free electron lasers with special reference to the FELIX project which operates with 3 ps long electron pulses.

Design of high-average-power near-millimeter free electron laser oscillators using short period wigglers and sheet electron beams

Abstract: The design and feasibility of a 1-MW continuous-wave (CW) free electron laser (FEL) oscillator are reviewed. The proposed configuration includes a short-period (I/sub w/ approximately 1 cm) planar wiggler, a sheet electron beam, a 0.5-1.0-MV thermionic electron gun, a hybrid waveguide/quasi-optical resonator, commercial DC power supplies, and a depressed collector. Cavity ohmic RF losses are estimated to be extremely low ( >

Free electron density in H I regions as an indicator of ionizing dark matter

Abstract: IT has recently been proposed1 that the free electrons giving rise to the observed dispersion of pulsar signals result mainly from photoionization induced by the decay of dark matter particles in the Galaxy. For reasons related to conservation of energy and momentum the decay photons would have a unique energy in the rest frame of the decaying particles, and in the proposed theory this energy is close to 14 eV. A crucial prediction of the theory is that in regions where the dark matter density is approximately constant, the free electron density ne in opaque H I regions should also be approximately constant, when averaged over several mean free paths for the ionizing photons. The determination of an accurate distance for the pulsar PSR1451-68 (ref. 2), in combination with its known dispersion measure and with previous observations of two other pulsars with accurate distances, leads to a successful test of this prediction.

Nonlinear theory and experiment of collective free electron lasers

Abstract: A theoretical and experimental study of the nonlinear performance of a free electron laser (FEL) amplifier operating in the collective (Raman) regime is reported. The FEL generates up to ∼100 kW of rf power at a frequency of 9.3 GHz and an efficiency of ∼10%. Power saturation, efficiency, and synchrotron oscillations are studied as a function of rf input power, electron beam energy, current, wiggler field amplitude, and axial distance within the helical wiggler. The influences of the nonlinear electron motion in the ponderomotive potential and space‐charge waves are studied by measurements of the dependence of gain and efficiency on the initial radiation intensity. Good agreement with a nonlinear theory that takes cognizance of electron trapping in the combined ponderomotive and space‐charge potential well is obtained.

Raman scattering study of the electron-phonon coupling in GaInAs-InP quantum wells

Abstract: Recent Raman scattering studies of Ga0.47In0.53As-InP quantum wells have shown that the form of the alloy optical phonons can be quite different from that observed in the bulk due to coupling between the phonons and the two-dimensional electron gas (2DEG). However, the limited range of samples studied so far has made the elucidation of the precise coupling mechanism difficult. The authors report here a Raman scattering study of the GaInAs optical phonons in a wide range of undoped GaInAs-InP quantum wells with well widths in the range 50-300 AA. The frequencies and intensities of the two LO phonons of the alloy are studied as a function of both well width and photoexcited carrier density and a resonant coupling between the electrons and the GaAs-like LO phonon is observed when the lowest electronic inter-subband spacing approaches the phonon energy. These results are discussed in terms of the coupling between phonons and intersubband collective excitations.

Free electron laser pumped by a powerful traveling electromagnetic wave

Abstract: A three-wave free-electron laser (FEL) was operated with a powerful 8.4-GHz electromagnetic pump wave replacing the usual magnetostatic wiggler. The presence of a uniform axial magnetic field B/sub 0/ produced cyclotron-harmonic idler waves. Peaks in the emission spectrum corresponding to cyclotron harmonics were observed covering a frequency range from 16.5 to 130 GHz. The frequency spectrum of this novel FEL mechanism was tuned continuously by the variation of B/sub 0/. >

High-energy angle-resolved secondary-electron emission spectroscopy of highly oriented pyrolytic graphite

Abstract: The angle-resolved secondary-electron emission (ARSEE) spectra of highly oriented pyrolytic graphite have been measured in the 7-70 eV electron kinetic energy range. A peak observed at approximately=59 eV with K/sub ///=1.35-1.65 AA-1 was tentatively explained within a semiempirical approach based on a free electron parabola with an inner potential of -16.2 eV (relative to the vacuum level) which crosses the M point at 59 eV. From the crossing of the free electron parabola with the zone centre peaks at 16 eV and 28.5 eV were independently described. These results suggest that the empty density of states for large kinetic energies are well described by a free electron parabola. In the energy range below 30 eV good agreement was obtained with previously published ARSEE experiments. XPS and AES measurements were performed to verify that the 59 eV peak did no originate from an impurity level. After argon ion bombardment the fine structure observed in the ARSEE spectra of HOPG was completely removed.

Electronic structure of liquid lithium and sodium from electron spectroscopy

Abstract: The electronic structure of liquid alkali metals is of particular interest since these group of metals represent the electronically most simple metals known in the periodic system. This also means that there is the greatest chance of finding an electronic structure close to the prediction of the free electron model (FEM) for these metals, in particular for lithium. So far there exists no published valence band investigation of liquid alkali metal surfaces which proved to be atomically clean. In this study liquid Li and Na have been prepared revealing excellent stability against recontamination as checked both by UPS and XPS. The valence band spectra of lithium are in agreement with a parabolic band whose width however proves to be significantly reduced compared to the FEM. In contrast the experimental band width of sodium exhibits a minor reduction from the FEM prediction whereas the valence band shape is rather triangular. Only minor changes of the relative s to p band state photoionization cross sections as a function of the excitation energy have been observed within the range of hv = 11.8 − 48.4 eV in contrast to the results on polyvalent metals. The use of HeII radiation enabled a variety of observations. For example in the case of sodium, besides the different valence band spectra with their typical plasmon structures, core states and Auger transitions have also been examined. The splitting of the 2p states can be observed by the high resolution UPS. The energetic position of the L2,3VV Auger transition is consistent with a negligible two hole final state energy.

Evidence for electron correlation in the two-electron continuum during double ionization in 300-keV H++He collisions.

Abstract: Absolute differential ionization probabilities for small impact parameter H{sup +}+He collision were measured as a function of the electron energy, electron ejection angle, and final recoil-ion charge state. Good agreement was found between an experimental angular distribution of ejected electrons and results of a coupled-channel calculation in the case of single ionization. In the case of double ionization, significant discrepancies to theoretical predictions are found, indicating a breakdown of the independent electron model. Furthermore, evidence is provided that dynamic correlation effects in the two-electron continuum strongly influence the angular distribution of ejected electrons for double ionization.

Electron Emission at Plastic Deformation of Colored Crystals

Abstract: Results are given of experimental and theoretical investigations of electron emission, observed under plastic deformation of colored alkali halide crystals. The emission is shown to be due to Auger process by the electrons pushed out of the surface from F-centres by dislocation. A comparison of theory and experiment provides some characteristics of deformed crystals, namely the exit velocity of dislocations on the surface and the density of deep surface traps. The mechanism of dislocation sensibilization of photo-stimulated electron emission of colored crystals is investigated. It is shown that the rise of the particle number emitted is due to the electric field created by moving of charged dislocations which leads to a drift of the free electrons excited by photoionization of the bulk F-centres. [Russian Text Ignored].

Bipolar Ion and Electron Diffusion Charging of Aerosol Particles in high purity argon and nitrogen

Abstract: The bipolar diffusion charging process is studied theoretically for aerosol particles in high purity argon and nitrogen. The Fuchs theory is extended by the incorporation of the free electron charging of the aerosol particles. The ion parameters for positive and negative ions are assumed to be identical. The influence of the free electrons explains the differences between the mean mobility and mean mass of negative and positive ions. The ratio of the electron number concentration to the number concentration of negative ions is used to fit the curves, calculated by the extended model, to the experimentally determined bipolar charge distribution in argon and nitrogen. The extended Fuchs model was found to be rather insensitive to variations in the mobility and mass compared with the model with four different ion parameters. Further experimental studies of the bipolar charge distribution in a gas mixture of pure nitrogen and sulfur hexafluoride, SF6, indicate the importance of the free electrons in the bipolar diffusion charging process.

Photoemission study of the final band in Ag(111)

Abstract: For direct transitions in angle-resolved photoemission from a single crystal, the final electronic states inside the crystal can be approximately described by a free-electron-like band for energies higher than approximately=30 eV above the Fermi level. The authors report a determination of the dispersion relation for this band in Ag along the (111) direction over a wide photon energy range (40-180 eV), from which the inner potential (average electrostatic potential in the solid within the Hartree approximation) and the energy-dependent electron effective mass are deduced. The effective mass is found to increase from approximately 1.00 m at 54 eV to 1.08 m at 149 eV above the Fermi level (m is the free electron mass). This result is compared with existing many-body model calculations. The present study is also relevant to the problem of surface state resonance in this system discussed previously.

Experimental study of the role of plasma polarization on line shape in dense laboratory plasmas.

Abstract: This paper presents an experimental study of the spectral line profiles emitted by multicharged ions in dense laser-produced plasmas. The time and/or space integrated spectra of the Lyman series of H-like lithium show a strong red asymmetry and, in some cases, a red shift of the top of the lines. It is shown that such profiles have to be interpreted as due to space and time integration of mostly symmetric profiles, which are red shifted by an amount that depends on the emitting plasma zone. Theoretically calculated profiles show the role, in the observed profiles, of the plasma polarization shift due to the penetration of atomic orbits by the plasma free electrons.

Two-dimensional analysis of beat wave current drive with intense microwave pulses

Abstract: Current drive in tokamak plasmas by a beat wave is considered in two-dimensional (2-D) geometry. The beat wave is excited by the non-linear interaction of two intense microwave pulses (free electron lasers) in the plasma. The three-wave non-linear interaction equations in steady state are solved numerically. The 2-D toroidal effect and the effect of finite spatial width of the pump microwave pulses are taken into account for the excitation of the beat mode. To illustrate the principle, two types of tokamak are considered: one is small, such as, typically, the Microwave Tokamak Experiment (MTX), and the other one is larger, such as the Joint European Torus (JET). In both cases, it is found that good beat wave coupling exists for a Langmuir beat wave with a phase velocity of around 2.0 to 4.0 times the thermal velocity of the electrons. The fraction of total input power of the right circularly polarized pump waves deposited in the beat mode can be as high as 29% in JET and 32% in MTX. In these cases, there is almost complete pump depletion of the higher frequency pump microwave. It is also found that, for the same input parameters, left circularly polarized pump waves are less efficient than right circularly polarized pump waves for depositing power in the beat mode.