Showing papers on "Free electron model published in 1984"

Band Structure in the High Temperature Phase of Fe 3 O 4

Abstract: Band structure of the ferrimagnetic Fe 3 O 4 was calculated in the high temperature cubic phase by means of self-consistent APW method. Though the obtained band structure corresponds closely with the ionic Neel model, it was shown that the itinerant electron model is more adequate than the localized electron model. The minority spin d e bands of B site iron have an electron Fermi surface at the \( \varGamma \) point and hole surfaces around the W point. Experimental data including photoelectron emission, optical reflectivity, neutron scattering and transport phenomena were discussed qualitatively in relation to the calculated band structure.

Magnetization of a two-dimensional electron gas

Abstract: The oscillations of magnetization of a two-dimensional free electron gas with field are derived first in the ideal situationT=0 and sharp Landau levels without electron spin and then for finiteT and for broadened levels. Explicit results are obtained in the limiting approximations that the broadening (or kT) is small or large compared with the separation of Landau levels, and the special situations of a field-independent Fermi energy and a field-independent electron density are discussed. The modifications introduced by taking account of electron spin are then considered, and finally the steady magnetic susceptibility superimposed on the oscillations is discussed.

Current distribution in the scanning vacuum tunnel microscope: a free-electron model

Abstract: Insight into the resolution of the recently developed technique of scanning tunnelling microscopy (STM) is achieved by considering the transmission of free electrons through a constant potential barrier with corrugated boundaries representing the sampled surface and probing tip, respectively. The amplitudes of the reflected and transmitted waves are calculated via an extension of the so-called GR-method developed to treat scattering from a corrugated hard wall. Results for the distribution of current density, for the dependence of the tunnelling current on the horizontal and vertical positions of the scanning tip and for the resulting equicurrent lines (STM images) are presented for a two-dimensional model. Simple analytical approximations are shown to reproduce computed trends versus tip-sample separation, tip curvature and average barrier height.

Transport properties of photoelectrons in Bi12SiO20

Abstract: An optical method for studying the transport properties of photogenerated electrons in the photoconducting electro‐optic crystal of Bi12SiO20 is described. The physical model in which the free electrons are assumed to hop from the donor sites to the trapping sites and thus create a space‐charge field has been developed. The crystal is mounted in a transverse configuration with the light beam propagating along the [110] axis and the electric field applied along the [110] axis. By measuring the photoinduced birefrigence variations, we determine the quantum efficiency φ of generating a free electron by an absorbed photon, the range μτ of a photoelectron, and the dark conductivity of the material. φ varies from 1.00 to 0.03 when the incident wavelength changes from 400 to 700 nm. The range μτ is found to be 5.2×10−6 cm2/V and the dark conductivity 2×10−15 (Ω cm)−1.

A note on the bonding, optical spectrum and composition of tetrahedrite

Abstract: Tetrahedrites of composition (Cu, Ag)10(Cu2, Fe, Zn)2(Sb, As)4S13 or Cu12Sb14/3S13 have 208 valence electrons per unit cell and are expected to be semiconductors. The bands are full in these cases, whereas compositions towards the classical formula Cu12Sb4S13 (204 valence electrons per unit cell) have only partially filled bands and are therefore expected to be metallic. These predictions are supported by new optical absorption spectra of tetrahedrites with 205 and 208 valence electrons per unit cell. The gap between valence and conduction bands of the semiconductor is about 1.7 (±0.2) eV. A further prediction based on a nearly-free electron model is that 208 valence electrons per unit cell represent a compositional limit for tetrahedrites, and that the stability increases as compositions approach this limit. Existing data indicate an exponential increase in the number of occurrences as the limit is approached.

Modulational Instability of Ion Wave in Plasma with Negative Ion

Abstract: A modulational instability of an ion wave in a plasma composed of electron, positive ion and negative ion has been investigated theoretically. We derive a nonlinear Schrodinger equation for ion wave and examine the signs of nonlinear and dispersion coefficients. The dispersion coefficient is negative irrespective of the wavenumber. The sign of the nonlinear coefficient changes at a critical wavenumber k c , above which the coefficient becomes negative and the ion wave is modulationally unstable. The critical wavenumber k c depends strongly on the negative ion density. At the critical density of negative ion where the nonlinear coefficient of the Korteweg-de Vries equation for ion wave vanishes, the critical wavenumber k c is reduced to zero and the wave of any wavenumber is modulationally unstable.

Coherent state of an electron in a homogeneous constant magnetic field and the zero magnetic field limit

Abstract: Coherent states of an electron embedded in a constant homogeneous magnetic field are constructed. The centres of the probability distributions belonging to these states gyrate along possible classical trajectories. Suitable packets of such coherent states are defined which reduce to properly normalised free electronic states in the zero magnetic field limit. A simple example is given to illustrate the dynamics of free electron localisation due to the presence of a magnetic field.

Radiation growth in a millimeter-wave free-electron laser operating in the collective regime

Abstract: Frequency‐resolved measurements of radiation growth have been performed on a millimeter‐wave free‐electron laser using an intense relativistic electron beam. These measurements have shown large radiation growth rates (approximately 2 dB/cm) over a broad instantaneous bandwidth (66–90 GHz), in good agreement with predictions of theory for operation in the collective regime. Growth narrowing and saturation effects have also been observed. In addition, a large increase in experimental power and efficiency has been observed to result from tapering the strength of the axial magnetic field in the sense that compensates for kinetic energy extraction from the electron beam. Direct calorimetric measurements indicate the production of ≳75 MW centered at 75 GHz with 6% experimental efficiency.

Integral-equation method for partially ionized plasmas

Abstract: A pseudoclassical integral-equation method for obtaining the equation of state, pair-distribution functions, and static structure factors for partially ionized plasmas is presented. The electron-ion charge distribution is separated into a localized, quantum-mechanical part and a delocalized, nearly classical part. The Planck-Larkin method is used to separate the electron distribution. Localized-electron distributions are obtained from solutions of the Schroedinger equation. Distribution functions for ions and free electrons, including weakly bound electrons, are obtained from solutions to the classical hypernetted-chain (HNC) integral equation. Pseudopotentials that have two-body quantum mechanics built in, but which preclude strong bound states, are used as input to the HNC. The results are applied to H and Ar plasmas.

The two-beam accelerator

Abstract: A two-beam accelerator, in which one of the beams is an intense low energy beam made to undergo free electron lasing and the other beam is a compact bunch of high energy electrons, is shown to be an interesting possibility for a linear collider.

Study of wave function tails and reactivity from exterior electron model

Abstract: The exterior electron model was found to provide a means to test wave function tails of individual molecular orbitals and also to study reactivity of molecules. Demonstrative ab initio calculations were made for NH3, CO, H2O, and H2S at various levels of basis sets.

Atomic physics for beam-target interactions

Abstract: This survey is devoted to a few basic atomic problems associated with the stopping of nonrelativistic pointlike ions in dense and hot matter, and also to the Stark broadening diagnostics of the resulting beam-produced plasmas. First, we consider the free electron contribution, taken in the RPA approximation with an exact dynamic dielectric function, valid at any temperature. Therefore, we obtain stopping power and straggling for any projectile velocity. The temperature dependence is of special relevance for a projectile energy smaller than 5 MeV/a.m.u. Next, we revise the Barkas effect ( Z 3 corrections) through a novel and compact formulation, which is based on an analogy with electron impact broadening theory. It facilitates inclusion of the non hydrogenic and electronic structure of the target ions, in a more selective way. The results may increase the usual Z 2 -stopping by 15 to 30 per cent corrections. Then, we show how the Stark broadening diagnostics of the compressed D + T fuel, seeded with high Z species, arising from the surrounding envelopes, may provide accurate determination of the electron number density n e . In this connection, it should be appreciated that the relatively long compression times (≃ 20 nsec) suggested by the HIBALL numerical simulation allow for a nearly Local Thermodynamic Equilibrium (LTE) state in the target, with T e ≃ T i . As a consequence, spectroscopic measurements are expected to be easier to implement in HIF targets, than in laser ones. A tentative proposal for the use of Stark broadening diagnostics of inflight excited and highly stripped ion projectiles is displayed in § 5. Experiments involving an HIB produced by a standard accelerator, and interacting with an independently produced coronal plasma are finally outlined.

Effects of screening by conduction electrons on the crystal electric field in REAl2 intermetallic compounds

Abstract: The fourth-order B4 and sixth-order B6 crystal electric field (CEF) parameters are evaluated for NdAl2, taking into account the contribution of the whole lattice of ions, by the use of reciprocal space according to the Duthie-Heine technique. Screening of the CEF by free conduction electrons gives a parameter B4 of opposite sign to the experimental value. This becomes correct if the screening is removed, the point-ion limit taken and the Al charge made zero. A simple explanation of this striking and unexpected result is offered by screening the rare-earth spheres with electrons with d character, while keeping the Al ones screened by free electrons. The situation for B6 is less satisfactory, the calculated value being one order of magnitude smaller than the experimental one. These conclusions can be extended to the rest of the REAl2 series (RE represents a rare earth). The effect of some internal structure on the ions is also considered.

A new (non‐copper‐induced) 1.35 eV emission band in n‐type GaAs. Origin and characteristics

Abstract: A new (non-copper induced) emission band peaked at hνm(77 K) = 135 eV is observed in n-type GaAs Pronounced features of this emission are: it disappears as copper atoms are introduced into GaAs and appears again as the copper atoms are afterwards removed from the GaAs interior Evidence is presented that the free electron transitions to acceptor-type (VGaTeAs) VAScomplexes are responsible for the non-trivial emission observed Temperature and doping variations of the peak positions, intensities, and half-widths of the new 135 eV emission band are presented An analysis of these results shows that the energy levels of the (VGaTeAs) VAS complexes are localized not far (e = 01 eV) from the top of the valence band; these complexes readily capture both, free electrons (capture coefficient cn = 10−11cm3/s) and holes (capture coefficient cn= 10−8cm3/s); rather strong electron-phonon coupling exists in (VGaTeAs) VAScomplexes – phonons are effectively emitted during the free electron radiative recombination [Russian Text Ignored]

Bright Electron Beams For Free Electron Lasers

Abstract: Bright Electron Beams for Free Electron LasersC. W. RobersonOffice of Naval ResearchArlington, VA 22217AbstractFree electron laser experiments and other high power radiation sources are verydemanding of beam quality for good efficiency at short wavelength. The approach toobtaining high current bright beams is examined. We conclude that very bright beamsrequire high current density cathodes and strong magnetic fields. A new definition ofbeam brightness for field immersed diodes is suggested by the data. We find thatperturbations on the electron beam are enhanced as a result of beam density.IntroductionThere has been considerable renewed interest in the generation of high powerradiation from free electrons in recent years. One of the most promising is the freeelectron laser. It is capable of high power and scales to short wavelengths. A primarylimiting factor is electron beam quality. The FEL requires a cold monoenergetic electronbeam of high current density. For high power operation one usually requires high currentas well. However, the effective temperature of the beam increases with current in r -fand induction linacs, which offsets the increase in gain and efficiency obtained by thehigher current. The increase in beam temperature is due, in part, to the increase intransverse emittance of the beam at the higher beam currents. A useful phenomenologicalrelation between the normalized transverse emittance and the current is the Lawson -Pennerrelation

Delocalisation of electrons in dense helium gas by external electric field

Abstract: A theory of the sharp increase of the electron mobility as a function of the external field in dense helium gas is proposed. Low mobility of thermalised electrons is shown to be caused by their localisation in bubbles. Free electrons as well as bound ones are supposed to exist in the gas. As the external field increases free electrons are heated and redistribution of the numbers of free and bound electrons occurs. The fraction of free electrons and experimentally measured drift velocity rise steeply. In high fields the drift velocity attains its gas-kinetic value.

Laser‐induced profile modification: Effect of electron response

Abstract: Physical processes which occur at the critical surface such as resonance absorption and scattering of the incident laser light are very sensitive to the exact form of the laser induced density profile. The presence of ‘‘overdense bumps’’ in density profiles has been seen experimentally and calculated theoretically using a simple isothermal model. These structures are important, in that they could, in an experimental situation, reflect a significant amount of light before it reaches the critical surface and considerably affect the physics occuring within a laser pellet target. A generalization of a previous calculation by using an arbitrary equation of state (density response of the electrons to the electrostatic potential φ) of the form ρ=ρ(φ) is obtained. In particular, two models for the electronic response (one including the effects of the presence of trapped electrons within the overdense shock structures) calculating the resulting effect on the produced density profiles are investigated. Qualitatively, it is found that the presence of overdense structures and their one‐dimensional stability within an underdense plasma is unaffected by the more general electronic response. Conditions for the existence of these structures are discussed and a detailed comparison with full particle in cell (PIC) code simulations is made, emphasizing the trapped electron model where excellent agreement is obtained.

Free electron scattering on a localised potential with cubic symmetry: application to AuNi

Abstract: The authors have calculated the resistivity, caused by the scattering of free electrons on an impurity described by a localised potential with cubic symmetry in terms of phase shifts (l

Guided-Wave Characteristics And Optical Damage In LiNb0 3 Waveguides

Abstract: High power optical performances of Ti-indiffused and ion-exchanged waveguides were investigated and discussed. It was experimentally confirmed that Ti-indiffused waveguide suffered a serious optical damage at the output power density less than 0.1 mW/mm at A = 0.6328 pm. For x-propagating guided waves in y-cut crystal, the throughput decay time, by which we could characterize the dependences of optical damage, was revealed to be inversely propor-tional to square of initial guided power, P2, for TE modes, and to P3 for TM modes, and also confirmed to increase with an exponential function of wavelength λ. No optical damage was found for z-propagating guided waves. Proton-exchanged waveguides exhibited to have a good damage resistance. Their surface acoustic wave(SAW) performances, however, were found to be degraded; an IDT's insertion loss of 45 dB was measured, which was much larger than 13 - 14 dB available for Ti-indiffused waveguides. According to the experimental results of infrared absorption spectra, we speculate that the deterioration of SAW performances is attributed to the formation of a HNbO3 phase. A preliminary study using electron spin resonance(ESR) at 77 K provided us a good evidence of the close relation between impurities such as iron and free electrons in the crystal under UV light irradiation.