Showing papers on "Free electron model published in 1991"

Magnetic g factor of electrons in GaAs/AlxGa1-xAs quantum wells.

Abstract: The magnitude and sign of the effective magnetic splitting factor g* for conduction electrons in GaAs/AlxGa(1-x)As quantum wells have been determined as a function of well width down to 5 nm. The experimental method is based on combined measurements of the decay time of photoluminescence and of the suppression of its circular polarization under polarized optical pumping in a magnetic field perpendicular to the growth axis (Hanle effect). Measurements as a function of hole sheet density in the wells reveal a transition from excitonic behavior with very small apparent g value for low density, to larger absolute values characteristic of free electrons at higher densities. For 20-nm wells g* for electrons is close to the bulk value (-0.44), and increases for narrower wells passing through zero for well width close to 5.5 nm. A theoretical analysis based on three-band k.p theory, including allowance for conduction-band nonparabolicity and for wave-function penetration into the barriers, gives a reasonable representation of the data, leading to the conclusion that g* in quantum wells has a value close to that of electrons in the bulk at the confinement energy above the band minimum.

Subband dependent mobilities and carrier saturation mechanisms in thin Si doping layers in GaAs in the high density limit

Abstract: Shubnikov-de Haas and persistent photoconductivity measurements have been performed as a function of hydrostatic pressure to study the saturation of the free electron concentration and the mobilities of the individual subbands at high doping densities in very thin sheets (2, 5, 10 nm) of silicon donors in MBE GaAs. The samples were grown at very low temperature (400 degrees C) in order to limit dopant diffusion, and silicon concentrations were close to the solubility limit at this temperature. As has been shown previously with spike-doped GaAs(Si), the relative occupancies and the mobilities of the lower subbands are very sensitive to the spreading of the dopant distribution. A routine was developed for the analysis of the Fourier transforms of the complex pattern of Shubnikov-de Haas peaks in order to provide quantitative values for the mobilities of the individual subbands. The results of this analysis are compared with values deduced from the magnetic field dependence of the resistivity and Hall effect. On applying hydrostatic pressures of the order of 15 kbar in the dark, a decrease of the free electron concentration of a factor of two was observed. This was accompanied by an increase in the mobility of all the subbands due to the change in the charge state of the silicon donors in the doping slab. With the two thinnest slabs the mobility at ambient pressure is so low in the i=0 subband that Shubnikov-de Haas peaks from this subband could not be detected at fields up to 15 T, although strong peaks could be observed from the higher order subbands. After illumination of the thinnest sample at high pressure the measured free electron concentration is not restored to the zero pressure value. One possibility is that the missing electrons occupy a localized non-metastable Si state resonant with the conduction band rather than a DX centre. The pressure coefficient of the carrier density yields an extrapolated position for the energy level for the Si localized state of 270+or-10 meV above the Gamma -conduction band edge at ambient pressure.

Long-Period Oscillating Interactions Between Ferromagnetic Layers Separated by a Nonmagnetic Metal: a Simple Physical Picture

Abstract: We suggest that, in certain conditions, the long period of the oscillating coupling between ferromagnetic layers separated by a nonmagnetic metal may result from an effect of vernier between the space lattice parameter, and the half Fermi wavelength period of a RKKY-type coupling. The predictions of a simple model of free electrons are compared to available experimental data.

Three-dimensional electrons and two-dimensional electric subbands in the transport properties of tin-doped n-type indium selenide: Polar and homopolar phonon scattering.

Abstract: Electron-scattering mechanisms in n-type indium selenide doped with different amounts of tin are studied by means of the Hall effect (30--300 K) and photo-Hall effect (300 K). The electron mobility at room temperature is found to increase with the free-electron concentration in samples with low tin content. The same behavior is observed when the electron concentration increases due to thermal annealing or photogeneration. That is explained through the presence of two kinds of free electrons contributing to the charge transport along the layers: high-mobility three-dimensional (3D) electrons in the conduction band, and low-mobility two-dimensional electrons in the electric subbands. These 2D subbands are proposed to exist in InSe due to size-quantization effects in thin layers located between two stacking faults. In these regions electron states become higher than conduction-band states. Electrons are transferred outside these regions and are confined in 2D subbands by the resulting electric field. In regard to the electron-scattering mechanisms, it is shown that LO polar phonons play an important role in the scattering of 3D electrons, whose mobility has been calculated by an iteration method. From a comparison with experimental results, we show that the coupling constant for the 3D electron-phonon deformation-potential interaction has been overestimated in previous calculations. The relaxation time for the scattering of 2D electrons by homopolar phonons is determined by using a variational wave function, and the calculated 2D mobility decreases when the localization of the 2D subbands along the c axis increases.

Defect-state occupation, Fermi-level pinning, and illumination effects on free semiconductor surfaces

Abstract: A comprehensive physical model for equilibrium and illuminated free semiconductor surfaces interacting with an arbitrary number of monoenergetic surface and bulk defect states is presented. The position of the surface Fermi energy is shown to be determined by the overall balance of surface charge and surface electric field, and is not merely set by alignment to the energy of the highest-density surface state. Boundary conditions are derived for nonequilibrium device analysis, which includes the full electrostatic and recombinative details of the surface states. An approximate analytical solution is given for the semiconductor transport equations with these boundary conditions and this model is used to describe illumination effects on the free surface. The solution can be represented in a graphical form, which allows the most significant contributions to be easily visualized. Fermi-level pinning on free surfaces is then classified according to the primary contributions to the charge balance and also according to the sensitivity of the surface potential to charge perturbations. The occupation factors for the surface states under illumination conditions are derived from the analytic model and are shown to both shift and saturate with increasing illumination intensity. This allows the origin of free-surface photovoltage, depletion-edge contraction, and band flattening to be reinterpreted as arising from a change in surface-state occupation rather than from a quenching of the depletion-region field by free electrons and holes.

Exciton states in coupled double quantum wells in a static electric field

Abstract: We calculate variationally the four lowest energy levels and oscillator strengths of 1s excitons in a coupled-double-quantum-well structure in an applied static electric field. We demonstrate the importance of employing a variational wave function, which allows for single-particle-state mixing, and which treats the in-plane radial dependence of the exciton states in a more sophisticated manner than the commonly used single exponential. We accomplish this by expanding the eigenstates in a basis consisting of exciton wave functions rather than the commonly used basis of free electron and hole wave functions. These basis wave functions are the ground states of the excitonic Hamiltonians where the electron is primarily confined to one layer and the hole to another---possibly the same---layer. We apply this method to symmetric coupled wells as well as to an asymmetric structure in which the electron and hole in the ground state are localized in separate layers even in the absence of an applied electric field. From an analysis of our results, we arrive at a general approach to qualitatively understand and classify the excitonic properties of these structures.

Recombination of free electrons with ions

Abstract: This paper reports a measurement of radiative recombination rates for U28+ ions and free electrons at energies from 0 to 2 e V. The results appear to contradict known theories.

Unified theory and comparative study of cyclotron masers, ion‐channel lasers, and free electron lasers

Abstract: A unified small‐signal amplification theory is developed to compare growth mechanisms responsible for a number of relativistic radiation generators. The theory is formulated from the basis that the electron resonance frequency produced by the external fields of the devices depends on γ−q, where γ is the beam Lorentz factor and q is a constant (q=1 for cyclotron masers, q=1/2 for ion‐channel lasers, and q=0 for free electron lasers). It is concluded that for wave amplification, the sign of the electron mismatch frequency is required to be the same as the sign of bunching parameter that is determined by the total bunching both axial and azimuthal; this depends on the q value. The two bunching mechanisms exist, not only in the single electron resonance regime, but also in the collective gain regime. Competition or reinforcement between the two bunching mechanisms is determined by the q value, the electron axial velocity, and the wave phase velocity.

Coupling of excitons with excitations of the Fermi sea in asymmetric quantum wells.

Abstract: In asymmetric quantum wells, optical transitions couple a hole state with several conduction-band states. The coupling of excitons associated with higher subbands and free electrons in the lowest subband are investigated using a two-band Mahan--Nozi\`eres--De Dominicis Hamiltonian. The emission spectrum for carriers in the lowest subband shows a very weak Fermi edge singularity at the Fermi level. When the Fermi level approaches the second subband, coupling of excitons with excitations of the Fermi sea leads to a large enhancement of the emission spectrum at the Fermi level.

Parametric (quasi-Cerenkov) X-ray free electron lasers

Abstract: The free electron laser on the basis of parametric (quasi-Cerenkov) instability is considered. Threshold conditions for this process are obtained. It is shown that rather rigid requirements for the particle beam are needed in the X-ray region.

Interaction of intense laser light with free electrons

Abstract: Stimulated Bremsstrahlung multiphoton stimulated Bremsstrahlung spontaneous and stimulated compton scattering ponderomotive potential, Kapitza-Dirac effect and transverse electron channeling in a standing wave free electron lasers laser acceleration of electrons wave packets and their free evolution wave packets in the theory of MSB free-free and bound-free field-induced coherent transitions in a single atom above-threshold ionization - perturbation theory and semi-classical approach, method of essential states, AC Stark shift and ponderomotive acceleration, ATI in the limit of extremely short pulses, harmonic generation interference suppression of photoionization from Rydberg levels in a strong ionizing field

Coverage dependence of surface optical second-harmonic generation from CO/Ni(110): Investigation with a nonlinear-interference technique

Abstract: A nonlinear-optical-interference technique in surface second-harmonic generation is developed to measure directly the variation of surface nonlinear susceptibility of CO/Ni(110) as a function of the CO coverage. It is found that some of the susceptibility elements vary linearly with the coverage and others nonlinearly. A simple model suggests that the former are dominated by contributions from nearly free electrons, while the latter by contributions from the more localized electrons.

Hole coupling resonator for free electron lasers

Abstract: The performance of two mirror resonators with holes for output coupling is studied for free electron laser application using a Fox-Li type code. The mode profiles inside and outside the cavity, the diffraction losses at the mirror edges and apertures, the amount of useful power coupled through the hole, etc., are calculated for the dominant mode for different hole and mirror dimensions. It is found that resonators in near concentric geometry can develop a mode degeneracy in certain cases, which should be avoided for the stability of the free electron laser gain and output. A resonator configuration for a free electron laser at Lawrence Berkeley Laboratory was found which can provide satisfactory performance over a wavelength range from 25 to 50 μm. The possibility of further increasing the tuning range by an adjustable intracavity aperture is discussed.

Quantum well, voltage‐induced quantum well, and quantum barrier electron waveguides: Mode characteristics and maximum current

Abstract: It is shown that finite‐potential heterostructure wells, homostructure voltage‐induced wells, and heterostructure barriers can act as waveguides for ballistic electrons and that waveguiding is described by a single dispersion relation and can occur at energies above all band edges The guided mode cutoffs, electron velocity, effective mass, density of states, and ballistic current density (applicable to 2D electron gases) are presented The maximum ballistic guided current flowing in a given direction for a 10 monolayer Ga075Al025As/GaAs/Ga09Al01As waveguide is found to be 23 mA per μm of waveguide width–allowing considerably greater currents than in single‐mode quantum wires

“Free” Electrons in Solids

Abstract: To a good approximation, the properties of solids can be divided into vibrational dynamics and electronic properties. This so-called adiabatic approximation (Chap. 4) is based on the fact that for the dynamics of the heavy nuclei, or of the nuclei together with their strongly bound core electrons (this combination is known as the “atomic core”), the energy can be expressed as a function of the nuclear or core coordinates in terms of a time-independent potential: the electron system, because of its very much smaller mass, follows the motion of the nuclei or cores almost instantaneously. From the viewpoint of the electron system this also means that for the electron dynamics one can regard the nuclear or core motion as extremely slow and, in the limiting case, as nonexistent. Within the adiabatic approximation one can then determine the excitation states of the electron system in the static potential of the positively charged, periodically arranged nuclei or atomic cores. In doing so, one neglects any interactions between the moving atomic cores and the remaining electrons of the crystal. In order to treat electronic transport phenomena (Sects. 9.3 – 9.5) in crystals, one has to reintroduce these so-called electron-lattice interactions in the form of a perturbation.

Theory of multiharmonic generation and multiphoton electron emission at a metal surface.

Abstract: The nonlinear-optical processes of multiharmonic generation and multiphoton electron emission at the surface of a metal are studied. Results are derived for arbitrary-order perturbation theory. Application is made to the Sommerfeld model, and estimates of the yields are given for simple free-electron-like metals.

The Role of the Electron‐Hole System in the Thermodynamics of YBa2Cu3O7−δGas Equilibrium

Abstract: The YBa2Cu3O7−δ-gas equilibrium is studied theoretically in terms of the approach proposed earlier by the authors. The contribution of the electron-hole system of the HTSC oxide to the equilibrium thermodynamics is studied for several models of the electronic structure. Oxygen—oxygen interactions in the basal plane are taken into account to calculate the chemical potential of oxygen ions in the oxide, distinct from the previous paper where these interactions are disregarded. It is shown that the equilibrium is well described for a sufficiently broad (much broader than kT) valence band with a hole effective mass mp ≈ 3 to 5 free electron masses. [Russian Text Ignored.]

Scattering of x rays by the bound and free electrons in dense plasma.

Abstract: We present a method for calculating the total x-ray-scattering cross section as a function of angle for kilo-electron-volt x rays scattered by partially ionized hot dense plasma. The calculational procedure is based on determining the separate contributions of bound and free electrons to the scattering cross sections. Basic distributions needed for the calculation are obtained by solving the Thomas-Fermi model in the correlation sphere, where both electrons and ions are accounted for. The results indicate significant changes in the intensity and angular distribution of the scattering cross sections between different plasma conditions and between plasma and cold material.

Electron scattering in insulating liquid mixtures

Abstract: A drift-mobility formula for quasifree electrons in nonpolar liquid mixture is derived. The theory is based on electron scattering by potential fluctuations due to concentration fluctuations in the mixture as well as density fluctuations. A comparison is made with the observed mobilities.

Temperature Dependence of Photoluminescence Decay Time in Tunneling Bi-Quantum-Well Structures

Abstract: We studied the nonresonant electron tunneling time in tunneling bi-quantum-well structures with a pump-probe photoluminescence (PL) system using optical mixing. The PL decay time of narrow wells for an AlxGa1-xAs (x=0.51) barrier of 4.0 nm, determined by tunneling, decreases from 40 ps to 24 ps as the temperature increases from 6 K to 132 K. We attribute this to electrons in an exciton state thermalizing into free electrons having a faster tunneling rate.

Chaotic behaviour of the storage ring free electron laser

Abstract: The macrotemporal structure of the storage ring free electron laser is studied in relation to deterministic chaos laws. Comparison between experimental and theoretical results shows that the laser obeys such laws. This can have some important consequences for the future operation of free electron lasers.

Free-electron lasers

Abstract: The Free-Electron Laser (FEL) principle has been known since the early 1970’s but for many years FEL’s have played a marginal role in comparison with conventional lasers. Only in recent years it has become clear that these devices have the potential of becoming exceedingly powerful light sources in the vacuum-ultraviolet (VUV) and X ray regime. In my talk I will first deal with undulator radiation since it is intimitely related to FEL radiation, then explain the low-gain FEL and finally treat the highgain FEL based on the principle of Self Amplified Spontaneous Emission (SASE). SASE-FEL’s are frequently considered as the fourth generation of accelerator-based light sources. In contrast to existing synchrotron radiation light sources which are mostly storage rings the FEL requirements on the electron beam quality in terms of low transverse emittance and small energy spread are so demanding that only linear accelerators can be used to provide the drive beam. In my one-hour talk it was not possible to go much into mathematical details. The high-gain FEL is therefore treated only qualitatively. For a thorough presentation of SASE FEL’s I refer to the book “The Physics of Free Electron Lasers” by Saldin, Schneidmiller and Yurkov and to the lectures by J. Rossbach at the CERN Accelerator School on Synchrotron Radiation.

Contribution of Auger processes to the particle-induced electron emission from nearly-free-electron metals

Abstract: Using the microscopic theory of particle-induced electron emission developed earlier [M. Rosler and W. Brauer, Phys. Status Solidi (b) 148 (1988) 213] the role of excitation of conduction electrons by Auger processes is investigated. Starting from a simple model of the Auger excitation process we have calculated the energy distribution of emerging electrons and the electron yield. It is shown that the excitation via Auger processes should be included in a quantitative description of particle-induced emission phenomena.

Experimental investigation of a plasma edge cathode scheme for pulsed electron beam extraction

Abstract: A scheme with the potential for generation of an electron beam with high brightness and several μs pulse duration for microwave generation, electron accelerators, or free electron lasers is investigated experimentally. An electron beam is extracted transversely to the flow of a plasma jet. The transverse boundary of the plasma allows extraction of a space charge limited electron current for 7 μs at a current density of 18 A/cm2. At present, a normalized microscopic brightness of 8×108 A m−2 rad−2 is achieved. Closure of the extraction gap by invasion of plasma has been observed with a typical velocity of 0.1 cm/μs. Higher current density and higher brightness are expected for higher plasma densities and larger extraction fields.