等离子体中集体效应对韧致辐射的影响

Birmingham和Dawson所导得的韧致辐射公式中丢掉了离子对场的屏蔽效应,这样做是不合适的。本文导得了包含离子场极化效应修正后的公式,在很多情况下,离子屏蔽效应能显著减少韧致辐射能量。 关键词：     

Quantum Kinetic Theory for Laser Plasmas. Dynamical Screening in Strong Fields

A quantum kinetic theory for correlated charged-particle systems in strong time-dependent electromagnetic fields is developed. Our approach is based on a systematic gauge-invariant nonequilibrium Green's functions formulation. Extending our previous analysis [1] we concentrate on the selfconsistent treatment of dynamical screening and electromagnetic fields which is applicable to arbitrary nonequilibrium situations. The resulting kinetic equation generalizes previous results to quantum plasmas with full dynamical screening and includes many-body effects. It is, in particular, applicable to the interaction of dense plasmas with strong electromagnetic fields, including laser fields and x-rays. Furthermore, results for the modification of the plasma screening and the longitudinal field fluctuations due to the electromagnetic field are presented.     

Experimental measurements of topological singularity screening in random paraxial scalar and vector optical fields

There exists a substantial body of theory that predicts mutual screening of signed topological singularities (topological charges) in random optical fields (speckle patterns). Such screening appears to be rather mysterious because there are neither energetic nor entropic reasons for its existence. We present the first experimental confirmation of mutual screening by the stationary points of the intensity, the canonical optical scalar field, and of mutual screening by C points in elliptically polarized light, the generic optical vector field. We also elucidate specific aspects of the geometry and topology of these fields that we argue give rise to screening.     

Screening effects on polar mode scattering at high electric fields

The effects of screening of polar optical mode of scattering at high electric fields are studied by performing Monte Carlo calculations on InSb at 77 K. The average carrier energy is found to be markedly affected by the inclusion of screening.     

Mechanisms for the generation of coherent longitudinal-optical phonons in GaAs /AlGaAs multiple quantum wells

We show that coherent optical phonons in GaAs multiple quantum wells are generated in a completely different way as compared to bulk GaAs. Unlike in bulk GaAs where the ultrafast screening of electric fields by photogenerated charge carriers is known to be dominant, three distinctive generation mechanisms contribute simultaneously in multiple quantum wells. The interplay between impulsive Raman scattering, forbidden Raman scattering, and screening of surface electric fields, whose relative strengths are determined by laser intensity, detuning from the exciton resonance, and the barrier width, generates a rich variety of new phenomena.     

Nuclear magnetic shielding of a hydrogen atom in an electric field

The nuclear magnetic screening constant of a hydrogen atom in a uniform electric field is calculated, neglecting any effect of electron spin. The screening is found to be reduced by the electric field for all directions of the applied magnetic field, the reduction being greatest if the two fields are perpendicular.     

High-resolution imaging using Hadamard encoding

High-resolution imaging techniques using noninvasive modalities such as magnetic resonance (MR) imaging are being pursued as in vivo cancer screening techniques in an attempt to eliminate the invasive nature of surgical biopsy. When acquiring high-resolution MR images for tissue screening, image fields of view have in the past been limited by the matrix sizes available in conventional MR scanners. We present here a technique that uses aliasing to produce high resolution images with larger matrix sizes than are currently available. The image is allowed to alias in both the frequency encoding and phase encoding dimensions, and the individual, aliased fields of view are recovered by Hadamard encoding methods. These fields may then be tiled to obtain a composite image with high spatial resolution and a large field of view. The technique is demonstrated using two-dimensional and three-dimensional in vivo imaging of the human brain and breast.     

Short- and long-range screening of optical phase singularities and C points

Screening of signed (charged) singularities-phase vortices in scalar fields, C points in vector fields, is discussed for paraxial optical fields with short- and long-range correlations. A circular region of radius R is assumed. Short-range screening is exemplified by a Gaussian field correlator, long-range screening by a J₀ Bessel function. The short-range screening length is obtained analytically; this is found to be in substantial agreement with recent experiments. For long-range screening, an accurate asymptotic formula suitable for quantitative comparison with data (numerical or laboratory) is derived for the variance of the net charge. A J₀ correlation function is not attainable in practice, but it is shown how to generate a pseudo-long-range optical field whose correlation function closely approximates this form; screening in such a field is well described by our theoretical results for J₀. The charge variance can be measured by three different methods: by counting positive and negative singularities inside the region of interest, by counting signed zero crossings on the perimeter of this region; or by measuring phase derivatives along the perimeter. For the first method, the charge variance is calculated by integration over the charge correlation function, for the second (third) by integration over the zero crossing (phase derivative) correlation function. It is proven explicitly that, as expected, all three calculations yield the same result. It is also shown analytically that for short-range screening the zero crossings can be counted along a straight line whose length is 2πR, but that for long-range screening this useful simplification no longer holds; for this case another formula is given that is suitable for data correction. The effects of boundary smoothing are discussed, and a class of generalized exponential smoothing functions is introduced. Analytical (numerical) results are given for the large R limit of the charge variance for the short (long) range case. Finally, it is shown that for realizable optical fields, both for the short and pseudo-long-range cases, for sufficiently small R the charge variance grows as R², whereas for sufficiently large R it grows as R.     

Screening of collective cyclotron resonance in inversion layers on InSb

Cyclotron Resonance (CR) of inversion electrons on InSb is studied in parallel magnetic fields with normally incident light. Whereas in this Voigt configuration collective CR is observed in thin bulk samples, we observe single particle CR in the case of inversion layers. The apparent screening of the collective mode is explained by the inhomogeneity of the inversion electron gas in crossed electric and magnetic fields.     

High-field cooperative transport phenomena and screening in phase-separated degenerate antiferromagnetic semiconductors

Response to a strong electric field is investigated of degenerate antiferromagnetic semiconductors EuTe spontaneously separated into antiferromagnetic and ferromagnetic phases. Whereas in weak fields their response is Ohmic, in strong fields a sequence of very high peaks appears over the Ohmic-type background beginning from a certain threshold field strength. Their spacing decreases with increasing field strength. These peaks are explained by cooperative motion of charged ferromagnetic microregions inside the antiferromagnetic host. The screening by these microregions is investigated theoretically.     

Magnetic field studies of the frictional drag between coupled two-dimensional electronic systems – Coulomb versus phonon coupling

The coupling between systems of two spatially separated two-dimensional (2D) electron gases and between systems of a separated 2D electron gas and a 2D hole gas is studied as a function of magnetic field . The small barrier (30 nm) separated coupled electron gases showed a transition from a phonon dominated interaction at to a Coulomb dominated one at quantising fields. For large barriers (190 nm) phonons have been found to be the dominant coupling mechanism both at zero and finite fields. However, for all barriers investigated we could observe novel screening effects manifested in a suppression of the coupling at half-filled Landau levels. For the coupled electron–hole gases we have investigated samples with large barriers ( ) so that the coupling is both in zero and finite fields dominated by phonon mediated processes. The enhanced screening effects could not be observed in those samples possibly due to the less pronounced quantisation of the hole gas.     

Screening of impurities in strong magnetic fields

Charged impurities inserted in an electron gas in strong magnetic fields and at low temperatures are considered. Using the random phase and the generalized Born approximations, a self-consistent calculation of the screening of the impurities and the broadening of the electronic energy levels due to the scattering by these impurities is presented. Concrete results obtained in numerical form show that for typical semiconductors the anisotropy of te screening induced by the magnetic field is strongly reduced by collisional damping. The screening length, however, depends rather strongly on the field.On leave of absence from the Department of Physics, University of Tokyo, Tokyo, Japan.     

Field-dependence of carrier screening in non-linear transport theory

Modifications to the high-field Boltzmann transport equation are obtained to include ab initio self-consistent non-equilibrium free-carrier screening of the scattering rates. For quasi-static homogeneous fields the screening involves a Lindhard-like dielectric function which is field-dependent via the carrier distribution function and the carrier energetics.     

Enhanced Raman scattering on metal surfaces

We present a theory which describes enhanced Raman scattering from molecules adsorbed on metals. The enhancement is due to the screening fields induced by the optically polarized adsorbate in the presence of the substrate. These fields interact coherently with the applied optical field and the vibrational motion of the adsorbed molecule. The enhancement is shown to be coverage dependent and also rather sensitive to the optical dielectric function of the substrate. Predicted enhancements compare well with experiment.     

Do columnar defects produce bulk pinning?

From magneto-optical imaging performed on heavy-ion-irradiated YBa(2)Cu(3)O(7-delta) single crystals, it is found that at fields and temperatures where strong single vortex pinning by individual irradiation-induced amorphous columnar defects is to be expected, vortex motion is limited by the nucleation of vortex kinks at the specimen surface. In the material bulk, vortex motion occurs through (easy) kink sliding. Depinning in the bulk determines the screening current only at fields comparable to or larger than the matching field, at which the majority of vortices is not trapped by an ion track.     

Observation of magnetic field-induced contraction of fission yeast cells using optical projection microscopy

The charges in live cells interact with or produce electric fields, which results in enormous dielectric responses, flexoelectricity, and related phenomena. Here we report on a contraction of Schizosaccharomyces pombe (fission yeast) cells induced by magnetic fields, as observed using a phase-sensitive projection imaging technique. Unlike electric fields, magnetic fields only act on moving charges. The observed behavior is therefore quite remarkable, and may result from a contractile Lorentz force acting on diamagnetic screening currents. This would indicate extremely high intracellular charge mobilities. Besides, we observed a large electro-optic response from fission yeast cells.     

Effect of superstrong magnetic fields on thermonuclear reaction rates on the surface of magnetars

A simple and efficient screening model for studying the effects of superstrong magnetic fields （such as those of magnetars） on thermonuclear reaction rates on magnetar surfaces is proposed in this paper. The most interesting thermonuclear reactions, including hydrogen burning by the CNO cycle and helium burning by the triple alpha reaction, are investigated on the surface ofmagnetars. We find that the superstrong magnetic fields can increase the thermonuclear reaction rates by many orders of magnitude. The enhancement may have a dramatic effect on the thermonuclear runaways and bursts on the surfaces of magnetars.     

Using the chern-simons action for a self-consistent determination of the magnetic screening mass in thermal QCD

We present a self-consistent determination of the screening mass for chromomagnetic fields in QCD within the framework of dimensional reduction. The three-dimensional Chern-Simons density is used as a mass term for a self-consistent perturbative calculation that yields a value of m approximately 1.604(g(2)N/2pi)T for the magnetic screening mass.     

The effect of applied electric fields on chemisorption

A simple model is used to study the effects of applied electric fields on the neutral chemisorption bond at a metal-vacuum interface. The adsorbate-substrate complex is represented by a model diatomic molecule in an electric field which varies spatially due to substrate screening. The field induced changes in binding energy, equilibrium position and vibrational frequency of the adsorbate are calculated. The latter changes are due to the dependence of the effective field coefficients on the adsorbate distance from the surface. Measurable effects, which reflect the bond characteristics, are predicted for experimentally attainable fields.     

Atomic-scale compensation phenomena at polar interfaces

The interfacial screening charge that arises to compensate electric fields of dielectric or ferroelectric thin films is now recognized as the most important factor in determining the capacitance or polarization of ultrathin ferroelectrics. Here we investigate using aberration-corrected electron microscopy and density-functional theory to show how interfaces cope with the need to terminate ferroelectric polarization. In one case, we show evidence for ionic screening, which has been predicted by theory but never observed. For a ferroelectric film on an insulating substrate, we found that compensation can be mediated by an interfacial charge generated, for example, by oxygen vacancies.