Development of a spin polarized low energy electron diffraction system

We have designed and constructed a spin polarized low energy electron diffraction system working in the reflected electron pulse counting mode. This system is capable of measuring asymmetries due to spin-orbit and exchange interactions. Photoemission from a strained GaAs/GaAsP super lattice is used as the source of spin polarized electrons. Spin-orbit asymmetry is evaluated for Ir(100) single crystal at various energies. Subsequently, exchange asymmetry has been evaluated on 40 monolayer Fe deposited on Ir(100). This instrument proves to be useful in understanding structure and magnetism at surfaces. (C) 2016 AIP Publishing LLC.

High-energy electron beam production by femtosecond laser interactions with exploding-foil plasmas

The interaction of an ultraintense, 30-fs laser pulse with a preformed plasma was investigated as a method of producing a beam of high-energy electrons. We used thin foil targets that are exploded by the laser amplified spontaneous emission preceding the main pulse. Optical diagnostics show that the main pulse interacts with a plasma whose density is well below the critical density. By varying the foil thickness, we were able to obtain a substantial emission of electrons in a narrow cone along the laser direction with a typical energy well above the laser ponderomotive potential. These results are explained in terms of wake-field acceleration.

DOMINANCE OF SHORT-RANGE-ORDER EFFECTS IN LOW-ENERGY ELECTRON-DIFFRACTION INTENSITY SPECTRA

Using low-energy electron-diffraction (LEED) formalism, we demonstrate theoretically that LEED I-V spectra are characterized mainly by short-range order. We also show experimentally that diffuse LEED (DLEED) I-V spectra can be accurately measured from a disordered system using a video-LEED system even at very low coverage. These spectra demonstrate that experimental DLEED I-V spectra from disordered systems may be used to determine local structures. As an example, it is shown that experimental DLEED I-V spectra from K/Co {1010BAR} at potassium coverages of 0.07, 0.1, and 0.13 monolayer closely resemble calculated and experimental LEED I-V spectra for a well-ordered Co{1010BAR}-c(2X2)-K superstructure, leading to the conclusion that at low coverages, potassium atoms are located in the fourfold-hollow sites and that there is no large bond-length change with coverage.

Generation of overdense and high-energy electron-positron-pair plasmas by irradiation of a thin foil with two ultraintense lasers

A scheme for enhanced quantum electrodynamics (QED) production of electron-positron-pair plasmas is proposed that uses two ultraintense lasers irradiating a thin solid foil from opposite sides. In the scheme, under a proper matching condition, in addition to the skin-depth emission of gamma-ray photons and Breit-Wheeler creation of pairs on each side of the foil, a large number of high-energy electrons and photons from one side can propagate through it and interact with the laser on the other side, leading to much enhanced gamma-ray emission and pair production. More importantly, the created pairs can be collected later and confined to the center by opposite laser radiation pressures when the foil becomes transparent, resulting in the formation of unprecedentedly overdense and high-energy pair plasmas. Two-dimensional QED particle-in-cell simulations show that electron-positron-pair plasmas with overcritical density 10(22) cm(-3) and a high energy of 100s of MeV are obtained with 10 PW lasers at intensities 10(23) W/cm(2), which are of key significance for laboratory astrophysics studies.

Molecular t-matrices for low-energy electron diffraction (TMOL v1.1)*1

We describe a FORTRAN-90 program that computes scattering t-matrices for a molecule. These can be used in a Low-Energy Electron Diffraction program to solve the molecular structural problem very efficiently. The intramolecular multiple scattering is computed within a Dyson-like approach, using free space Green propagators in a basis of spherical waves. The advantage of this approach is related to exploiting the chemical identity of the molecule, and to the simplicity to translate and rotate these t-matrices without performing a new multiple-scattering calculation for each configuration. FORTRAN-90 routines for rotating the resulting t-matrices using Wigner matrices are also provided.

A FORTRAN-90 low-energy electron diffraction program (LEED90 v1.1)

We describe a FORTRAN-90 program to compute low-energy electron diffraction I(V) curves. Plane-waves and layer doubling are used to compute the inter-layer multiple-scattering, while the intra-layer multiple-scattering is computed in the standard way expanding the wavefield on a basis of spherical waves. The program is kept as general as possible, in order to allow testing different parts of multiple-scattering calculations. In particular, it can handle non-diagonal t-matrices describing the scattering of non-spherical potentials, anisotropic vibrations, anharmonicity, etc. The program does not use old FORTRAN flavours, and has been written keeping in mind the advantage for parallelism brought forward by FORTRAN-90.

In situ annealing during the growth of relaxed SiGe

In this paper, a graded Si1-xGex buffer and thereafter the Si0.8Ge0.2 uniform layer were grown at a little lower temperature to keep the surface smooth, which will provide the gliding dislocations a wider channel and less low energy nucleation sites on the surface. Therefore, the dislocation density may be reduced. However, the motion of the existing threading dislocations cannot retain equilibrium at lower temperature, strain will accumulate and be in favor of the nucleation of dislocation. In situ annealing was used to reduce the residual strain in the sample during the low-temperature growth of SiGe. A fully relaxed Si0.8Ge0.2 layer was obtained with the surface dislocation density of 3x10(5)cm(-2).

The role of Sb in the molecular beam epitaxy growth of 1.30-1.55 mu m wavelength GaInNAs/GaAs quantum well with high indium content

Sb-assisted GaInNAs/GaAs quantum wells (QWs) with high (42.5%) indium content were investigated systematically. Transmission electron microscopy, reflection high-energy electron diffraction and photoluminescence (PL) measurements reveal that Sb acts as a surfactant to suppress three-dimensional growth. The improvement in the 1.55 mu m range is much more apparent than that in the 1.3 mu m range.. which can be attributed to the difference in N composition. The PL intensity and the full-width at half maximum of the 1.55 mu m single-QW were comparable with that of the 1.3 Am QWs. (c) 2006 Elsevier B.V. All rights reserved.

Investigation of periodicity fluctuations in strained (GaNAs)(1)(GaAs)(m) superlattices by the kinematical simulation of x-ray diffraction

Periodicity fluctuations of layer thickness and composition in a superlattice not only decrease the intensity, they also broaden the width of the satellite peaks in the x-ray diffraction pattern. In this letter, we develop a method that is dependent on the width of satellite peaks to assess periodicity fluctuations of a superlattice quickly. A linear relation of the magnitude of fluctuations, peak width and peak order has been derived from x-ray diffraction kinematical theory. By means of this method, periodicity fluctuations in strained (GaNAs)(1)(GaAs)(m) superlattices grown on GaAs substrates by molecular beam epitaxy have been studied. Distinct satellite peaks indicate that the superlattices are of high quality. The N composition of 0.25 and its fluctuation of 20% in a strained GaNxAs1-x monolayer are obtained from simulations of the measured diffraction pattern. The x-ray simulations and in situ observation results of reflection high-energy electron diffraction are in good agreement. (C) 1999 American Institute of Physics. [S0003-6951(99)00828-1].