The spin dependence of the conductance of an asymmetric double-barrier InGaAs device is studied within the multiband k(.)p and envelope function approximations. The spin-dependent transmission probability for electrons across the structure is obtained using transfer matrices and the low bias conductance per unit area is calculated as a function of the Fermi energy (or doping) in the contacts. The possibility to obtain spin polarized currents in such devices is demonstrated, however, the resulting degree of polarization is rather small (a few percent) in the specific InGaAs structures considered here.
A small-mass system has been developed for monitoring the flux of neutrons with energy up to 1 MeV at the new time-of-flight facility at CERN, n_TOF. The monitor is based on a thin Mylar foil with a Li-6 deposit, placed in the neutron beam, and an array of Silicon detectors, placed outside the beam, for detecting the products of the Li-6(n, alpha)H-3 reaction. The small amount of material on the beam ensures a minimal perturbation of the flux and minimizes the background related to scattered neutrons. Moreover, a further reduction of the gamma-ray background has been obtained by constructing the scattering chamber hosting the device in carbon fibre. A detailed description of the flux monitor is here presented, together with the characteristics of the device, in terms of efficiency, resolution and induced background. The use of the monitor in the measurement of neutron capture cross-sections at n_TOF is discussed.
Thin-film ZnO/CdS/CuIn1-xGaxSe2 solar cells are manufactured with similar to 20% solar-cell conversion efficiency. The CuIn1-xGaxSe2 (CIGS) absorber exhibits rather different physical properties compared with conventional semiconductors (e.g. Si, GaAs and ZnSe). For instance, (i) the valence-band maximum in CIGS consists of cation-d-anion-p hybridized states. (ii) Cation vacancies have low formation energies and high mobility at room temperature. (iii) The most stable surface of CuIn1-xGaxSe2 is the reconstructed, and otherwise polar, (112) surface. (iv) Solar cells with absorbers containing grain-boundaries outperform cells with crystalline absorbers. In this work, the fundamental physical properties of CIGS, like the electronic structure, the defect formation energies, as well as surface properties are discussed from a theoretical perspective.
Electronic band-edge structure and optical properties of Si 1-xGex are investigated theoretically emloying a full-potential linearized augmented plane wave (FPLAPW) method. The exchange-correlation potential in the local density approximation (LDA) is corrected by an on-site Coulomb potential (i.e., within the LDA.+USIC approach) acting asymmetrically on the atomic-like orbitals in the muffin-tin spheres. The electronic structure of the Si1-xGex is calculated self-consistently, assuming a Td symmetrized Hamiltonian and a linear behavior of the valence-band eigenfunctions for Si, SiGe, and Ge with respect to Ge composition x, assuming randomly alloyed crystal structure, i.e., a "virtual-crystal like" approximation (VGA). We show that this approach yields accurate band-gap energies, effective masses, dielectric function, and optical properties of Si1-xGex. We perform absorption measurements showing the band-gap energy for x < 0.25.
We propose the local density approximation (LDA) plus an on-site Coulomb self-interaction-like correction (SIC) potential for describing sp-hybridized bonds in semiconductors and insulators. We motivate the present LDA+U-SIC scheme by comparing the exact exchange (EXX) hole with the LDA exchange hole. The LDA+U-SIC method yields good band-gap energies E-g and dielectric constants (omega approximate to 0) of Si, Ge, GaAs, and ZnSe. We also show that LDA consistently underestimates the Gamma-point effective electron m, and light-hole M-lh masses, and the underlying reason for this is a too strong light-hole-electron coupling within LDA. The advantages of the LDA+USIC approach are a computational time of the same order as the ordinary LDA, the orbital dependent LDA+USIC exchange-correlation interaction is asymmetric analogously to the EXX potential, and the method can be used for materials and compounds involving localized d- and f-orbitals.
General metrics of large aspect ratio tokamaks is used in the paper. General expressions for the neoclassical poloidal plasma rotation V-2(theta) and radial ion heat flux Gamma(Ti) are obtained. Their dependence 2 on the squared Mach number alpha = U-zetai(2)/c(5)(2) is analyzed (here U-zetai is the ion toroidal velocity and c(s) is the sound velocity, respectively). Some interesting peculiarities of this dependence are emphasized.
The concept of ExB flow velocity shear suppression is utterly fundamental in modem fusion research. It is asserted that there are models enabling to understand the physics involved in LH transitions. To improve the understanding of the mechanisms leading to the formation of Transport Barriers, especially the relation between Internal and Edge barriers it is necessary to invoke the issue of electric fields. Edge transport barriers are the feature of the H-mode, the baseline regime of ITER, whereas Internal Transport Barriers are used to develop regimes that might be employed for steady state operation of ITER, definitely beneficial for design and operation of fusion power plants in the future. Their synergy will be addressed. Plasma flows are closely connected to electric fields. Therefore, their role is crucial for understanding of tokamaks aimed at the achievement of fusion energy. This appears in the well known neoclassical theory as the most accomplished and selfconsistent basis for understanding of fusion plasmas. It pertains to the novel concept of zonal flows emerging from the recent development of gyro-kinetic transport codes. The equilibrium poloidal and toroidal flows are also crucial for the concept of the electric field shear suppression of plasma turbulence in tokamaks. Yet, this timely and topical issue has remained largely unaddressed experimentally because of great difficulties in measuring flows in plasmas.