Exchange between deep donors in semiconductors: a quantum defect approach

Exchange interactions among defects in semiconductors are commonly treated within effective-mass theory using a scaled hydrogenic wave-function. However such a wave-function is only applicable to shallow impurities; here we present a simple but robust generalization to treat deep donors, in which we treat the long-range part of the wavefunction using the well established quantum defect theory, and include a model central-cell correction to fix the bound-state eigenvalue at the experimentally observed value. This allows us to compute the effect of binding energy on exchange interactions as a function of donor distance; this is a significant quantity given recent proposals to carry out quantum information processing using deep donors. As expected, exchange interactions are suppressed (or increased), compared to the hydrogenic case, by the greater localization (or delocalization) of the wavefunctions of deep donors (or `super-shallow' donors with binding energy less then the hydrogenic value). The calculated results are compared with a simple scaling of the Heitler-London hydrogenic exchange; the scaled hydrogenic results give the correct order of magnitude but fail to reproduce quantitatively our calculations. We calculate the donor exchange in silicon including inter-valley interference terms for donor pairs along the $\{100\}$ direction, and also show the influence of the donor type on the distribution of nearest-neighbour exchange constants at different concentrations. Our methods can be used to compute the exchange interactions between two donor electrons with arbitrary binding energy.Comment: 11 pages, 10 figures, RevTeX

"Portugiesische Redewendungen" : Fraseologia para aprendizes de L. E.

Rezension zu Stefan Ettinger, Manuela Nunes: Portugiesische Redewendungen – Ein Wörter- und Übungsbuch für Fortgeschrittene. Helmut Buske Verlag Hamburg, 2006, 151 S

Ultraviolet avalanche in anisotropic non-Abelian plasmas

We present solutions of coupled particle-field evolution in classical U(1) and SU(2) gauge theories in real time on three-dimensional lattices. For strongly anisotropic particle momentum distributions, we find qualitatively different behavior for the two theories when the field strength is high enough that non-Abelian self-interactions matter for SU(2). It appears that the energy drained by a Weibel-like plasma instability from the particles does not build up exponentially in transverse magnetic fields but instead returns, isotropically, to the hard scale via a rapid avalanche into the ultraviolet.Comment: 22 pages, 10 figures; v3: small textual changes; updated to correspond with version to appear in publicatio

KNO scaling of fluctuations in pp and pA, and eccentricities in heavy-ion collisions

Multiplicity fluctuations at midrapidity in p+p collisions at high energies are described by a negative binomial distribution and exhibit approximate Koba-Nielsen-Olesen (KNO) scaling. We find that these KNO fluctuations are important also for reproducing the multiplicity distribution in d+Au collisions observed at RHIC, adding to the Glauber fluctuations of the number of binary collisions or participants. We predict that the multiplicity distribution in p+Pb collisions at the LHC also deviates little from the KNO scaling function. Finally, we analyze various moments of the eccentricity of the particle production zone in A+A collisions at RHIC and LHC and find that particle production fluctuations increase fluctuation dominated moments such as the triangularity epsilon_3 substantially.Comment: 7 pages, 6 figures; v2: updated references and a few minor modifications, to appear in PR

Branching ratio change in K- absorption at rest and the nature of the Lambda(1405)

We investigate in-medium corrections to the branching ratio in K- absorption at rest and their effect on the (positively and negatively) charged pion spectrum. The in-medium corrections are due to Pauli blocking, which arises if the Lambda(1405) is assumed to be a $\bar{K}$-nucleon bound state and leads to a density and momentum dependent mass shift of the Lambda(1405). Requiring that the optical potential as well as the branching ratio are derived from the same elementary T-matrix, we find that the in-medium corrected, density dependent T-matrix gives a better description of the K- absorption reaction than the free, density-independent one. This result suggests that the dominant component of the Lambda(1405) wave function is the $\bar{K}N$ bound state.Comment: 8 Pages, Revtex with epsf, and embedded 8 ps figure

Caracterização geotécnica dos solos da Região de Bom Jesus, Luanda, Angola

Este trabalho tem como principal objectivo, contribuir para a caracterização geológica e geotécnica dos maciços terrosos da região de Bom Jesus, uma das principais áreas interessadas pela futura expansão urbana da cidade de Luanda em Angola, com a finalidade de identificar as potenciais zonas de risco associadas à instabilidade de taludes. A área de estudo fica situada na região de Bom Jesus, província de Luanda, município do Icolo-Bengo, tendo como acesso a estrada nacional (EN120) que liga as cidades de Luanda e N’dalatando, e a estrada principal de Bom Jesus, a qual atravessa a área em estudo. Na área ocorre uma série sedimentar com uma espessura que admite-se poder ser superior a 4000 m e que está assente em discordância sobre o soco cristalino Pré-Câmbrico, sendo constituída por distintas sequências correspondentes a diferentes condições na deposição das formações sedimentares da Bacia do Kwanza, com idades entre o Cretácico inferior e o Quaternário [Morais, 1993]