Numerical Computations with H(div)-Finite Elements for the Brinkman Problem

The H(div)-conforming approach for the Brinkman equation is studied numerically, verifying the theoretical a priori and a posteriori analysis in previous work of the authors. Furthermore, the results are extended to cover a non-constant permeability. A hybridization technique for the problem is presented, complete with a convergence analysis and numerical verification. Finally, the numerical convergence studies are complemented with numerical examples of applications to domain decomposition and adaptive mesh refinement.Comment: Minor clarifications, added references. Reordering of some figures. To appear in Computational Geosciences, final article available at http://www.springerlink.co

ESR observations of paramagnetic centers in intrinsic hydrogenated microcrystalline silicon

Paramagnetic centers in hydrogenated microcrystalline silicon, µc-Si:H have been studied using dark and light-induced electron-spin resonance (ESR). In dark ESR measurements only one center is observed. The g values obtained empirically from powder-pattern line-shape simulations are g=2.0096 and g'=2.0031. We suggest that this center may be due to defects in the crystalline phase. During illumination at low temperatures, an additional ESR signal appears. This signal is best described by two powder patterns indicating the presence of two centers. One center is asymmetric (gi=1.999, g'=1.996), while the other is characterized by large, unresolved broadening such that unique g values cannot be obtained. The average g value for this center is 1.998. The light-induced signal, which we interpret as coming from carriers trapped in the band tails at the crystalline grain boundaries, remains for at least several minutes after the light is turned off. Although the time scales of the decay curves are very different for two samples prepared by different techniques, both decays can be fitted using the assumption of recombination due to distant pairs of electrons and holes trapped in localized band-tail states

Defects and recombination in microcrystalline silicon

The paper addresses the defect structure of microcrystalline silicon, muc Si H. Electron spin resonance ESR techniques are employed to study the nature and energy distribution of paramagnetic states in a large variety of undoped and doped muc Si H samples prepared by various methods under different deposition conditions. A qualitative model for the density of states distribution in the energy gap is developed which is dominated by two kinds of dangling bonds and bandtail states at both band edges. In phosphorus doped samples, ESR reveals a metal insulator transition at a phosphorus concentration of 4 x 10 18 cm 3 . Light induced ESR and electrically detected magnetic resonance show that at low temperatures recombination is dominated by tunneling transitions from the bandtail states into neutral dangling bonds. Apparently, at higher temperatures, direct capture from conducting states prevails. The results are discussed considering the heterogeneous structure of this materia

Quantum-beat recombination echoes

A recombination echo phenomenon of weakly coupled electron-hole pairs in disordered semiconductors is suggested. The time-domain calculation of spin-dependent recombination within such pairs indicates that dephasing beat oscillations can take place in a magnetic field and a phase recovery due to a resonant microwave pulse is possible. This effect, which we call the quantum-beat recombination echo, is illustrated for different pulse sequences. Applications for material characterisation and quantum computing are suggested