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Self-consistent Solution (self-consistent + solution)

Distribution by Scientific Domains

Physics and Astronomy	40%

Selected Abstracts

Biogeochemical modelling of the rise in atmospheric oxygen

GEOBIOLOGY, Issue 4 2006
M. W. CLAIRE
ABSTRACT Understanding the evolution of atmospheric molecular oxygen levels is a fundamental unsolved problem in Earth's history. We develop a quantitative biogeochemical model that simulates the Palaeoproterozoic transition of the Earth's atmosphere from a weakly reducing state to an O2 -rich state. The purpose is to gain an insight into factors that plausibly control the timing and rapidity of the oxic transition. The model uses a simplified atmospheric chemistry (parameterized from complex photochemical models) and evolving redox fluxes in the Earth system. We consider time-dependent fluxes that include organic carbon burial and associated oxygen production, reducing gases from metamorphic and volcanic sources, oxidative weathering, and the escape of hydrogen to space. We find that the oxic transition occurs in a geologically short time when the O2 -consuming flux of reducing gases falls below the flux of organic carbon burial that produces O2. A short timescale for the oxic transition is enhanced by a positive feedback due to decreasing destruction of O2 as stratospheric ozone forms, which is captured in our atmospheric chemistry parameterization. We show that one numerically self-consistent solution for the rise of O2 involves a decline in flux of reducing gases driven by irreversible secular oxidation of the crust caused by time-integrated hydrogen escape to space in the preoxic atmosphere, and that this is compatible with constraints from the geological record. In this model, the timing of the oxic transition is strongly affected by buffers of reduced materials, particularly iron, in the continental crust. An alternative version of the model, where greater fluxes of reduced hydrothermal cations from the Archean seafloor consume O2, produces a similar history of O2 and CH4. When climate and biosphere feedbacks are included in our model of the oxic transition, we find that multiple ,Snowball Earth' events are simulated under certain circumstances, as methane collapses and rises repeatedly before reaching a new steady-state. [source]

Comparative study of unscreened and screened molecular static linear polarizability in the Hartree,Fock, hybrid-density functional, and density functional models

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2008
Rajendra R. Zope
Abstract The sum-over-states (SOS) polarizabilities are calculated within approximate mean-field electron theories such as the Hartree,Fock approximation and density functional models using the eigenvalues and orbitals obtained from the self-consistent solution of the single-particle equations. The SOS polarizabilities are then compared with those calculated using the finite-field (FF) method. Three widely used mean-field models are as follows: (1) the Hartree,Fock (HF) method, (2) the three parameter hybrid generalized gradient approximation (GGA) (B3LYP), and (3) the parameter-free generalized gradient approximation due to Perdew,Burke,Ernzerhof (PBE). The comparison is carried out for polarizabilities of 142 molecules calculated using the 6-311++G(d,p) basis set at the geometries optimized at the B3LYP/6-311G** level. The results show that the SOS method almost always overestimates the FF polarizabilities in the PBE and B3LYP models. This trend is reversed in the HF method. A few exceptions to these trends are found. The mean absolute errors (MAE) in the screened (FF) and unscreened (SOS) polarizability are 0.78, 1.87, and 3.44 Å3 for the HF, B3LYP, and PBE-GGA methods, respectively. Finally, a simple scheme is devised to obtain FF quality polarizability from the SOS polarizability. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source]

Modulation bandwidth of semiconductor lasers based on coupled quantum wells

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 4 2004
M. S. Wartak
Abstract The effect of well coupling on differential gain and maximum-modulation bandwidth for semiconductor lasers based on coupled quantum wells is analyzed using the K -factor method. We determine differential gain in coupled quantum wells within the self-consistent solution of the Poisson, Schroedinger, and 4 × 4 Luttinger,Kohn equations. The multiple-body effects of bandgap renormalization, coulombic scattering interactions, and a nonMarkovian distribution are also included. The analysis has been performed for coupled wells at 1.55 ,m in an InGaAsP/InP lattice-matched system grown in the [001] direction. Our results suggest that in order to maximize modulation bandwidth and differential gain, one should design structures with barrier widths larger than 40 Å. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 272,274, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20275 [source]

A disc-wind model with correct crossing of all magnetohydrodynamic critical surfaces

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2000
N. Vlahakis
The classical Blandford & Payne model for the magneto-centrifugal acceleration and collimation of a disc-wind is revisited and refined. In the original model, the gas is cold and the solution is everywhere subfast magnetosonic. In the present model the plasma has a finite temperature and the self-consistent solution of the MHD equations starts with a subslow magnetosonic speed which subsequently crosses all critical points, at the slow magnetosonic, Alfvén and fast magnetosonic separatrix surfaces. The superfast magnetosonic solution thus satisfies MHD causality. Downstream of the fast magnetosonic critical point the poloidal streamlines overfocus towards the axis and the solution is terminated. The validity of the model to disc winds associated with young stellar objects is briefly discussed. [source]

Electronic structure of three-dimensional triangular torus-shaped quantum rings under external magnetic fields

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2003
Yiming Li
Abstract In this paper, we calculate the electron,hole energy states and the magnetization for InAs/GaAs triangular torus-shaped (TTS) quantum rings in a magnetic field. Our three-dimensional (3D) model considers (i) the effective one-band Hamiltonian approximation, (ii) the position- and energy-dependent quasi-particle effective mass approximation, (iii) the finite hard wall confinement potential, and (iv) the Ben Daniel-Duke boundary conditions. This model is solved numerically with the nonlinear iterative method to obtain the "self-consistent" solutions. We investigate the electron-hole energy spectra versus magnetic field for two different ring widths: R0 = 20 and 50 nm, and find that they strongly depend on the ring shape and size. Since the magnetic field penetrates into the inside region of the nonsimply connected ring, the electron (hole) transition energy between the lowest states versus magnetic field oscillates nonperiodically and is different from that of quantum dots. We find the magnetization at zero temperature is a negative function, saturates, and oscillates nonperiodically when the magnetic field increases. [source]