57 research outputs found
Photoexcited transients in disordered semiconductors: Quantum coherence at very short to intermediate times
We study theoretically electron transients in semiconductor alloys excited by
light pulses shorter than 100 femtoseconds and tuned above the absorption edge
during and shortly after the pulse, when disorder scattering is dominant.
We use non-equilibrium Green functions employing the field-dependent
self-consistent Born approximation. The propagators and the particle
correlation function are obtained by a direct numerical solution of the Dyson
equations in differential form. For the purely elastic scattering in our model
system the solution procedures for the retarded propagator and for the
correlation function can be decoupled.The propagator is used as an input in
calculating the correlation function. Numerical results combined with a
cumulant expansion permit to separate in a consistent fashion the dark and the
induced parts of the self-energy. The dark behavior reduces to propagation of
strongly damped quasi-particles; the field induced self-energy leads to an
additional time non-local coherence. The particle correlation function is
formed by a coherent transient and an incoherent back-scattered component. The
particle number is conserved only if the field induced coherence is fully
incorporated. The transient polarization and the energy balance are also
obtained and interpreted.Comment: Accepted for publication in Phys. Rev. B; 37 pages,17 figure
Internal Accounting Rules for Small and Medium Enterprises
Hlavním cílem této bakalářské práce je vytvoření vnitropodnikových účetních směrnic pro podnik, který dosud žádné směrnice vytvořeny nemá. Práce je zaměřena na aplikování teoretických poznatků do praxe. V teoretické části se zabývám analýzou vnitropodnikových směrnic a legislativou. Zmiňuji zásady pro vypracování směrnic včetně podstatných náležitostí. V praktické části se zabývám konkrétní tvorbou stěžejních vnitropodnikových směrnic pro daný podnik.The main focus of my Bachelor Thesis is to make internal accounting guidelines for a business where there have not been guidelines created yet. My work focuses on application of theoretical knowledge into an old every day practice. In the theoretical part of my work I focus on the analysis of the internal guidelines and the legislation. I mention the principles for the development of guidelines, including the essential requirements. In the practical part of my work I focus on the specific crucial production of the internal business guidelines for given business
Generalized gradient expansions in quantum transport equations
Gradient expansions in quantum transport equations of a Kadanoff-Baym form
have been reexamined. We have realized that in a consistent approach the
expansion should be performed also inside of the self-energy in the scattering
integrals of these equations. In the first perturbation order this internal
expansion gives new correction terms to the generalized Boltzman equation.
These correction terms are found here for several typical systems. Possible
corrections to the theory of a linear response to weak electric fields are also
discussed.Comment: 20 pages, latex, to appear in Journal of Statistical Physics, March
(1997
Relationship between the expansion of drylands and the intensification of Hadley circulation during the late twentieth century
The changes in coverage by arid climate and intensity of the Hadley circulation during the second half of the twentieth century were examined using observations and the multi-model ensemble (MME) mean of Twentieth-Century Coupled Climate Model (20C3M) simulations. It was found that the area of dry climate, which comprises steppe and desert climates following the Köppen climate classification, expanded to an appreciable extent in observation and, to a lesser degree, in MME simulation. The areal extent of steppe climate (the outer boundary of arid climate) tends to encroach on the surrounding climate groups, which, in turn, feeds desert climate (the inner part of arid climate) and causes it to grow. This result indicates the importance of accurate prediction for climate regimes that border steppe climate. Concomitant with the expansion of drylands, the observed intensity of the Hadley cell is persistently enhanced, particularly during boreal winter, suggesting the validity of a self-induction of deserts through a positive biogeophysical feedback (also known as Charney’s cycle). In comparison, the simulated Hadley circulation in the MME mean remains invariant in time. The current climate models, therefore, disagree with the observation in the long-term linkage between desertification and Hadley cell. Finally, the implication of such discrepancy is discussed as a possible guidance to improve models
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