23,197 research outputs found
Excitation Induced Dephasing in Semiconductor Quantum Dots
A quantum kinetic theory is used to compute excitation induced dephasing in
semiconductor quantum dots due to the Coulomb interaction with a continuum of
states, such as a quantum well or a wetting layer. It is shown that a frequency
dependent broadening together with nonlinear resonance shifts are needed for a
microscopic explanation of the excitation induced dephasing in such a system,
and that excitation induced dephasing for a quantum-dot excitonic resonance is
different from quantum-well and bulk excitons.Comment: 6 pages, 4 figures. Extensively revised text, two figures change
The Hierarchical - Trajectory by Perturbation Theory in a Running Coupling and its Logarithm
We compute the hierarchical -trajectory in terms of perturbation
theory in a running coupling. In the three dimensional case we resolve a
singularity due to resonance of power counting factors in terms of logarithms
of the running coupling. Numerical data is presented and the limits of validity
explored. We also compute moving eigenvalues and eigenvectors on the trajectory
as well as their fusion rules.Comment: 24 pages, 9 pictures included, uuencoded compressed postscript fil
Influence of Anomalous Dispersion on Optical Characteristics of Quantum Wells
Frequency dependencies of optical characteristics (reflection, transmission
and absorption of light) of a quantum well are investigated in a vicinity of
interband resonant transitions in a case of two closely located excited energy
levels. A wide quantum well in a quantizing magnetic field directed normally to
the quantum-well plane, and monochromatic stimulating light are considered.
Distinctions between refraction coefficients of barriers and quantum well, and
a spatial dispersion of the light wave are taken into account. It is shown that
at large radiative lifetimes of excited states in comparison with nonradiative
lifetimes, the frequency dependence of the light reflection coefficient in the
vicinity of resonant interband transitions is defined basically by a curve,
similar to the curve of the anomalous dispersion of the refraction coefficient.
The contribution of this curve weakens at alignment of radiative and
nonradiative times, it is practically imperceptible at opposite ratio of
lifetimes . It is shown also that the frequency dependencies similar to the
anomalous dispersion do not arise in transmission and absorption coefficients.Comment: 10 pages, 6 figure
Two Sets of Simple Formulae to Estimating Fractal Dimension of Irregular Boundaries
Irregular boundary lines can be characterized by fractal dimension, which
provides important information for spatial analysis of complex geographical
phenomena such as cities. However, it is difficult to calculate fractal
dimension of boundaries systematically when image data is limited. An
approximation estimation formulae of boundary dimension based on square is
widely applied in urban and ecological studies. However, the boundary dimension
is sometimes overestimated. This paper is devoted to developing a series of
practicable formulae for boundary dimension estimation using ideas from
fractals. A number of regular figures are employed as reference shapes, from
which the corresponding geometric measure relations are constructed; from these
measure relations, two sets of fractal dimension estimation formulae are
derived for describing fractal-like boundaries. Correspondingly, a group of
shape indexes can be defined. A finding is that different formulae have
different merits and spheres of application, and the second set of boundary
dimensions is a function of the shape indexes. Under condition of data
shortage, these formulae can be utilized to estimate boundary dimension values
rapidly. Moreover, the relationships between boundary dimension and shape
indexes are instructive to understand the association and differences between
characteristic scales and scaling. The formulae may be useful for the
pre-fractal studies in geography, geomorphology, ecology, landscape science,
and especially, urban science.Comment: 28 pages, 2 figures, 9 table
Closed Contour Fractal Dimension Estimation by the Fourier Transform
This work proposes a novel technique for the numerical calculus of the
fractal dimension of fractal objects which can be represented as a closed
contour. The proposed method maps the fractal contour onto a complex signal and
calculates its fractal dimension using the Fourier transform. The Fourier power
spectrum is obtained and an exponential relation is verified between the power
and the frequency. From the parameter (exponent) of the relation, it is
obtained the fractal dimension. The method is compared to other classical
fractal dimension estimation methods in the literature, e. g.,
Bouligand-Minkowski, box-couting and classical Fourier. The comparison is
achieved by the calculus of the fractal dimension of fractal contours whose
dimensions are well-known analytically. The results showed the high precision
and robustness of the proposed technique
- …