499 research outputs found
Frenkel Excitons in Random Systems With Correlated Gaussian Disorder
Optical absorption spectra of Frenkel excitons in random one-dimensional
systems are presented. Two models of inhomogeneous broadening, arising from a
Gaussian distribution of on-site energies, are considered. In one case the
on-site energies are uncorrelated variables whereas in the second model the
on-site energies are pairwise correlated (dimers). We observe a red shift and a
broadening of the absorption line on increasing the width of the Gaussian
distribution. In the two cases we find that the shift is the same, within our
numerical accuracy, whereas the broadening is larger when dimers are
introduced. The increase of the width of the Gaussian distribution leads to
larger differences between uncorrelated and correlated disordered models. We
suggest that this higher broadening is due to stronger scattering effects from
dimers.Comment: 9 pages, REVTeX 3.0, 3 ps figures. To appear in Physical Review
FIBONACCI SUPERLATTICES OF NARROW-GAP III-V SEMICONDUCTORS
We report theoretical electronic structure of Fibonacci superlattices of
narrow-gap III-V semiconductors. Electron dynamics is accurately described
within the envelope-function approximation in a two-band model.
Quasiperiodicity is introduced by considering two different III-V semiconductor
layers and arranging them according to the Fibonacci series along the growth
direction. The resulting energy spectrum is then found by solving exactly the
corresponding effective-mass (Dirac-like) wave equation using tranfer-matrix
techniques. We find that a self-similar electronic spectrum can be seen in the
band structure. Electronic transport properties of samples are also studied and
related to the degree of spatial localization of electronic envelope-functions
via Landauer resistance and Lyapunov coefficient. As a working example, we
consider type II InAs/GaSb superlattices and discuss in detail our results in
this system.Comment: REVTeX 3.0, 16 pages, 8 figures available upon request. To appear in
Semiconductor Science and Technolog
Fluorescence decay in aperiodic Frenkel lattices
We study motion and capture of excitons in self-similar linear systems in
which interstitial traps are arranged according to an aperiodic sequence,
focusing our attention on Fibonacci and Thue-Morse systems as canonical
examples. The decay of the fluorescence intensity following a broadband pulse
excitation is evaluated by solving the microscopic equations of motion of the
Frenkel exciton problem. We find that the average decay is exponential and
depends only on the concentration of traps and the trapping rate. In addition,
we observe small-amplitude oscillations coming from the coupling between the
low-lying mode and a few high-lying modes through the topology of the lattice.
These oscillations are characteristic of each particular arrangement of traps
and they are directly related to the Fourier transform of the underlying
lattice. Our predictions can be then used to determine experimentally the
ordering of traps.Comment: REVTeX 3.0 + 3PostScript Figures + epsf.sty (uuencoded). To appear in
Physical Review
Experimental evidence of delocalized states in random dimer superlattices
We study the electronic properties of GaAs-AlGaAs superlattices with
intentional correlated disorder by means of photoluminescence and vertical dc
resistance. The results are compared to those obtained in ordered and
uncorrelated disordered superlattices. We report the first experimental
evidence that spatial correlations inhibit localization of states in disordered
low-dimensional systems, as our previous theoretical calculations suggested, in
contrast to the earlier belief that all eigenstates are localized.Comment: 4 pages, 5 figures. Physical Review Letters (in press
Exciton Optical Absorption in Self-Similar Aperiodic Lattices
Exciton optical absorption in self-similar aperiodic one-dimensional systems
is considered, focusing our attention on Thue-Morse and Fibonacci lattices as
canonical examples. The absorption line shape is evaluated by solving the
microscopic equations of motion of the Frenkel-exciton problem on the lattice,
in which on-site energies take on two values, according to the Thue-Morse or
Fibonacci sequences. Results are compared to those obtained in random lattices
with the same stechiometry and size. We find that aperiodic order causes the
occurrence of well-defined characteristic features in the absorption spectra
which clearly differ from the case of random systems, indicating a most
peculiar exciton dynamics. We successfully explain the obtained spectra in
terms of the two-center problem. This allows us to establish the origin of all
the absorption lines by considering the self-similar aperiodic lattices as
composed of two-center blocks, within the same spirit of the renormalization
group ideas.Comment: 16 pages in REVTeX 3.0. 2 figures on request to F. D-A
([email protected]
Education to gain sustainability : analysis and approach from the experience obtained from two high school systems in mexican rural communities
The decade “Education for Sustainability” is just emerging and one of its goals emphasized the importance of considering the currícula in a transdisciplinary and community based programs. This includes recognizing local and regional interests as relevant topics into the currícula. “Education for sustainability” does not appear to have a strong basis on their actions as its theoretical discourse does. The study we present here about content analysis in the curricula of two high schools systems in rural Mexican communities is an example. In this research we analyzed: 1) how the curricula is oriented towards forest management and the way is connected to the social reality of the communities; 2) how does the learning process develops in the classroom and its dynamics with teachers and students and 3) how does the environmental learning take place. Results revealed that more research is needed with adolescents in order to change the educational structure in rural Mexican high schools.<br /
Three-dimensional effects on extended states in disordered models of polymers
We study electronic transport properties of disordered polymers in the
presence of both uncorrelated and short-range correlated impurities. In our
procedure, the actual physical potential acting upon the electrons is replaced
by a set of nonlocal separable potentials, leading to a Schr\"odinger equation
that is exactly solvable in the momentum representation. We then show that the
reflection coefficient of a pair of impurities placed at neighboring sites
(dimer defect) vanishes for a particular resonant energy. When there is a
finite number of such defects randomly distributed over the whole lattice, we
find that the transmission coefficient is almost unity for states close to the
resonant energy, and that those states present a very large localization
length. Multifractal analysis techniques applied to very long systems
demonstrate that these states are truly extended in the thermodynamic limit.
These results reinforce the possibility to verify experimentally theoretical
predictions about absence of localization in quasi-one-dimensional disordered
systems.Comment: 16 pages, REVTeX 3.0, 5 figures on request from FDA
([email protected]). Submitted to Phys. Rev. B. MA/UC3M/09/9
Photonic realization of the relativistic Kronig-Penney model and relativistic Tamm surface states
Photonic analogues of the relativistic Kronig-Penney model and of
relativistic surface Tamm states are proposed for light propagation in fibre
Bragg gratings (FBGs) with phase defects. A periodic sequence of phase slips in
the FBG realizes the relativistic Kronig-Penney model, the band structure of
which being mapped into the spectral response of the FBG. For the semi-infinite
FBG Tamm surface states can appear and can be visualized as narrow resonance
peaks in the transmission spectrum of the grating
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Early neuronal accumulation of DNA double strand breaks in Alzheimer's disease.
The maintenance of genomic integrity is essential for normal cellular functions. However, it is difficult to maintain over a lifetime in postmitotic cells such as neurons, in which DNA damage increases with age and is exacerbated by multiple neurological disorders, including Alzheimer's disease (AD). Here we used immunohistochemical staining to detect DNA double strand breaks (DSBs), the most severe form of DNA damage, in postmortem brain tissues from patients with mild cognitive impairment (MCI) or AD and from cognitively unimpaired controls. Immunostaining for γH2AX-a post-translational histone modification that is widely used as a marker of DSBs-revealed increased proportions of γH2AX-labeled neurons and astrocytes in the hippocampus and frontal cortex of MCI and AD patients, as compared to age-matched controls. In contrast to the focal pattern associated with DSBs, some neurons and glia in humans and mice showed diffuse pan-nuclear patterns of γH2AX immunoreactivity. In mouse brains and primary neuronal cultures, such pan-nuclear γH2AX labeling could be elicited by increasing neuronal activity. To assess whether pan-nuclear γH2AX represents DSBs, we used a recently developed technology, DNA damage in situ ligation followed by proximity ligation assay, to detect close associations between γH2AX sites and free DSB ends. This assay revealed no evidence of DSBs in neurons or astrocytes with prominent pan-nuclear γH2AX labeling. These findings suggest that focal, but not pan-nuclear, increases in γH2AX immunoreactivity are associated with DSBs in brain tissue and that these distinct patterns of γH2AX formation may have different causes and consequences. We conclude that AD is associated with an accumulation of DSBs in vulnerable neuronal and glial cell populations from early stages onward. Because of the severe adverse effects this type of DNA damage can have on gene expression, chromatin stability and cellular functions, DSBs could be an important causal driver of neurodegeneration and cognitive decline in this disease
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