275 research outputs found
Dynamical phenomena in Fibonacci Semiconductor Superlattices
We present a detailed study of the dynamics of electronic wavepackets in
Fibonacci semiconductor superlattices, both in flat band conditions and subject
to homogeneous electric fields perpendicular to the layers. Coherent
propagation of electrons is described by means of a scalar Hamiltonian using
the effective-mass approximation. We have found that an initial Gaussian
wavepacket is filtered selectively when passing through the superlattice. This
means that only those components of the wavepacket whose wavenumber belong to
allowed subminibands of the fractal-like energy spectrum can propagate over the
entire superlattice. The Fourier pattern of the transmitted part of the
wavepacket presents clear evidences of fractality reproducing those of the
underlying energy spectrum. This phenomenon persists even in the presence of
unintentional disorder due to growth imperfections. Finally, we have
demonstrated that periodic coherent-field induced oscillations (Bloch
oscillations), which we are able to observe in our simulations of periodic
superlattices, are replaced in Fibonacci superlattices by more complex
oscillations displaying quasiperiodic signatures, thus sheding more light onto
the very peculiar nature of the electronic states in these systems.Comment: 7 pagex, RevTex, 5 Postscript figures. Physical Review B (in press
Physical nature of critical wave functions in Fibonacci systems
We report on a new class of critical states in the energy spectrum of general
Fibonacci systems. By introducing a transfer matrix renormalization technique,
we prove that the charge distribution of these states spreads over the whole
system, showing transport properties characteristic of electronic extended
states. Our analytical method is a first step to find out the link between the
spatial structure of these critical wave functions and the quasiperiodic order
of the underlying lattice.Comment: REVTEX 3.0, 11 pages, 2 figures available upon request. To appear in
Phys. Rev. Let
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
Riemann solvers and undercompressive shocks of convex FPU chains
We consider FPU-type atomic chains with general convex potentials. The naive
continuum limit in the hyperbolic space-time scaling is the p-system of mass
and momentum conservation. We systematically compare Riemann solutions to the
p-system with numerical solutions to discrete Riemann problems in FPU chains,
and argue that the latter can be described by modified p-system Riemann
solvers. We allow the flux to have a turning point, and observe a third type of
elementary wave (conservative shocks) in the atomistic simulations. These waves
are heteroclinic travelling waves and correspond to non-classical,
undercompressive shocks of the p-system. We analyse such shocks for fluxes with
one or more turning points.
Depending on the convexity properties of the flux we propose FPU-Riemann
solvers. Our numerical simulations confirm that Lax-shocks are replaced by so
called dispersive shocks. For convex-concave flux we provide numerical evidence
that convex FPU chains follow the p-system in generating conservative shocks
that are supersonic. For concave-convex flux, however, the conservative shocks
of the p-system are subsonic and do not appear in FPU-Riemann solutions
Estudio experimental de perfiles de temperatura en elementos de fábrica cerámica sometidos a altas temperaturas
This article discusses heat transfer through a brick element in order to know the thermal behavior of onedimensional brickwork masonry samples exposed to high temperatures.
The object of the tests is to build time-temperature curves according to different thermal steps in transient to experimentally determine the temperature profiles in the interior of a wall.
Through this study, it is possible to demonstrate absolute moisture of a factory item from 300 °C (variation of temperatures in the interior of the element), avoid the associated phenomenon of evaporation of water during the thermal process as well as to obtain profiles of temperatures that help calculate the cross section of a factory element subjected to high temperatures.En este artículo se analiza la transferencia de calor a través de un elemento de fábrica de ladrillo con el fin de conocer el comportamiento térmico de secciones de fábrica unidimensionales expuestas a altas temperaturas.
El objeto de los ensayos es construir curvas tiempo-temperatura en función de diversos escalones térmicos en régimen transitorio para determinar experimentalmente los perfiles de temperatura en el interior de un muro.
A través de este estudio es posible evidenciar el contenido de humedad absoluta de un elemento de fábrica a partir de los 300 ºC (variación de las temperaturas en el interior del elemento), evitar el fenómeno asociado de la evaporación del agua durante el proceso térmico así como obtener perfiles de temperaturas que ayuden a calcular la sección eficaz de un elemento de fábrica sometido a altas temperaturas
Recent Advances in the Catalytic Enantioselective Reformatsky Reaction
This paper reviews the present state of the catalytic enantioselective Reformatsky reaction. Advancements in asymmetric versions of this reaction have recently led to a considerable extension of its scope and applicability, principally due to the use of highly active chiral ligands and very specific reaction conditions.M. A. F. I. is grateful for financial support from the Ministerio de Economía y Competitividad (MINECO) (CTQ 2012-35790) and the Consejería de Educación de la Comunidad de Madrid (programme AVANCAT, S2009/PPQ-1634). B. M. thanks the European Commission for a Marie Curie Integration Grant (FP7-PEOPLE-2012-CIG). I. M. P. and D. A. A. are grateful for financial support from the Spanish Ministerio de Ciencia e Innovación (MICINN) (project numbers CTQ2011-24165 and CTQ2010-20387, respectively) and from the University of Alicante
Energy spectra of quasiperiodic systems via information entropy
We study the relationship between the electronic spectrum structure and the
configurational order of one-dimensional quasiperiodic systems. We take the
Fibonacci case as an specific example, but the ideas outlined here may be
useful to accurately describe the energy spectra of general quasiperiodic
systems of technological interest. Our main result concerns the {\em
minimization} of the information entropy as a characteristic feature associated
to quasiperiodic arrangements. This feature is shown to be related to the
ability of quasiperiodic systems to encode more information, in the Shannon
sense, than periodic ones. In the conclusion we comment on interesting
implications of these results on further developments on the issue of
quasiperiodic order.Comment: REVTeX 3.0, 8 pages, 3 figures available on request from FD-A
([email protected]), Phys Rev E submitted, MA/UC3M/02/9
Optimizing the thermoelectric efficiency of icosahedral quasicrystals and related complex alloys
Trace and antitrace maps for aperiodic sequences, their extensions and applications
We study aperiodic systems based on substitution rules by means of a
transfer-matrix approach. In addition to the well-known trace map, we
investigate the so-called `antitrace' map, which is the corresponding map for
the difference of the off-diagonal elements of the 2x2 transfer matrix. The
antitrace maps are obtained for various binary, ternary and quaternary
aperiodic sequences, such as the Fibonacci, Thue-Morse, period-doubling,
Rudin-Shapiro sequences, and certain generalizations. For arbitrary
substitution rules, we show that not only trace maps, but also antitrace maps
exist. The dimension of the our antitrace map is r(r+1)/2, where r denotes the
number of basic letters in the aperiodic sequence. Analogous maps for specific
matrix elements of the transfer matrix can also be constructed, but the maps
for the off-diagonal elements and for the difference of the diagonal elements
coincide with the antitrace map. Thus, from the trace and antitrace map, we can
determine any physical quantity related to the global transfer matrix of the
system. As examples, we employ these dynamical maps to compute the transmission
coefficients for optical multilayers, harmonic chains, and electronic systems.Comment: 13 pages, REVTeX, now also includes applications to electronic
systems, some references adde
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