5,646 research outputs found
Influence of static disorder and polaronic band formation on interfacial electron transfer in organic photovoltaic devices
Understanding the interfacial charge-separation mechanism in organic
photovoltaics requires, due to its high level of complexity, bridging between
chemistry and physics. To elucidate the charge separation mechanism, we present
a fully quantum dynamical simulation of a generic one-dimensional Hamiltonian,
which physical parameters model prototypical PCBM or acceptor
systems. We then provide microscopic evidence of the influence random static
and dynamic potentials have on the interfacial charge-injection rate. In
particular, we unveil that dynamic potentials, due to strong electron-vibration
interactions, can lead to the formation of polaronic bands. Such dynamical
potentials, when compared to random static potentials, can provide the main
detrimental influence on the efficiency of the process of interfacial
charge-separation
Aspects of Discrete Breathers and New Directions
We describe results concerning the existence proofs of Discrete Breathers
(DBs) in the two classes of dynamical systems with optical linear phonons and
with acoustic linear phonons. A standard approach is by continuation of DBs
from an anticontinuous limit. A new approach, which is purely variational, is
presented. We also review some numerical results on intraband DBs in random
nonlinear systems. Some non-conventional physical applications of DBs are
suggested. One of them is understanding slow relaxation properties of glassy
materials. Another one concerns energy focusing and transport in biomolecules
by targeted energy transfer of DBs. A similar theory could be used for
describing targeted charge transfer of nonlinear electrons (polarons) and, more
generally, for targeted transfer of several excitations (e.g. Davydov soliton).Comment: to appear in the Proceedings of NATO Advanced Research Workshop
"Nonlinearity and Disorder: Theory and Applications",
Tashkent,Uzbekistan,October 1-6, 200
Harmonic Solid Theory of Photoluminescence in the High Field Two-Dimensional Wigner Crystal
Motivated by recent experiments on radiative recombination of two-dimensional
electrons in acceptor doped GaAs-AlGaAs heterojunctions as well as the success
of a harmonic solid model in describing tunneling between two-dimensional
electron systems, we calculate within the harmonic approximation and the time
dependent perturbation theory the line shape of the photoluminescence spectrum
corresponding to the recombination of an electron with a hole bound to an
acceptor atom. The recombination process is modeled as a sudden perturbation of
the Hamiltonian for the in-plane degrees of freedom of the electron. We include
in the perturbation, in addition to changes in the equilibrium positions of
electrons, changes in the curvatures of the harmonically approximated
potential. The computed spectra have line shapes similar to that seen in a
recent experiment. The spectral width, however, is roughly a factor of 3
smaller than that seen in experiment if one assumes a perfect Wigner crystal
for the initial state state of the system, whereas a simple random disorder
model yields a width a factor of 3 too large. We speculate on the possible
mechanisms that may lead to better quantitative agreement with experiment.Comment: 22 pages, RevTex, 8 figures. Submitted to the Physical Review
Tight-binding approximation for semi-infinite solids. Application of a transform method and of delta function normalization
A transform method for treating semi‐infinite solids in the tight‐binding (TB) approximation is introduced. The difference equations for the TB orbital coefficients are converted, thereby, to convenient algebraic equations. For this purpose, a Dirac delta function normalization for the wave function is also introduced, instead of the usual box one. Single and coupled bands are treated, and the methods are applied elsewhere to electron transfer problems at interfaces
London force and energy transportation between interfacial surfaces
With appropriately selected optical frequencies, pulses of radiation propagating through a system of chemically distinct and organized components can produce areas of spatially selective excitation. This paper focuses on a system in which there are two absorptive components, each one represented by surface adsorbates arrayed on a pair of juxtaposed interfaces. The adsorbates are chosen to be chemically distinct from the material of the underlying surface. On promotion of any adsorbate molecule to an electronic excited state, its local electronic environment is duly modified, and its London interaction with nearest neighbor molecules becomes accommodated to the new potential energy landscape. If the absorbed energy then transfers to a neighboring adsorbate of another species, so that the latter acquires the excitation, the local electronic environment changes and compensating motion can be expected to occur. Physically, this is achieved through a mechanism of photon absorption and emission by molecular pairs, and by the engagement of resonance transfer of energy between them. This paper presents a detailed analysis of the possibility of optically effecting such modifications to the London force between neutral adsorbates, based on quantum electrodynamics (QED). Thus, a precise link is established between the transfer of excitation and ensuing mechanical effects
On the Complexity of the Word Problem for Automaton Semigroups and Automaton Groups
In this paper, we study the word problem for automaton semigroups and
automaton groups from a complexity point of view. As an intermediate concept
between automaton semigroups and automaton groups, we introduce
automaton-inverse semigroups, which are generated by partial, yet invertible
automata. We show that there is an automaton-inverse semigroup and, thus, an
automaton semigroup with a PSPACE-complete word problem. We also show that
there is an automaton group for which the word problem with a single rational
constraint is PSPACE-complete. Additionally, we provide simpler constructions
for the uniform word problems of these classes. For the uniform word problem
for automaton groups (without rational constraints), we show NL-hardness.
Finally, we investigate a question asked by Cain about a better upper bound for
the length of a word on which two distinct elements of an automaton semigroup
must act differently
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