290 research outputs found
Localization properties of one-dimensional Frenkel excitons: Gaussian versus Lorentzian diagonal disorder
We compare localization properties of one-dimensional Frenkel excitons with
Gaussian and Lorentzian uncorrelated diagonal disorder. We focus on the states
of the Lifshits tail, which dominate the optical response and low-temperature
energy transport in molecular J-aggregates. The absence of exchange narrowing
in chains with Lorentzian disorder is shown to manifest itself in the disorder
scaling of the localization length distribution. Also, we show that the local
exciton level structure of the Lifshits tail differs substantially for these
two types of disorder: In addition to the singlets and doublets of localized
states near the bare band edge, strongly resembling those found for Gaussian
disorder, for Lorentzian disorder two other types of states are found in this
energy region as well, namely multiplets of three or four states localized on
the same chain segment and isolated states localized on short segments.
Finally, below the Lifshits tail, Lorentzian disorder induces strongly
localized exciton states, centered around low energy sites, with localization
properties that strongly depend on energy. For Gaussian disorder with a
magnitude that does not exceed the exciton bandwidth, the likelihood to find
such very deep states is exponentially small.Comment: 9 two-column pages, 4 figures, to appear in Phys. Rev.
Excitons in Molecular Aggregates with L\'evy Disorder: Anomalous Localization and Exchange Broadening of Optical Spectra
We predict the existence of exchange broadening of optical lineshapes in
disordered molecular aggregates and a nonuniversal disorder scaling of the
localization characteristics of the collective electronic excitations
(excitons). These phenomena occur for heavy-tailed L\'evy disorder
distributions with divergent second moments - distributions that play a role in
many branches of physics. Our results sharply contrast with aggregate models
commonly analyzed, where the second moment is finite. They bear a relevance for
other types of collective excitations as well
Response to the Comment on "Excitons in Molecular Aggregates with L\'evy Disorder: Anomalous Localization and Exchange Broadening of Optical Spectra"
In previous work, we have predicted novel effects, such as exchange
broadening, anomalous scaling of the localization length and a blue shift of
the absorption spectrum with increasing disorder strength, for static disorder
models described by stable distributions with stability index {\alpha}<1. The
main points of the Comment are that the outliers introduced by heavy tails in
the disorder distribution (i) do not lead to deviations from the conventional
scaling law for the half width at half maximum (HWHM) of the absorption
spectrum and (ii) do not lead to non-universality of the distribution of
localization lengths. We show below that the findings reported by us in the
Letter are correct and that the wrong conclusions of the Comment arise from
focusing on small {\sigma} values.Comment: Based on our response submitted to Physical Review Letters on January
20, 2012. We now also take into account the modifications made to the Comment
upon resubmission of the Comment. The Reply has been accepted in Physical
Review Letter
Nonmonotonic energy harvesting efficiency in biased exciton chains
We theoretically study the efficiency of energy harvesting in linear exciton
chains with an energy bias, where the initial excitation is taking place at the
high-energy end of the chain and the energy is harvested (trapped) at the other
end. The efficiency is characterized by means of the average time for the
exciton to be trapped after the initial excitation. The exciton transport is
treated as the intraband energy relaxation over the states obtained by
numerically diagonalizing the Frenkel Hamiltonian that corresponds to the
biased chain. The relevant intraband scattering rates are obtained from a
linear exciton-phonon interaction. Numerical solution of the Pauli master
equation that describes the relaxation and trapping processes, reveals a
complicated interplay of factors that determine the overall harvesting
efficiency. Specifically, if the trapping step is slower than or comparable to
the intraband relaxation, this efficiency shows a nonmonotonic dependence on
the bias: it first increases when introducing a bias, reaches a maximum at an
optimal bias value, and then decreases again because of dynamic (Bloch)
localization of the exciton states. Effects of on-site (diagonal) disorder,
leading to Anderson localization, are addressed as well.Comment: 9 pages, 6 figures, to appear in Journal of Chemical Physic
Signature of Anomalous Exciton Localization in the Optical Response of Self-Assembled Organic Nanotubes
We show that the disorder scaling of the low-temperature optical absorption linewidth of tubular molecular assemblies sharply contrasts with that known for one-dimensional aggregates. The difference can be explained by an anomalous localization of excitons, which arises from the combination of long-range intermolecular interactions and the tube's higher-dimensional geometry. As a result, the exciton density of states near the band bottom drops to zero, leading to a strong suppression of exciton localization. Our results explain the strong linear dichroism and weak exciton-exciton scattering in tubular J aggregates observed in experiments and suggest that for nanoscale wirelike applications a tubular shape is to be preferred over a truly one-dimensional chain
Magnetic Correlations in the Two Dimensional Anderson-Hubbard Model
The two dimensional Hubbard model in the presence of diagonal and
off-diagonal disorder is studied at half filling with a finite temperature
quantum Monte Carlo method. Magnetic correlations as well as the electronic
compressibility are calculated to determine the behavior of local magnetic
moments, the stability of antiferromagnetic long range order (AFLRO), and
properties of the disordered phase. The existence of random potentials
(diagonal or ``site'' disorder) leads to a suppression of local magnetic
moments which eventually destroys AFLRO. Randomness in the hopping elements
(off-diagonal disorder), on the other hand, does not significantly reduce the
density of local magnetic moments. For this type of disorder, at half-filling,
there is no ``sign-problem'' in the simulations as long as the hopping is
restricted between neighbor sites on a bipartite lattice. This allows the study
of sufficiently large lattices and low temperatures to perform a finite-size
scaling analysis. For off-diagonal disorder AFLRO is eventually destroyed when
the fluctuations of antiferromagnetic exchange couplings exceed a critical
value. The disordered phase close to the transition appears to be
incompressible and shows an increase of the uniform susceptibility at low
temperatures.Comment: 10 pages, REVTeX, 14 figures included using psfig.st
"Optical conductance fluctuations: diagrammatic analysis in Landauer approach and non-universal effects"
The optical conductance of a multiple scattering medium is the total
transmitted light of a diffuse incoming beam. This quantity, very analogous to
the electronic conductance, exhibits universal conductance fluctuations. We
perform a detailed diagrammatic analysis of these fluctuations. With a
Kadanoff-Baym technique all the leading diagrams are systematically generated.
A cancellation of the short distance divergencies occurs, that yields a well
behaved theory. The analytical form of the fluctuations is calculated and
applied to optical systems. Absorption and internal reflections reduce the
fluctuations significantly.Comment: 25 pages Revtex 3.0, 18 seperate postscript figure
Dynamical Mean-Field Solution for a Model of Metal-Insulator Transitions in Moderately Doped Manganites
We propose that a specific spatial configuration of lattice sites that
energetically favor {\it 3+} or {\it 4+} Mn ions in moderately doped manganites
constitutes approximately a spatially random two-energy-level system. Such an
effect results in a mechanism of metal-insulator transitions that appears to be
different from both the Anderson transition and the Mott-Hubbard transition.
Correspondingly, a disordered Kondo lattice model is put forward, whose
dynamical mean-field solution agrees reasonably with experiments.Comment: 4 pages, 2 eps figures, Revtex. First submitted to PRL on May 16,
199
Drude weight and dc-conductivity of correlated electrons
The Drude weight and the dc-conductivity of strongly
correlated electrons are investigated theoretically. Analytic results are
derived for the homogeneous phase of the Hubbard model in
dimensions, and for spinless fermions in this limit with -corrections
systematically included to lowest order. It is found that is
finite for all , displaying Fermi liquid behavior, , at low temperatures. The validity of this result for finite dimensions
is examined by investigating the importance of Umklapp scattering processes and
vertex corrections. A finite dc-conductivity for is argued to be a
generic feature of correlated lattice electrons in not too low dimensions.Comment: 15 pages, uuencoded compressed PS-fil
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