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 $D$ and the dc-conductivity $\sigma_{dc} (T)$ of strongly correlated electrons are investigated theoretically. Analytic results are derived for the homogeneous phase of the Hubbard model in $d = \infty$ dimensions, and for spinless fermions in this limit with $1/d$-corrections systematically included to lowest order. It is found that $\sigma_{dc}(T)$ is finite for all $T > 0$, displaying Fermi liquid behavior, $\sigma_{dc} \propto 1/T^2$, 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 $T > 0$ is argued to be a generic feature of correlated lattice electrons in not too low dimensions.Comment: 15 pages, uuencoded compressed PS-fil