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

Graphene nanoring as a tunable source of polarized electrons

We propose a novel spin filter based on a graphene nanoring fabricated above a ferromagnetic strip. The exchange interaction between the magnetic moments of the ions in the ferromagnet and the electron spin splits the electronic states, and gives rise to spin polarization of the conductance and the total electric current. We demonstrate that both the current and its polarization can be controlled by a side-gate voltage. This opens the possibility to use the proposed device as a tunable source of polarized electrons.Comment: 12 pages, 7 figures, accepted in Nanotechnolog

Collective dynamics of colloids at fluid interfaces

The evolution of an initially prepared distribution of micron sized colloidal particles, trapped at a fluid interface and under the action of their mutual capillary attraction, is analyzed by using Brownian dynamics simulations. At a separation \lambda\ given by the capillary length of typically 1 mm, the distance dependence of this attraction exhibits a crossover from a logarithmic decay, formally analogous to two-dimensional gravity, to an exponential decay. We discuss in detail the adaption of a particle-mesh algorithm, as used in cosmological simulations to study structure formation due to gravitational collapse, to the present colloidal problem. These simulations confirm the predictions, as far as available, of a mean-field theory developed previously for this problem. The evolution is monitored by quantitative characteristics which are particularly sensitive to the formation of highly inhomogeneous structures. Upon increasing \lambda\ the dynamics show a smooth transition from the spinodal decomposition expected for a simple fluid with short-ranged attraction to the self-gravitational collapse scenario.Comment: 13 pages, 12 figures, revised, matches version accepted for publication in the European Physical Journal

Transverse depinning and melting of a moving vortex lattice in driven periodic Josephson junction arrays

We study the effect of thermal fluctuations in a vortex lattice driven in the periodic pinning of a Josephson junction array. The phase diagram current ($I$) vs. temperature ($T$) is studied. Above the critical current $I_c(T)$ we find a moving vortex lattice (MVL) with anisotropic Bragg peaks. For large currents $I\gg I_c(T)$, there is a melting transition of the MVL at $T_M(I)$. When applying a small transverse current to the MVL, there is no dissipation at low $T$. We find an onset of transverse vortex motion at a transverse depinning temperature $T_{tr}(I)<T_M(I)$.Comment: 4 pages, 4 figures, Figure 2 changed, added new reference

When are entrepreneurs more environmentally oriented? An analysis of stakeholders' pressures at different stages of evolution of the venture

Entrepreneurs are subjected to increased institutional pressures that encourage them to include environmental issues in their overall business objectives. Despite this, entrepreneurs do not always place the same importance on environmental issues in the overall objectives, but some are more environmentally oriented than others. We contend that these differences are explained by two factors: the stage of evolution of the venture and the intensity of coercive and normative environmental pressures on entrepreneurs. Using a sample of 9781 entrepreneurs from 27 countries, our research shows that entrepreneurs are more environmentally oriented (1) in early stages of evolution, (2) in countries with high coercive pressures, and (3) in countries with high normative pressures. Additionally, our results indicate that the differences in the environmental orientation in the early and late stages are reduced in countries with high normative pressures and that these differences are not influenced by the intensity of coercive pressures

Pyreneplax basaensisnew genus, new species (Decapoda, Brachyura, Vultocinidae) from the Priabonian (Late Eocene) of the Pyrenees of Huesca (Aragón, Spain), and remarks on the genus Lobonotus A. Milne-Edwards, 1863

P. Pyreneplax basaensis nou gènere, nova espècie (Decapoda, Brachyura, Vultocinidae) del Priabonià (Eocè Superior) del Pirineu d’Osca (Aragó, Espanya), i observacions sobre el gènere Lobonotus A. Milne-Edwards, 1863. Els nous braquiürs fòssils trobats a la Formació Margas de Arguís (Priabonià, Eocè superior) dels Pirineus Centrals (Aragó), permeten la descripció d’un nou gènere y una nova espècie ( Pyreneplax basaensis), així com la revisió d’alguns tàxons previament assignats al gènere Lobonotus A. Milne-Edwards, 1863 (Carpilioidea: Tumidocarcinidae). Tenint en compte la gran similitud existent entre Pyreneplax n. gen. i les espècies Lobonotus sandersi(Blow i Manning, 1997), L. granosus(Beschin, Busulini, De Angeli i Tessier, 2002) i L. sommarugai Beschin, Busulini i Tessier, 2009, aquestes són també assignades al nou gènere. A més, un conjunt de caràcters tant dorsals com ventrals, compartits amb la espècie actual Vultocinus anfractusNg i Manuel-Santos, 2007, suggereixen la inclusió del nou gènere dins la família Vultocinidae Ng i Manuel-Santos, 2007 (Goneplacoidea). Al mateix temps es discuteixen alguns aspectes de la posició sistemàtica del gènere Lobonotus A. Milne-Edwards, 1863.New fossil brachyuran finds in the Margas de Arguís Formation (Priabonian, Late Eocene) in the Central Pyrenees of Aragon (Spain), allow the description of a new genus and a new species ( Pyreneplax basaensis), and the revision of some taxa previously assigned to the genus Lobonotus A. Milne-Edwards, 1863 (Carpilioidea: Tumidocarcinidae). According to the close similarity between Pyreneplax n. gen. and Lobonotus sandersi (Blow and Manning, 1997), L. granosus (Beschin, Busulini, De Angeli and Tessier, 2002) and L. sommarugai Beschin, Busulini and Tessier, 2009, these species are here assigned to the new genus. In addition, an important set of dorsal and ventral characters shared with the extant species Vultocinus anfractus Ng and Manuel-Santos, 2007, suggests the inclusion of the new genus in the family Vultocinidae Ng and Manuel-Santos, 2007 (Goneplacoidea). Remarks on the genus Lobonotusare also discussed herein.P. Pyreneplax basaensisnuevo género, nueva especie (Decapoda, Brachyura, Vultocinidae) del Priaboniense (Eoceno Superior) del Pirineo de Huesca (Aragón, España), y observaciones sobre el género Lobonotus A. Milne-Edwards, 1863. Nuevos braquiuros fósiles encontrados en la Formación Margas de Arguís (Priaboniense, Eoceno superior), en los Pirineos Centrales (Aragón), permiten la descripción de un nuevo género y una nueva especie ( Pyreneplax basaensis), así como la revisión de algunos taxones previamente asignados al género Lobonotus A. Milne-Edwards, 1863 (Carpilioidea: Tumidocarcinidae). De acuerdo con la notable similitud entre Pyreneplax n. gen., y las especies Lobonotus sandersi (Blow y Manning, 1997), L. granosus(Beschin, Busulini, De Angeli y Tessier, 2002) y L. sommarugaiBeschin, Busulini y Tessier, 2009, éstas se asignan al nuevo género. Asimismo, un destacado conjunto de caracteres dorsales y ventrales compartidos con la especie actual Vultocinus anfractus Ng y Manuel-Santos, 2007, sugiere la inclusión del nuevo género en la familia Vultocinidae Ng y Manuel-Santos, 2007 (Goneplacoidea). También se efectúan algunas observaciones sobre el género LobonotusA. Milne-Edwards, 1863

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

Anomalous optical absorption in a random system with scale-free disorder

We report on an anomalous behavior of the absorption spectrum in a one-dimensional lattice with long-range-correlated diagonal disorder with a power-like spectrum in the form S(k) ~ 1/k^A. These type of correlations give rise to a phase of extended states at the band center, provided A is larger than a critical value A_c. We show that for A < A_c the absorption spectrum is single-peaked, while an additional peak arises when A > A_c, signalling the occurrence of the Anderson transition. The peak is located slightly below the low-energy mobility edge, providing a unique spectroscopic tool to monitor the latter. We present qualitative arguments explaining this anomaly.Comment: 4 pages, 4 postscript figures, uses revtex

Full capacitance-matrix effects in driven Josephson-junction arrays

We study the dynamic response to external currents of periodic arrays of Josephson junctions, in a resistively capacitively shunted junction (RCSJ) model, including full capacitance-matrix effects}. We define and study three different models of the capacitance matrix $C_{\vec{r},\vec{r}'}$: Model A includes only mutual capacitances; Model B includes mutual and self capacitances, leading to exponential screening of the electrostatic fields; Model C includes a dense matrix $C_{\vec{r},\vec{r}'}$ that is constructed approximately from superposition of an exact analytic solution for the capacitance between two disks of finite radius and thickness. In the latter case the electrostatic fields decay algebraically. For comparison, we have also evaluated the full capacitance matrix using the MIT fastcap algorithm, good for small lattices, as well as a corresponding continuum effective-medium analytic evaluation of a finite voltage disk inside a zero-potential plane. In all cases the effective $C_{\vec{r},\vec{r}'}$ decays algebraically with distance, with different powers. We have then calculated current voltage characteristics for DC+AC currents for all models. We find that there are novel giant capacitive fractional steps in the I-V's for Models B and C, strongly dependent on the amount of screening involved. We find that these fractional steps are quantized in units inversely proportional to the lattice sizes and depend on the properties of $C_{\vec{r},\vec{r}'}$. We also show that the capacitive steps are not related to vortex oscillations but to localized screened phase-locking of a few rows in the lattice. The possible experimental relevance of these results is also discussed.Comment: 12 pages 18 Postscript figures, REVTEX style. Paper to appear in July 1, Vol. 58, Phys. Rev. B 1998 All PS figures include

Exact-Diagonalization Studies of Inelastic Light Scattering in Self-Assembled Quantum Dots

We report exact diagonalization studies of inelastic light scattering in few-electron quantum dots under the strong confinement regime characteristic of self-assembled dots. We apply the orthodox (second-order) theory for scattering due to electronic excitations, leaving for the future the consideration of higher-order effects in the formalism (phonons, for example), which seem relevant in the theoretical description of available experiments. Our numerical results stress the dominance of monopole peaks in Raman spectra and the breakdown of selection rules in open-shell dots. The dependence of these spectra on the number of electrons in the dot and the incident photon energy is explicitly shown. Qualitative comparisons are made with recent experimental results.Comment: 11 pages, 11 figure