Roughness-induced energetic disorder at the metal/organic interface

The amplitude of the roughness-induced energetic disorder at the metal/organic interface is calculated. It was found that for moderately rough electrodes, the correction to the electrostatic image potential at the charge location is small. For this reason, roughness-induced energetic disorder cannot noticeably affect charge carrier injection, contrary to the recent reports.This work was supported by the ISTC Grant No. 2207 and RFBR grants 05-03-33206 and 03-03-33067. The research described in this publication was made possible in part by Award No. RE2-2524-MO-03 of the U.S. Civilian Research & Development Foundation for the Independent States of the Former Soviet Union (CRDF)

Structures and waves in a nonlinear heat-conducting medium

The paper is an overview of the main contributions of a Bulgarian team of researchers to the problem of finding the possible structures and waves in the open nonlinear heat conducting medium, described by a reaction-diffusion equation. Being posed and actively worked out by the Russian school of A. A. Samarskii and S.P. Kurdyumov since the seventies of the last century, this problem still contains open and challenging questions.Comment: 23 pages, 13 figures, the final publication will appear in Springer Proceedings in Mathematics and Statistics, Numerical Methods for PDEs: Theory, Algorithms and their Application

On the nature of the fourth generation neutrino and its implications

We consider the neutrino sector of a Standard Model with four generations. While the three light neutrinos can obtain their masses from a variety of mechanisms with or without new neutral fermions, fourth-generation neutrinos need at least one new relatively light right-handed neutrino. If lepton number is not conserved this neutrino must have a Majorana mass term whose size depends on the underlying mechanism for lepton number violation. Majorana masses for the fourth generation neutrinos induce relative large two-loop contributions to the light neutrino masses which could be even larger than the cosmological bounds. This sets strong limits on the mass parameters and mixings of the fourth generation neutrinos.Comment: To be published. Few typos corrected, references update

Large-order NSPT for lattice gauge theories with fermions:the plaquette in massless QCD

Numerical Stochastic Perturbation Theory (NSPT) allows for perturbative computations in quantum field theory. We present an implementation of NSPT that yields results for high orders in the perturbative expansion of lattice gauge theories coupled to fermions. The zero-momentum mode is removed by imposing twisted boundary conditions; in turn, twisted boundary conditions require us to introduce a smell degree of freedom in order to include fermions in the fundamental representation. As a first application, we compute the critical mass of two flavours of Wilson fermions up to order $O(\beta^{-7})$ in a ${\mathrm{SU}}(3)$ gauge theory. We also implement, for the first time, staggered fermions in NSPT. The residual chiral symmetry of staggered fermions protects the theory from an additive mass renormalisation. We compute the perturbative expansion of the plaquette with two flavours of massless staggered fermions up to order $O(\beta^{-35})$ in a ${\mathrm{SU}}(3)$ gauge theory, and investigate the renormalon behaviour of such series. We are able to subtract the power divergence in the Operator Product Expansion (OPE) for the plaquette and estimate the gluon condensate in massless QCD. Our results confirm that NSPT provides a viable way to probe systematically the asymptotic behaviour of perturbative series in QCD and, eventually, gauge theories with fermions in higher representations.Comment: 49 pages, 28 figures. Revised version, to be published in EPJC. Some references added, typos corrected, and improved discussion on finite-volume effect

Roughness-induced energetic disorder at the metal/organic interface

The amplitude of the roughness-induced energetic disorder at the metal/organic interface was calculated. It was found that for moderately rough electrodes, the correction to the electrostatic image potential at the charge location is small. As a result, roughness-induced energetic disorder does not noticeably affect charge carrier injection. © 2006 The American Physical Society

Energetic disorder at the interface between disordered organic material and metal electrode

The physics of organic disordered materials is dominated by the effects of energetic disorder. We show that image forces reduce the electrostatic component of the total energetic disorder near an interface with a metal electrode. Typically, the standard deviation of energetic disorder is dramatically reduced at the first few layers of organic semiconductor molecules adjacent to the metal electrode. This means that the use of bulk disorder parameters (such as standard deviation of disorder) for description of the energetic disorder at the interface is poorly justified even in the case of identical spatial and chemical structure of the organic material at the interface and in the bulk of the transport layer. Implications for charge injection into organic semiconductors are discussed. © 2007 Old City Publishing. Inc

Energetic disorder at the metal-organic semiconductor interface

The physics of organic semiconductors is dominated by the effects of energetic disorder. We show that image forces reduce the electrostatic component of the total energetic disorder near an interface with a metal electrode. Typically, the variance of energetic disorder is dramatically reduced at the first few layers of organic semiconductor molecules adjacent to the metal electrode. Implications for charge injection into organic semiconductors are discussed. © 2006 The American Physical Society

Transversal and longitudinal diffusion in polar disordered organic materials

Longitudinal and transversal diffusion of charge carriers in disordered polar organic materials has been studied using Monte Carlo simulation. It was found that the simple Einstein relation is valid for the transversal diffusion coefficient, while neither the simple nor the modified Einstein relation is valid for the longitudinal diffusion coefficient. Effect of the field dependence of the transversal diffusion coefficient on the ultimate resolution of organic devices has been discussed. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA

Transport energy in disordered organic materials

Applicability of the concept of the transport energy to charge carrier transport in disordered organic materials has been reexamined. We showed that because of a strong spatial correlation in the distribution of the site energies in organic materials the transport energy does not exist. © 2006 WILEY-VCH Verlag GmbH & Co, KGaA