Effect Of Diagonal Disorder Research Articles

Effects of disorder on charge transport in semico++ nducting polymers

Polaron transport process in semiconducting polymers with disordered structure and morphology is simulated using a nonadiabatic evolution method. The simulations are performed within the framework of an extended version of the Su–Schrieffer–Heeger model modified to include diagonal disorder, off-diagonal disorder and an external electric field. It is found that the polaron transport mechanism is determined by the competition between the disorder and the electric field. For the case of pure diagonal disorder, the polaron transport undergoes a crossover from adiabatic drift to nonadiabatic hopping under a large scope of electric field (E<2.0mV/Å), whereas the field-assisted tunneling dominates charge transport for a higher electric field. For the combined disorder, it is demonstrated that the competition effects and the synergetic effects between the diagonal and off-diagonal disorder are equally important. The effects of diagonal disorder are negative in most cases, and the energetically easier pathways for charge transport are mainly opened by the off-diagonal disorder.

Effects of diagonal disorder on charge density wave and superconductivity in local pair systems

We analyse the influence of diagonal disorder (random site energy) on charge density wave (CDW) and superconductivity (SS) in local pair systems which are described by the model of hard core charged bosons on a lattice. This problem was previously studied within the mean field approximation for the case of half-filled band ( n=1). Here we extend that investigation to the case of arbitrary particle concentration (0< n<2) and examine the phase diagrams of the model and the behaviour of superfluid density ( ρ s) as a function of n and the increasing disorder. Depending on the strength of random on-site energies, the intersite density–density repulsion and the concentration, the model can exhibit several various phases, including homogeneous phases: CDW, SS and Bose-glass (NO) as well as the phase separated states: CDW–SS, CDW–NO and particle droplets. The obtained results for SS phase are in qualitative agreement with the available Monte-Carlo calculations for two-dimensional lattice. Also, in a definite range of parameters, the system exhibits the phenomena which we call a disorder induced superconductivity and a disorder induced charge ordering.

Disorder effects on resonant tunneling in double-barrier quantum wells.

We analyze, within the framework of a one-band tight-binding model, the effect of diagonal disorder on the electronic states of a chainlike double-barrier quantum-well structure. Tunneling transmission probabilities together with local densities of states are evaluated in order to study quantum-size effects as a function of disorder in a quantum well.

EFFECT OF DIAGONAL DISORDER ON THE UPPER CRITICAL FIELD OF THE LOCAL ELECTRON PAIR SYSTEM

The effect of diagonal disorder on the upper critical field of the local electron pair system is studied in the framework of the mean field approximation. It is obtained that this kind of disorder greatly changes the temperature behavior of H(c2). Comparing che upper critical field of this system with those of heavy fermion superconductors CeCu2Si2 and UPt3, we find that it could have the similar unusual temperature dependence.

EFFECT OF DIAGONAL DISORDER ON SUPERCONDUCTIVITY IN THE TRIPLET BIPOLARONIC SYSTEM

The effect of diagonal disorder on superconductivity of the triplet bipolaronic system with a two-delta random site energy distribution is analysed in the framework of the mean-field approximation. It is found that this kind of disorder strongly affects superconductivity of the system. When the triplet bipolaron concentration is near 1/2, the transition temperature decreased seriously; while when the concentration is small, the normal random site energy can not impede the occurence of superconductivity completely. This results from the tunneling motion of triplet bipolarons. It is therefore expected that superconductivity may appear in some amorphous semiconductors with low bipolaron concentration.

Effects of off-diagonal disorder on localization of electronic excitations in mixed molecular solids

In this paper we explore the effects of off-diagonal disorder on electronic energy transfer (EET) in an impurity band of an isotopically-mixed, organic solid at low temperatures. We have considered the localization of an elementary excitation in a system characterized by both diagonal disorder, originating from inhomogeneous broadening W of the site-excitation energies, and of off-diagonal disorder arising from the energetic spread σ of the transfer integrals. We have utilized an exact expression for the self-energy of a disordered system where both the site-excitation energies and the transfer integrals are characterized by a Lorentzian distribution, together with the localization function method of Liciardello and Economou to establish the localization condition in the center of the impurity band. Model calculations were performed for a Bethe lattice and for the Hubbard density of states, demonstrating the enhancement of delocalization due to off-diagonal disorder, whereupon the Anderson transition (AT) will be exhibited at higher values of W than in the original Anderson model (OAM), when σ=0. For large values of the ratio W/σ≳12 the effects of off-diagonal disorder are negligible. Numerical calculations of σ were performed for a random distribution of impurities, while W was roughly estimated for recent spectroscopic measurements. These data, together with the results of the model calculations for a Bethe lattice, established the existence of the critical impurity concentration ? for EET in the impurity band. Off-diagonal disorder results in the lowering of ? relative to the OAM; however, the effect of diagonal disorder is dominant in determining the termination of EET in the impurity band.