Composition dependent magnetic properties of iron oxide - polyaniline nanoclusters

Gamma - Iron Oxide prepared by sol -gel process was used to produce nanocomposites with polyaniline of varying aniline concentrations. TEM shows the presence of chain like structure for lower polyaniline concentration. The room temperature hysteresis curves show finite coercivity of 160 Oe for all the composites while the saturation magnetization was found to decrease with increasing polymer content. ZFC - FC magnetisation measurements indicate high blocking temperatures. It is believed that this indicates a strongly interacting system, which is also shown by our TEM results. Monte Carlo simulations performed on a random anisotropy model with dipolar and exchange inteactions match well with experimental results.Comment: 9 (nine) pages, 6 figures (jpeg and eps

Role of defects in transport through a quantum dot single electron transistor

The effect of a single dotlike defect on the transport through a quantum dot single electron transistor weakly coupled to external leads is studied. It is found that the conductance profile is changed significantly by the quantum mechanical tunneling between the dot and the defect and the interactions between them, both of which are dependent on the distance between the dot and the defect, as also by the morphology of the defect. In particular, we find that even a very small strength of interdot interaction has a major influence on the transport and must be taken into account in device fabrication

Magnetic properties of polypyrrole - coated iron oxide nanoparticles

Iron oxide nanoparticles were prepared by sol -gel process. Insitu polymerization of pyrrole monomer in the presence of oxygen in iron oxide ethanol suspension resulted in a iron oxide - polypyrrole nanocomposite. The structure and magnetic properties were investigated for varying pyrrole concentrations. The presence of the gamma - iron oxide phase and polypyrrole were confirmed by XRD and FTIR respectively. Agglomeration was found to be comparatively much reduced for the coated samples, as shown by TEM. AC susceptibility measurements confirmed the superparamagnetic behaviour. Numerical simulations performed for an interacting model system are performed to estimate the anisotropy and compare favourably with experimental results.Comment: 11 pages,8 figure

Single domain magnetic arrays: role of disorder and interactions

The hysteresis of an array of interacting single domain magnetic particles is studied, where the particles interact via exchange and dipolar interactions. The dependence of the magnetic properties on the distribution of grain sizes, the density of the grains, the anisotropy energy and the exchange interactions in the array is investigated through Monte Carlo simulations for $\gamma- {\rm Fe_2 O}_3$ nanoparticle systems. We also present some experimental results on the $\gamma - {\rm Fe_2 O}_3$-polypyrrole nanocomposite system which agree with the trends observed in our simulations

Variable-range hopping: role of Coulomb interactions

The effect of Coulomb interactions on hopping conduction in the variable-range hopping regime is analyzed within a linear-response formalism. Here the conductivity and the dielectric function are related to the density-density response function for which a generalized master equation (GME) can be derived using the Mori-Zwanzig projector formalism. The GME can be thought of as a random resistor network with frequency-dependent internode conductances, whose values can be determined from a function related to the current-current correlator at the two nodes. We evaluate the internode conductances using a diagrammatic perturbation formalism. For a single electron hop with all the other charges frozen, we obtain hop rates correct to all orders in Coulomb interaction. This gives us a finite temperature generalization of existing results for the interacting system. We then incorporate relaxation effects that accompany electron hops, using a dynamical model of the Coulomb gap. We argue that the parameter that governs the local relaxation is related to the conductivity itself. These internode conductances are then used to calculate the dc conductivity of the network by effective- medium approximation. We show that a crossover from Efros-Shklovskii's T<SUP>&#189;</SUP> behavior to Mott's T<SUP>&#188;</SUP> behavior occurs due to the relaxation effects, as the temperature is increased. At low temperatures the relaxation is slow so that electrons hop in a frozen charge background and thereby sense the Coulomb gap. This gives the T<SUP>&#189;</SUP> behavior. At higher temperatures the relaxation gets faster and the Coulomb gap is alleviated leading to Mott's behavior

Temperature dependent photoluminescence in self assembled InAs quantum dot arrays

In this paper we investigate the competing effects of thermally assisted hopping and radiative recombination of disorder induced spatially localized excitons on the temperature dependent PL spectrum of a self assembled InAs quantum dot array on GaAs. The stationary photoluminescence spectrum due to the lowest exciton state is calculated from a Monte Carlo simulation. We have also included the effect of the narrowing of the band gap with the increase of temperature. Our results on variation of the peak position and linewidth of the PL spectrum with temperature are in agreement with existing experimental results on InAs/GaAs dot arrays

Dielectric properties of the electron glass

The dielectric-constant matrix of a system of electrons that have been localized by disorder is calculated as a function of frequency and distance. It is shown that the basic polarization process in these systems is quite different from those with extended electrons, as the electronic transitions can only occur by phonon-mediated hops between localized states. It is also argued that the random-phase approximation is not adequate for such systems, as the correct account for the excitation energies of the system requires inclusion of electron-hole interaction. Our analysis provides an expression for screening length that shows a basic departure from the Thomas-Fermi approximation. On the basis of our results for polarizability, we also argue that a soft Coulomb gap for electron-hole excitations should exist to provide dielectric stability to the system

Role of defects in transport through a quantum dot single electron transistor

The effect of a single dotlike defect on the transport through a quantum dot single electron transistor weakly coupled to external leads is studied. It is found that the conductance profile is changed significantly by the quantum mechanical tunneling between the dot and the defect and the interactions between them, both of which are dependent on the distance between the dot and the defect, as also by the morphology of the defect. In particular, we find that even a very small strength of interdot interaction has a major influence on the transport and must be taken into account in device fabrication

Ferromagnetism of anderson localized electrons: application to cluster compounds

A study of the electrical transport and magnetic properties of a series of cluster compounds with the generic formula A<SUB>0.5</SUB>M<SUB>2</SUB>X<SUB>4</SUB> suggests that the electrons at the Fermi surface are localized, and the ferromagnetism seen in these compounds arises from these electrons. The magnetism of these compounds shows some features characteristic of itinerant models and others which are characteristic of localized models. We construct a model which has a nondegenerate band of localized states with on-site repulsion. Further, the singly occupied states interact via direct exchange interaction which is ferromagnetic. Using a mean-field approximation we calculate the various magnetic properties, which are in qualitative accord with the observed behavior. In particular, we find that the single-particle excitations play a dominant role in the magnetism of these compounds, even though the electrons are localized. We also analyze the spin-wave excitations in this model and discuss their effect on low-temperature thermodynamics