Inelastic electron transport in polymer nanofibers

In this paper we present theoretical analysis of the electron transport in conducting polymers. We concentrate on the study of the effects of temperature on characteristics of the transport. We treat a conducting polymers in a metal state as a network of metallic-like grains connected by electron quantum tunneling via intermediate state localized on a polymer chain between the grains. To analyze the effects of temperature on this kind of electron intergrain transport we represent the thermal environment as a phonon bath coupled to the intermediate state. The electron transmission is computed using the Buttiker model within the scattering matrix formalism. This approach is further developed, and the dephasing parameter is expessed in terms of relevant energies including the thermal energy. It is shown that temperature dependencies of both current and conductance associated with the above transport mechanism differ from those typical for other conduction mechanisms in conducting polymers. This could be useful to separate out the contribution from the intergrain electron tunneling to the net electric current in transport experiments on various polymer nanofibers. The proposed model could be used to analyze inelastic electron transport through molecular junctions.Comment: 8 pages, 5 pictures; text added, figures adde

Abundance gradients in spiral disks: is the gradient inversion at high redshift real?

We compute the abundance gradients along the disk of the Milky Way by means of the two-infall model: in particular, the gradients of oxygen and iron and their temporal evolution. First, we explore the effects of several physical processes which influence the formation and evolution of abundance gradients. They are: i) the inside-out formation of the disk, ii) a threshold in the gas density for star formation, iii) a variable star formation efficiency along the disk, iv) radial flows and their speed, and v) different total surface mass density (gas plus stars) distributions for the halo. We are able to reproduce at best the present day gradients of oxygen and iron if we assume an inside-out formation, no threshold gas density, a constant efficiency of star formation along the disk and radial gas flows. It is particularly important the choice of the velocity pattern for radial flows and the combination of this velocity pattern with the surface mass density distribution in the halo. Having selected the best model, we then explore the evolution of abundance gradients in time and find that the gradients in general steepen in time and that at redshift z~3 there is a gradient inversion in the inner regions of the disk, in the sense that at early epochs the oxygen abundance decreases toward the Galactic center. This effect, which has been observed, is naturally produced by our models if an inside-out formation of the disk and and a constant star formation efficiency are assumed. The inversion is due to the fact that in the inside-out formation a strong infall of primordial gas, contrasting chemical enrichment, is present in the innermost disk regions at early times. The gradient inversion remains also in the presence of radial flows, either with constant or variable speed in time, and this is a new result.Comment: 15 pages, 19 figures, accepted for publication in MNRA

Metals get an awkward cousin

A newly predicted state of matter is a simple theoretical example of a phase that conducts electricity but is not smoothly connected to our conventional model of metals. A viewpoint on arXiv:1201.5998.Comment: Physics 5, 82 (2012

Electron-Hole Asymmetry in GdBaCo_{2}O_{5+x}: Evidence for Spin Blockade of Electron Transport in a Correlated Electron System

In RBaCo_{2}O_{5+x} compounds (R is rare earth) variability of the oxygen content allows precise doping of CoO_2 planes with both types of charge carriers. We study transport properties of doped GdBaCo_{2}O_{5+x} single crystals and find a remarkable asymmetry in the behavior of holes and electrons doped into a parent insulator GdBaCo_{2}O_{5.5}. Doping dependences of resistivity, Hall response, and thermoelectric power reveal that the doped holes greatly improve the conductivity, while the electron-doped samples always remain poorly conducting. This doping asymmetry provides strong evidence for a spin blockade of the electron transport in RBaCo_{2}O_{5+x}.Comment: 4 pages, 5 figures, accepted for publication in PR

Metal-Insulator transitions in the periodic Anderson model

We solve the Periodic Anderson model in the Mott-Hubbard regime, using Dynamical Mean Field Theory. Upon electron doping of the Mott insulator, a metal-insulator transition occurs which is qualitatively similar to that of the single band Hubbard model, namely with a divergent effective mass and a first order character at finite temperatures. Surprisingly, upon hole doping, the metal-insulator transition is not first order and does not show a divergent mass. Thus, the transition scenario of the single band Hubbard model is not generic for the Periodic Anderson model, even in the Mott-Hubbard regime.Comment: 5 pages, 4 figure

Low temperature terahertz spectroscopy of n-InSb through a magnetic field driven metal-insulator transition

We use fiber-coupled photoconductive emitters and detectors to perform terahertz (THz) spectroscopy of lightly-doped n-InSb directly in the cryogenic (1.5 K) bore of a high-field superconducting magnet. We measure transmission spectra from 0.1-1.1 THz as the sample is driven through a metal-insulator transition (MIT) by applied magnetic field. In the low-field metallic state, the data directly reveal the plasma edge and magneto-plasmon modes. With increasing field, a surprisingly broad band (0.3-0.8 THz) of low transmission appears at the onset of the MIT. This band subsequently collapses and evolves into the sharp 1s -> 2p- transition of electrons `frozen' onto isolated donors in the insulating state.Comment: 4 pages, 3 figure

Lithium abundance and 6Li/7Li ratio in the active giant HD123351 I. A comparative analysis of 3D and 1D NLTE line-profile fits

Current three-dimensional (3D) hydrodynamical model atmospheres together with NLTE spectrum synthesis, permit to derive reliable atomic and isotopic chemical abundances from high-resolution stellar spectra. Not much is known about the presence of the fragile 6Li isotope in evolved solar-metallicity RGB stars, not to mention its production in magnetically active targets like HD123351. From fits of the observed CFHT spectrum with synthetic line profiles based on 1D and 3D model atmospheres, we seek to estimate the abundance of the 6Li isotope and to place constraints on its origin. We derive A(Li) and the 6Li/7Li isotopic ratio by fitting different synthetic spectra to the Li-line region of a high-resolution CFHT spectrum (R=120 000, S/R=400). The synthetic spectra are computed with four different line lists, using in parallel 3D hydrodynamical CO5BOLD and 1D LHD model atmospheres and treating the line formation of the lithium components in non-LTE (NLTE). We find A(Li)=1.69+/-0.11 dex and 6Li/7Li=8.0+/-4.4 % in 3D-NLTE, using the line list of Mel\'endez et al. (2012), updated with new atomic data for V I, which results in the best fit of the lithium line profile of HD123351. Two other line lists lead to similar results but with inferior fit qualities. Our 2-sigma detection of the 6Li isotope is the result of a careful statistical analysis and the visual inspection of each achieved fit. Since the presence of a significant amount of 6Li in the atmosphere of a cool evolved star is not expected in the framework of standard stellar evolution theory, non-standard, external lithium production mechanisms, possibly related to stellar activity or a recent accretion of rocky material, need to be invoked to explain the detection of 6Li in HD123351.Comment: 16 pages, 11 figures. Accepted for publication in A&

Wave functions in the neighborhood of a toroidal surface; hard vs. soft constraint

The curvature potential arising from confining a particle initially in three-dimensional space onto a curved surface is normally derived in the hard constraint $q \to 0$ limit, with $q$ the degree of freedom normal to the surface. In this work the hard constraint is relaxed, and eigenvalues and wave functions are numerically determined for a particle confined to a thin layer in the neighborhood of a toroidal surface. The hard constraint and finite layer (or soft constraint) quantities are comparable, but both differ markedly from those of the corresponding two dimensional system, indicating that the curvature potential continues to influence the dynamics when the particle is confined to a finite layer. This effect is potentially of consequence to the modelling of curved nanostructures.Comment: 4 pages, no fig

Theory of Anisotropic Hopping Transport due to Spiral Correlations in the Spin-Glass Phase of Underdoped Cuprates

We study the in-plane resistivity anisotropy in the spin-glass phase of the high-$T_{c}$ cuprates, on the basis of holes moving in a spiral spin background. This picture follows from analysis of the extended $t-J$ model with Coulomb impurities. In the variable-range hopping regime the resistivity anisotropy is found to have a maximum value of around 90%, and it decreases with temperature, in excellent agreement with experiments in La$_{2-x}$Sr$_x$CuO$_4$. In our approach the transport anisotropy is due to the non-collinearity of the spiral spin state, rather than an intrinsic tendency of the charges to self-organize.Comment: 5 pages, 4 figures; expanded versio

Insulator-metal-insulator transition and selective spectral weight transfer in a disordered strongly correlated system

We investigate the metal insulator transitions at finite temperature for the Hubbard model with diagonal alloy disorder. We solve the dynamical mean field theory equations with the non crossing approximation and we use the coherent potential approximation to handle disorder. The excitation spectrum is given for various correlation strength $U$ and disorder. Two successive metal insulator transitions are observed at integer filling values as $U$ is increased. An important selective transfer of spectral weight arises upon doping. The strong influence of the temperature on the low energy dynamics is studied in details.Comment: submitted to Phys. Rev.