11,302 research outputs found
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 limit, with 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
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 and disorder. Two successive metal
insulator transitions are observed at integer filling values as 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.
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- cuprates, on the basis of holes moving in a spiral spin
background. This picture follows from analysis of the extended 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
LaSrCuO. 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
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