944 research outputs found
Electron Transport in Nanogranular Ferromagnets
We study electronic transport properties of ferromagnetic nanoparticle arrays
and nanodomain materials near the Curie temperature in the limit of weak
coupling between the grains. We calculate the conductivity in the Ohmic and
non-Ohmic regimes and estimate the magnetoresistance jump in the resistivity at
the transition temperature. The results are applicable for many emerging
materials, including artificially self-assembled nanoparticle arrays and a
certain class of manganites, where localization effects within the clusters can
be neglected.Comment: 4 pages, 2 figure
Density of States and Conductivity of Granular Metal or Array of Quantum Dots
The conductivity of a granular metal or an array of quantum dots usually has
the temperature dependence associated with variable range hopping within the
soft Coulomb gap of density of states. This is difficult to explain because
neutral dots have a hard charging gap at the Fermi level. We show that
uncontrolled or intentional doping of the insulator around dots by donors leads
to random charging of dots and finite bare density of states at the Fermi
level. Then Coulomb interactions between electrons of distant dots results in
the a soft Coulomb gap. We show that in a sparse array of dots the bare density
of states oscillates as a function of concentration of donors and causes
periodic changes in the temperature dependence of conductivity. In a dense
array of dots the bare density of states is totally smeared if there are
several donors per dot in the insulator.Comment: 13 pages, 15 figures. Some misprints are fixed. Some figures are
dropped. Some small changes are given to improve the organizatio
Anomalously large capacitance of a plane capacitor with a two-dimensional electron gas
In electronic devices where a two-dimensional electron gas (2DEG) comprises
one or both sides of a plane capacitor, the resulting capacitance can be
larger than the "geometric capacitance" determined by the physical
separation between electrodes. This larger capacitance is known to result
from the Coulomb correlations between individual electrons within the low
density 2DEG, which lead to a negative thermodynamic density of states
(negative compressibility). Experiments on such systems generally operate in
the regime where the average spacing between electrons in the 2DEG
is smaller than , and these experiments observe by only a few
percent. A recent experiment [1], however, has observed larger than
by almost 40% while operating in the regime . In this paper we argue
that at correlations between the electronic charge of opposite
electrodes become important. We develop a theory of the capacitance for the
full range of . We show that, in the absence of disorder, the capacitance
can be times larger than the geometric value, where is the
electron Bohr radius. Our results compare favorably with the experiment of Ref.
[1] without the use of adjustable parameters.Comment: 8 pages, 6 figures; revised discussion of the zero density limit;
some typos fixe
Non-Ohmic variable-range hopping transport in one-dimensional conductors
We investigate theoretically the effect of a finite electric field on the
resistivity of a disordered one-dimensional system in the variable-range
hopping regime. We find that at low fields the transport is inhibited by rare
fluctuations in the random distribution of localized states that create
high-resistance ``breaks'' in the hopping network. As the field increases, the
breaks become less resistive. In strong fields the breaks are overrun and the
electron distribution function is driven far from equilibrum. The logarithm of
the resistance initially shows a simple exponential drop with the field,
followed by a logarithmic dependence, and finally, by an inverse square-root
law.Comment: Version accepted to Phys. Rev. Let
Solution of the tunneling-percolation problem in the nanocomposite regime
We noted that the tunneling-percolation framework is quite well understood at
the extreme cases of percolation-like and hopping-like behaviors but that the
intermediate regime has not been previously discussed, in spite of its
relevance to the intensively studied electrical properties of nanocomposites.
Following that we study here the conductivity of dispersions of particle
fillers inside an insulating matrix by taking into account explicitly the
filler particle shapes and the inter-particle electron tunneling process. We
show that the main features of the filler dependencies of the nanocomposite
conductivity can be reproduced without introducing any \textit{a priori}
imposed cut-off in the inter-particle conductances, as usually done in the
percolation-like interpretation of these systems. Furthermore, we demonstrate
that our numerical results are fully reproduced by the critical path method,
which is generalized here in order to include the particle filler shapes. By
exploiting this method, we provide simple analytical formulas for the composite
conductivity valid for many regimes of interest. The validity of our
formulation is assessed by reinterpreting existing experimental results on
nanotube, nanofiber, nanosheet and nanosphere composites and by extracting the
characteristic tunneling decay length, which is found to be within the expected
range of its values. These results are concluded then to be not only useful for
the understanding of the intermediate regime but also for tailoring the
electrical properties of nanocomposites.Comment: 13 pages with 8 figures + 10 pages with 9 figures of supplementary
material (Appendix B
Cyclotron enhancement of tunneling
A state of an electron in a quantum wire or a thin film becomes metastable,
when a static electric field is applied perpendicular to the wire direction or
the film surface. The state decays via tunneling through the created potential
barrier. An additionally applied magnetic field, perpendicular to the electric
field, can increase the tunneling decay rate for many orders of magnitude. This
happens, when the state in the wire or the film has a velocity perpendicular to
the magnetic field. According to the cyclotron effect, the velocity rotates
under the barrier and becomes more aligned with the direction of tunneling.
This mechanism can be called cyclotron enhancement of tunneling
Numerical studies of variable-range hopping in one-dimensional systems
Hopping transport in a one-dimensional system is studied numerically. A fast
algorithm is devised to find the lowest-resistance path at arbitrary electric
field. Probability distribution functions of individual resistances on the path
and the net resistance are calculated and fitted to compact analytic formulas.
Qualitative differences between statistics of resistance fluctuations in Ohmic
and non-Ohmic regimes are elucidated. The results are compared with prior
theoretical and experimental work on the subject.Comment: 12 pages, 12 figures. Published versio
Transport properties of chemically synthesized polypyrrole thin films
The electronic transport in polypyrrole thin films synthesized chemically
from the vapor phase is studied as a function of temperature as well as of
electric and magnetic fields. We find distinct differences in comparison to the
behavior of both polypyrrole films prepared by electrochemical growth as well
as of the bulk films obtained from conventional chemical synthesis. For small
electric fields F, a transition from Efros-Shklovskii variable range hopping to
Arrhenius activated transport is observed at 30 K. High electric fields induce
short range hopping. The characteristic hopping distance is found to be
proportional to F^(-1/2). The magnetoresistance R(B) is independent of F below
a critical magnetic field, above which F counteracts the magnetic field induced
localization.Comment: 6 pages, 5 figure
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