30,294 research outputs found
Strong disorder renormalization group on fractal lattices: Heisenberg models and magnetoresistive effects in tight binding models
We use a numerical implementation of the strong disorder renormalization
group (RG) method to study the low-energy fixed points of random Heisenberg and
tight-binding models on different types of fractal lattices. For the Heisenberg
model new types of infinite disorder and strong disorder fixed points are
found. For the tight-binding model we add an orbital magnetic field and use
both diagonal and off-diagonal disorder. For this model besides the gap spectra
we study also the fraction of frozen sites, the correlation function, the
persistent current and the two-terminal current. The lattices with an even
number of sites around each elementary plaquette show a dominant
periodicity. The lattices with an odd number of sites around each elementary
plaquette show a dominant periodicity at vanishing diagonal
disorder, with a positive weak localization-like magnetoconductance at infinite
disorder fixed points. The magnetoconductance with both diagonal and
off-diagonal disorder depends on the symmetry of the distribution of on-site
energies.Comment: 19 pages, 20 figure
Interfering directed paths and the sign phase transition
We revisit the question of the "sign phase transition" for interfering
directed paths with real amplitudes in a random medium. The sign of the total
amplitude of the paths to a given point may be viewed as an Ising order
parameter, so we suggest that a coarse-grained theory for system is a dynamic
Ising model coupled to a Kardar-Parisi-Zhang (KPZ) model. It appears that when
the KPZ model is in its strong-coupling ("pinned") phase, the Ising model does
not have a stable ferromagnetic phase, so there is no sign phase transition. We
investigate this numerically for the case of {\ss}1+1 dimensions, demonstrating
the instability of the Ising ordered phase there.Comment: 4 pages, 4 figure
Automated Real-Time Testing (ARTT) for Embedded Control Systems (ECS)
Developing real-time automated test systems for embedded control systems has
been a real problem. Some engineers and scientists have used customized
software and hardware as a solution, which can be very expensive and time
consuming to develop. We have discovered how to integrate a suite of
commercially available off-the-shelf software tools and hardware to develop a
scalable test platform that is capable of performing complete black-box testing
for a dual-channel real-time Embedded-PLC-based control system
(www.aps.anl.gov). We will discuss how the Vali/Test Pro testing methodology
was implemented to structure testing for a personnel safety system with large
quantities of requirements and test cases.
This work was supported by the U.S. Department of Energy, Basic Energy
Sciences, under Contract No. W-31-109-Eng-38.Comment: 6 pages, 8 figures, ICALEPCS 2001, Poster Sessio
Origin of conductivity cross over in entangled multi-walled carbon nanotube network filled by iron
A realistic transport model showing the interplay of the hopping transport
between the outer shells of iron filled entangled multi-walled carbon nanotubes
(MWNT) and the diffusive transport through the inner part of the tubes, as a
function of the filling percentage, is developed. This model is based on
low-temperature electrical resistivity and magneto-resistance (MR)
measurements. The conductivity at low temperatures showed a crossover from
Efros-Shklovski (E-S) variable range hopping (VRH) to Mott VRH in 3 dimensions
(3D) between the neighboring tubes as the iron weight percentage is increased
from 11% to 19% in the MWNTs. The MR in the hopping regime is strongly
dependent on temperature as well as magnetic field and shows both positive and
negative signs, which are discussed in terms of wave function shrinkage and
quantum interference effects, respectively. A further increase of the iron
percentage from 19% to 31% gives a conductivity crossover from Mott VRH to 3D
weak localization (WL). This change is ascribed to the formation of long iron
nanowires at the core of the nanotubes, which yields a long dephasing length
(e.g. 30 nm) at the lowest measured temperature. Although the overall transport
in this network is described by a 3D WL model, the weak temperature dependence
of inelastic scattering length expressed as L_phi ~T^-0.3 suggests the
possibility for the presence of one-dimensional channels in the network due to
the formation of long Fe nanowires inside the tubes, which might introduce an
alignment in the random structure.Comment: 29 pages,10 figures, 2 tables, submitted to Phys. Rev.
Model for Anisotropic Directed Percolation
We propose a simulation model to study the properties of directed percolation
in two-dimensional (2D) anisotropic random media. The degree of anisotropy in
the model is given by the ratio between the axes of a semi-ellipse
enclosing the bonds that promote percolation in one direction. At percolation,
this simple model shows that the average number of bonds per site in 2D is an
invariant equal to 2.8 independently of . This result suggests that
Sinai's theorem proposed originally for isotropic percolation is also valid for
anisotropic directed percolation problems. The new invariant also yields a
constant fractal dimension for all , which is the same
value found in isotropic directed percolation (i.e., ).Comment: RevTeX, 9 pages, 3 figures. To appear in Phys.Rev.
Anomalous Aharonov-Bohm conductance oscillations from topological insulator surface states
We study transport properties of a topological insulator nanowire when a
magnetic field is applied along its length. We predict that with strong surface
disorder, a characteristic signature of the band topology is revealed in
Aharonov Bohm (AB) oscillations of the conductance. These oscillations have a
component with anomalous period , and with conductance maxima at
odd multiples of , i.e. when the AB phase for surface electrons
is . This is intimately connected to the band topology and a surface
curvature induced Berry phase, special to topological insulator surfaces. We
discuss similarities and differences from recent experiments on BiSe
nanoribbons, and optimal conditions for observing this effect.Comment: 7 pages, 2 figure
Negative Magnetoresistance in the Nearest-neighbor Hopping Conduction
We propose a size effect which leads to the negative magnetoresistance in
granular metal-insulator materials in which the hopping between two nearest
neighbor clusters is the main transport mechanism. We show that the hopping
probability increases with magnetic field. This is originated from the level
crossing in a few-electron cluster. Thus, the overlap of electronic states of
two neighboring clusters increases, and the negative magnetoresistance is
resulted.Comment: Latex file, no figur
Magnetoresistance in semiconductor structures with hopping conductivity: effects of random potential and generalization for the case of acceptor states
We reconsider the theory of magnetoresistance in hopping semiconductors.
First, we have shown that the random potential of the background impurities
affects significantly preexponential factor of the tunneling amplitude which
becomes to be a short-range one in contrast to the long-range one for purely
Coulomb hopping centers. This factor to some extent suppresses the negative
interference magnetoresistance and can lead to its decrease with temperature
decrease which is in agreement with earlier experimental observations. We have
also extended the theoretical models of positive spin magnetoresistance, in
particular, related to a presence of doubly occupied states (corresponding to
the upper Hubbard band) to the case of acceptor states in 2D structures. We
have shown that this mechanism can dominate over classical wave-shrinkage
magnetoresistance at low temperatures. Our results are in semi-quantitative
agreement with experimental data.Comment: 19 pages, 3 figure
Out of equilibrium electronic transport properties of a misfit cobaltite thin film
We report on transport measurements in a thin film of the 2D misfit Cobaltite
. Dc magnetoresistance measurements obey the modified
variable range hopping law expected for a soft Coulomb gap. When the sample is
cooled down, we observe large telegraphic-like fluctuations. At low
temperature, these slow fluctuations have non Gaussian statistics, and are
stable under a large magnetic field. These results suggest that the low
temperature state is a glassy electronic state. Resistance relaxation and
memory effects of pure magnetic origin are also observed, but without aging
phenomena. This indicates that these magnetic effects are not glassy-like and
are not directly coupled to the electronic part.Comment: accepted in Phys Rev B, Brief report
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