32,234 research outputs found
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
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
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.
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
On Artifacts in Limited Data Spherical Radon Transform: Curved Observation Surface
In this article, we consider the limited data problem for spherical mean
transform. We characterize the generation and strength of the artifacts in a
reconstruction formula. In contrast to the third's author work [Ngu15b], the
observation surface considered in this article is not flat. Our results are
comparable to those obtained in [Ngu15b] for flat observation surface. For the
two dimensional problem, we show that the artifacts are orders smoother
than the original singularities, where is vanishing order of the smoothing
function. Moreover, if the original singularity is conormal, then the artifacts
are order smoother than the original singularity. We provide
some numerical examples and discuss how the smoothing effects the artifacts
visually. For three dimensional case, although the result is similar to that
[Ngu15b], the proof is significantly different. We introduce a new idea of
lifting the space
Joint space-time trellis decoding and channel estimation in correlated fading channels
Copyright © 2004 IEEEThis letter addresses the issue of joint space-time trellis decoding and channel estimation in time-varying fading channels that are spatially and temporally correlated. A recursive space-time receiver which incorporates per-survivor processing (PSP) and Kalman filtering into the Viterbi algorithm is proposed. This approach generalizes existing work to the correlated fading channel case. The channel time-evolution is modeled by a multichannel autoregressive process, and a bank of Kalman filters is used to track the channel variations. Computer simulation results show that a performance close to the maximum likelihood receiver with perfect channel state information (CSI) can be obtained. The effects of the spatial correlation on the performance of a receiver that assumes independent fading channels are examined.Van Khanh Nguyen and Langford B. Whit
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
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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