1,699 research outputs found
CT Image Reconstruction by Spatial-Radon Domain Data-Driven Tight Frame Regularization
This paper proposes a spatial-Radon domain CT image reconstruction model
based on data-driven tight frames (SRD-DDTF). The proposed SRD-DDTF model
combines the idea of joint image and Radon domain inpainting model of
\cite{Dong2013X} and that of the data-driven tight frames for image denoising
\cite{cai2014data}. It is different from existing models in that both CT image
and its corresponding high quality projection image are reconstructed
simultaneously using sparsity priors by tight frames that are adaptively
learned from the data to provide optimal sparse approximations. An alternative
minimization algorithm is designed to solve the proposed model which is
nonsmooth and nonconvex. Convergence analysis of the algorithm is provided.
Numerical experiments showed that the SRD-DDTF model is superior to the model
by \cite{Dong2013X} especially in recovering some subtle structures in the
images
Shot noise of spin current and spin transfer torque
We report the theoretical investigation of noise spectrum of spin current and
spin transfer torque for non-colinear spin polarized transport in a spin-valve
device which consists of normal scattering region connected by two
ferromagnetic electrodes. Our theory was developed using non-equilibrium
Green's function method and general non-linear and
relations were derived as a function of angle between magnetization of
two leads. We have applied our theory to a quantum dot system with a resonant
level coupled with two ferromagnetic electrodes. It was found that for the MNM
system, the auto-correlation of spin current is enough to characterize the
fluctuation of spin current. For a system with three ferromagnetic layers,
however, both auto-correlation and cross-correlation of spin current are needed
to characterize the noise spectrum of spin current. Furthermore, the spin
transfer torque and the torque noise were studied for the MNM system. For a
quantum dot with a resonant level, the derivative of spin torque with respect
to bias voltage is proportional to when the system is far away
from the resonance. When the system is near the resonance, the spin transfer
torque becomes non-sinusoidal function of . The derivative of noise
spectrum of spin transfer torque with respect to the bias voltage
behaves differently when the system is near or far away from the resonance.
Specifically, the differential shot noise of spin transfer torque is a
concave function of near the resonance while it becomes convex
function of far away from resonance. For certain bias voltages, the
period becomes instead of . For small , it
was found that the differential shot noise of spin transfer torque is very
sensitive to the bias voltage and the other system parameters.Comment: 15pages, 6figure
On Beltrami Model of de Sitter Spacetime
Based on some important properties of space, we present a Beltrami model
that may shed light on the observable puzzle of space
and the paradox between the special relativity principle and cosmological
principle. In , there are inertial-type coordinates and
inertial-type observers. Thus, the classical observables can be defined for
test particles and light signals. In addition, by choosing the definition of
simultaneity the Beltrami metric is transformed to the Robertson-Walker-like
metric. It is of positive spatial curvature of order . This is more or
less indicated already by the CMB power spectrum from WMAP and should be
further confirmed by its data in large scale.Comment: 4 page
New universal gates for topological quantum computation with Fibonacci- composite Majorana edge modes on topological superconductor multilayers
We propose a new design of universal topological quantum computer device
through a hybrid of the 1-, 2- and 7-layers of chiral topological
superconductor (TSC) thin films. Based on the coset
construction, strongly correlated Majorana fermion edge modes on the 7-layers
of TSC are factorized into the composite of the Fibonacci -anyon
and -anyon modes in the tricritical Ising model. Furthermore, the
deconfinement of and via the interacting potential gives
the braiding of either or . Topological phase gates are
assembled by the braidings. With these topological phase gates, we find a set
of fully topological universal gates for the composite
Majorana-Ising-type quantum computation. Because the Hilbert space still
possesses a tensor product structure of quibts and is characterized by the
fermion parities, encoding quantum information in this machine is more
efficient and substantial than that with Fibonacci anyons. The computation
results is easier to be read out by electric signals, so are the initial data
inputted.Comment: 6 pages, 3 figues, revised versio
Snyder's Model -- de Sitter Special Relativity Duality and de Sitter Gravity
Between Snyder's quantized space-time model in de Sitter space of momenta and
the \dS special relativity on \dS-spacetime of radius with Beltrami
coordinates, there is a one-to-one dual correspondence supported by a minimum
uncertainty-like argument. Together with Planck length , should be a fundamental constant. They lead to a
dimensionless constant . These indicate that physics at these two scales should be dual to
each other and there is in-between gravity of local \dS-invariance
characterized by . A simple model of \dS-gravity with a gauge-like action on
umbilical manifolds may show these characters. It can pass the observation
tests and support the duality.Comment: 32 page
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