35,520 research outputs found
Video Compressive Sensing for Dynamic MRI
We present a video compressive sensing framework, termed kt-CSLDS, to
accelerate the image acquisition process of dynamic magnetic resonance imaging
(MRI). We are inspired by a state-of-the-art model for video compressive
sensing that utilizes a linear dynamical system (LDS) to model the motion
manifold. Given compressive measurements, the state sequence of an LDS can be
first estimated using system identification techniques. We then reconstruct the
observation matrix using a joint structured sparsity assumption. In particular,
we minimize an objective function with a mixture of wavelet sparsity and joint
sparsity within the observation matrix. We derive an efficient convex
optimization algorithm through alternating direction method of multipliers
(ADMM), and provide a theoretical guarantee for global convergence. We
demonstrate the performance of our approach for video compressive sensing, in
terms of reconstruction accuracy. We also investigate the impact of various
sampling strategies. We apply this framework to accelerate the acquisition
process of dynamic MRI and show it achieves the best reconstruction accuracy
with the least computational time compared with existing algorithms in the
literature.Comment: 30 pages, 9 figure
Radiation-Induced "Zero-Resistance State" and the Photon Assisted Transport
We demonstrate that the radiation induced "zero-resistance state" observed in
a two-dimensional electron gas is a result of the non-trivial structure of the
density of states of the systems and the photon assisted transport. A toy model
of a structureless quantum tunneling junction where the system has oscillatory
density of states catches most of the important features of the experiments. We
present a generalized Kubo-Greenwood conductivity formula for the photon
assisted transport in a general system, and show essentially the same nature of
the transport anomaly in a uniform system.Comment: 4 pages, 3 figures. Please send comment to [email protected]. This
version added a paragraph to discuss the implication of negative conductanc
Spin-polarized tunneling spectroscopic studies of the intrinsic heterogeneity and pseudogap phenomena in colossal magnetoresistive manganite La_{0.7}Ca_{0.3}MnO_{3}
Spatially resolved tunneling spectroscopic studies of colossal
magnetoresistive (CMR) manganite (LCMO) epitaxial
films on substrate are investigated as
functions of temperature, magnetic field and spin polarization by means of
scanning tunneling spectroscopy. Systematic surveys of the tunneling spectra
taken with Pt/Ir tips reveal spatial variations on the length scale of a few
hundred nanometers in the ferromagnetic state, which may be attributed to the
intrinsic heterogeneity of the manganites due to their tendency towards phase
separation. The electronic heterogeneity is found to decrease either with
increasing field at low temperatures or at temperatures above all magnetic
ordering temperatures. On the other hand, spectra taken with Cr-coated tips are
consistent with convoluted electronic properties of both LCMO and Cr. In
particular, for temperatures below the magnetic ordering temperatures of both
Cr and LCMO, the magnetic-field dependent tunneling spectra may be
quantitatively explained by the scenario of spin-polarized tunneling in a
spin-valve configuration. Moreover, a low-energy insulating energy gap eV commonly found in the tunneling conductance spectra of bulk metallic
LCMO at may be attributed to a surface ferromagnetic insulating (FI)
phase, as evidenced by its spin filtering effect at low temperatures and
vanishing gap value above the Curie temperature. Additionally, temperature
independent pseudogap (PG) phenomena existing primarily along the boundaries of
magnetic domains are observed in the zero-field tunneling spectra. The PG
becomes strongly suppressed by applied magnetic fields at low temperatures when
the tunneling spectra of LCMO become highly homogeneous. These findings suggest
that the occurrence PG is associated with the electronic heterogeneity of the
manganites.Comment: 15 pages, 15 figures. Published in Physical Review B. Corresponding
author: Nai-Chang Yeh (E-mail: [email protected]
Crossover from Rate-Equation to Diffusion-Controlled Kinetics in Two-Particle Coagulation
We develop an analytical diffusion-equation-type approximation scheme for the
one-dimensional coagulation reaction A+A->A with partial reaction probability
on particle encounters which are otherwise hard-core. The new approximation
describes the crossover from the mean-field rate-equation behavior at short
times to the universal, fluctuation-dominated behavior at large times. The
approximation becomes quantitatively accurate when the system is initially
close to the continuum behavior, i.e., for small initial density and fast
reaction. For large initial density and slow reaction, lattice effects are
nonnegligible for an extended initial time interval. In such cases our
approximation provides the correct description of the initial mean-field as
well as the asymptotic large-time, fluctuation-dominated behavior. However, the
intermediate-time crossover between the two regimes is described only
semiquantitatively.Comment: 21 pages, plain Te
Cooling a mechanical resonator via coupling to a tunable double quantum dot
We study the cooling of a mechanical resonator (MR) that is capacitively
coupled to a double quantum dot (DQD). The MR is cooled by the dynamical
backaction induced by the capacitive coupling between the DQD and the MR. The
DQD is excited by a microwave field and afterwards a tunneling event results in
the decay of the excited state of the DQD. An important advantage of this
system is that both the energy level splitting and the decay rate of the DQD
can be well tuned by varying the gate voltage. We find that the steady average
occupancy, below unity, of the MR can be achieved by changing both the decay
rate of the excited state and the detuning between the transition frequency of
the DQD and the microwave frequency, in analogy to the laser sideband cooling
of an atom or trapped ion in atomic physics. Our results show that the cooling
of the MR to the ground state is experimentally implementable.Comment: 10 pages, 5 figure
The conduction pathway of potassium channels is water free under physiological conditions.
Ion conduction through potassium channels is a fundamental process of life. On the basis of crystallographic data, it was originally proposed that potassium ions and water molecules are transported through the selectivity filter in an alternating arrangement, suggesting a "water-mediated" knock-on mechanism. Later on, this view was challenged by results from molecular dynamics simulations that revealed a "direct" knock-on mechanism where ions are in direct contact. Using solid-state nuclear magnetic resonance techniques tailored to characterize the interaction between water molecules and the ion channel, we show here that the selectivity filter of a potassium channel is free of water under physiological conditions. Our results are fully consistent with the direct knock-on mechanism of ion conduction but contradict the previously proposed water-mediated knock-on mechanism
High-temperature superconductivity from fine-tuning of Fermi-surface singularities in iron oxypnictides
In the family of the iron-based superconductors, the FeAsO-type compounds
(with being a rare-earth metal) exhibit the highest bulk superconducting
transition temperatures () up to and thus hold
the key to the elusive pairing mechanism. Recently, it has been demonstrated
that the intrinsic electronic structure of SmFeCoAsO
() is highly nontrivial and consists of multiple
band-edge singularities in close proximity to the Fermi level. However, it
remains unclear whether these singularities are generic to the FeAsO-type
materials and if so, whether their exact topology is responsible for the
aforementioned record . In this work, we use angle-resolved
photoemission spectroscopy (ARPES) to investigate the inherent electronic
structure of the NdFeAsOF compound with a twice higher
. We find a similarly singular Fermi surface and
further demonstrate that the dramatic enhancement of superconductivity in this
compound correlates closely with the fine-tuning of one of the band-edge
singularities to within a fraction of the superconducting energy gap
below the Fermi level. Our results provide compelling evidence that the
band-structure singularities near the Fermi level in the iron-based
superconductors must be explicitly accounted for in any attempt to understand
the mechanism of superconducting pairing in these materials.Comment: Open access article available online at
http://www.nature.com/articles/srep1827
Energy transfer, pressure tensor and heating of kinetic plasma
Kinetic plasma turbulence cascade spans multiple scales ranging from
macroscopic fluid flow to sub-electron scales. Mechanisms that dissipate large
scale energy, terminate the inertial range cascade and convert kinetic energy
into heat are hotly debated. Here we revisit these puzzles using fully kinetic
simulation. By performing scale-dependent spatial filtering on the Vlasov
equation, we extract information at prescribed scales and introduce several
energy transfer functions. This approach allows highly inhomogeneous energy
cascade to be quantified as it proceeds down to kinetic scales. The pressure
work, , can
trigger a channel of the energy conversion between fluid flow and random
motions, which is a collision-free generalization of the viscous dissipation in
collisional fluid. Both the energy transfer and the pressure work are strongly
correlated with velocity gradients.Comment: 28 pages, 10 figure
Observation of Weyl nodes in robust type-II Weyl semimetal WP2
Distinct to type-I Weyl semimetals (WSMs) that host quasiparticles described
by the Weyl equation, the energy dispersion of quasiparticles in type-II WSMs
violates Lorentz invariance and the Weyl cones in the momentum space are
tilted. Since it was proposed that type-II Weyl fermions could emerge from
(W,Mo)Te2 and (W,Mo)P2 families of materials, a large numbers of experiments
have been dedicated to unveil the possible manifestation of type-II WSM, e.g.
the surface-state Fermi arcs. However, the interpretations of the experimental
results are very controversial. Here, using angle-resolved photoemission
spectroscopy supported by the first-principles calculations, we probe the
tilted Weyl cone bands in the bulk electronic structure of WP2 directly, which
are at the origin of Fermi arcs at the surfaces and transport properties
related to the chiral anomaly in type-II WSMs. Our results ascertain that due
to the spin-orbit coupling the Weyl nodes originate from the splitting of
4-fold degenerate band-crossing points with Chern numbers C = 2 induced by
the crystal symmetries of WP2, which is unique among all the discovered WSMs.
Our finding also provides a guiding line to observe the chiral anomaly which
could manifest in novel transport properties.Comment: 13 pages, 3 figure
The Strong Decay Patterns of the Exotic Hybrid Mesons
We calculate the coupling constants of the decay modes
within the framework of the light-cone QCD sum rule. Then we calculate the
partial width of these decay channels, which differ greatly from the existing
calculations using phenomenological models. For the isovector state,
the dominant decay modes are . For its isoscalar partner, its
dominant decay mode is . We also discuss the possible search of the
state at BESIII, for example through the decay chains or where can
be reconstructed through the decay modes
or . Hopefully the present work will be helpful to the
experimental establishment of the hybrid meson.Comment: 14 pages, 10 figure
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