5,775 research outputs found
Effects of Rashba spin-orbit coupling and a magnetic field on a polygonal quantum ring
Using standard quantum network method, we analytically investigate the effect
of Rashba spin-orbit coupling (RSOC) and a magnetic field on the spin transport
properties of a polygonal quantum ring. Using Landauer-Buttiker formula, we
have found that the polarization direction and phase of transmitted electrons
can be controlled by both the magnetic field and RSOC. A device to generate a
spin-polarized conductance in a polygon with an arbitrary number of sides is
discussed. This device would permit precise control of spin and selectively
provide spin filtering for either spin up or spin down simply by interchanging
the source and drain
Quantum spin Hall effect induced by electric field in silicene
We investigate the transport properties in a zigzag silicene nanoribbon in
the presence of an external electric field. The staggered sublattice potential
and two kinds of Rashba spin-orbit couplings can be induced by the external
electric field due to the buckled structure of the silicene. A bulk gap is
opened by the staggered potential and gapless edge states appear in the gap by
tuning the two kinds of Rashba spin-orbit couplings properly. Furthermore, the
gapless edge states are spin-filtered and are insensitive to the non-magnetic
disorder. These results prove that the quantum spin Hall effect can be induced
by an external electric field in silicene, which may have certain practical
significance in applications for future spintronics device.Comment: 4 pages, 5 figure
Intertwined dipolar and multipolar order in the triangular-lattice magnet TmMgGaO
A phase transition is often accompanied by the appearance of an order
parameter and symmetry breaking. Certain magnetic materials exhibit exotic
hidden-order phases, in which the order parameters are not directly accessible
to conventional magnetic measurements. Thus, experimental identification and
theoretical understanding of a hidden order are difficult. Here we combine
neutron scattering and thermodynamic probes to study the newly discovered
rare-earth triangular-lattice magnet TmMgGaO. Clear magnetic Bragg peaks at
K points are observed in the elastic neutron diffraction measurements. More
interesting, however, is the observation of sharp and highly dispersive spin
excitations that cannot be explained by a magnetic dipolar order, but instead
is the direct consequence of the underlying multipolar order that is "hidden"
in the neutron diffraction experiments. We demonstrate that the observed
unusual spin correlations and thermodynamics can be accurately described by a
transverse field Ising model on the triangular lattice with an intertwined
dipolar and ferro-multipolar order.Comment: Published versio
An hourglass model for the flare of HST-1 in M87
To explain the multi-wavelength light curves (from radio to X-ray) of HST-1
in the M87 jet, we propose an hourglass model that is a modified two-zone
system of Tavecchio & Ghisellini (hereafter TG08): a slow hourglass-shaped or
Laval nozzle-shaped layer connected by two revolving exponential surfaces
surrounding a fast spine, through which plasma blobs flow. Based on the
conservation of magnetic flux, the magnetic field changes along the axis of the
hourglass. We adopt the result of TG08---the high-energy emission from GeV to
TeV can be produced through inverse Compton by the two-zone system, and the
photons from radio to X-ray are mainly radiated by the fast inner zone system.
Here, we only discuss the light curves of the fast inner blob from radio to
X-ray. When a compressible blob travels down the axis of the first bulb in the
hourglass, because of magnetic flux conservation, its cross section experiences
an adiabatic compression process, which results in particle acceleration and
the brightening of HST-1. When the blob moves into the second bulb of the
hourglass, because of magnetic flux conservation, the dimming of the knot
occurs along with an adiabatic expansion of its cross section. A similar broken
exponential function could fit the TeV peaks in M87, which may imply a
correlation between the TeV flares of M87 and the light curves from radio to
X-ray in HST-1. The Very Large Array (VLA) 22 GHz radio light curve of HST-1
verifies our prediction based on the model fit to the main peak of the VLA 15
GHz radio light curve.Comment: 14 pages, 2 figures, accepted for publication in A
Temporally Constrained Group Sparse Learning for Longitudinal Data Analysis in Alzheimer's Disease
Sparse learning has been widely investigated for analysis of brain images to assist the diagnosis of Alzheimer’s disease (AD) and its prodromal stage, i.e., mild cognitive impairment (MCI). However, most existing sparse learning-based studies only adopt cross-sectional analysis methods, where the sparse model is learned using data from a single time-point. Actually, multiple time-points of data are often available in brain imaging applications, which can be used in some longitudinal analysis methods to better uncover the disease progression patterns. Accordingly, in this paper we propose a novel temporally-constrained group sparse learning method aiming for longitudinal analysis with multiple time-points of data. Specifically, we learn a sparse linear regression model by using the imaging data from multiple time-points, where a group regularization term is first employed to group the weights for the same brain region across different time-points together. Furthermore, to reflect the smooth changes between data derived from adjacent time-points, we incorporate two smoothness regularization terms into the objective function, i.e., one fused smoothness term which requires that the differences between two successive weight vectors from adjacent time-points should be small, and another output smoothness term which requires the differences between outputs of two successive models from adjacent time-points should also be small. We develop an efficient optimization algorithm to solve the proposed objective function. Experimental results on ADNI database demonstrate that, compared with conventional sparse learning-based methods, our proposed method can achieve improved regression performance and also help in discovering disease-related biomarkers
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