On the chordality of polynomial sets in triangular decomposition in top-down style

In this paper the chordal graph structures of polynomial sets appearing in triangular decomposition in top-down style are studied when the input polynomial set to decompose has a chordal associated graph. In particular, we prove that the associated graph of one specific triangular set computed in any algorithm for triangular decomposition in top-down style is a subgraph of the chordal graph of the input polynomial set and that all the polynomial sets including all the computed triangular sets appearing in one specific simply-structured algorithm for triangular decomposition in top-down style (Wang's method) have associated graphs which are subgraphs of the the chordal graph of the input polynomial set. These subgraph structures in triangular decomposition in top-down style are multivariate generalization of existing results for Gaussian elimination and may lead to specialized efficient algorithms and refined complexity analyses for triangular decomposition of chordal polynomial sets.Comment: 20 page

Saliency detection based on structural dissimilarity induced by image quality assessment model

The distinctiveness of image regions is widely used as the cue of saliency. Generally, the distinctiveness is computed according to the absolute difference of features. However, according to the image quality assessment (IQA) studies, the human visual system is highly sensitive to structural changes rather than absolute difference. Accordingly, we propose the computation of the structural dissimilarity between image patches as the distinctiveness measure for saliency detection. Similar to IQA models, the structural dissimilarity is computed based on the correlation of the structural features. The global structural dissimilarity of a patch to all the other patches represents saliency of the patch. We adopt two widely used structural features, namely the local contrast and gradient magnitude, into the structural dissimilarity computation in the proposed model. Without any postprocessing, the proposed model based on the correlation of either of the two structural features outperforms 11 state-of-the-art saliency models on three saliency databases.Comment: For associated source code, see https://github.com/yangli-xjtu/SD

Quantum dynamics of two capacitively coupled superconducting islands via Josephson junctions

In this paper, we consider a system consisting of two capacitively coupled superconducting islands via Josephson junctions. We show that it can be reduced to two coupling harmonic oscillators under certain conditions, and solved exactly in terms of a displacing transformation, a beam-splitter-like transformation, and a squeezing transformation. It is found that the system evolves by a rotated-squeezed-coherent state when the system is initially in a coherent state. Quantum dynamics of the Cooper pairs in the two superconducting islands is investigated. It is shown that the number of the Cooper pairs in the two islands evolves periodically.Comment: 4 pages, 1 figur

Electronic structure and optic absorption of phosphorene under strain

We studied the electronic structure and optic absorption of phosphorene (monolayer of black phosphorus) under strain. Strain was found to be a powerful tool for the band structure engineering. The in-plane strain in armchair or zigzag direction changes the effective masse components along both directions, while the vertical strain only has significant effect on the effective mass in the armchair direction. The band gap is narrowed by compressive in-plane strain and tensile vertical strain. Under certain strain configurations, the gap is closed and the energy band evolutes to the semi-Dirac type: the dispersion is linear in the armchair direction and is gapless quadratic in the zigzag direction. The band-edge optic absorption is completely polarized along the armchair direction, and the polarization rate is reduced when the photon energy increases. Strain not only changes the absorption edge, but also the absorption polarization.Comment: 5 pages, 5 figure

Numerical Study on Outflows in Seyfert Galaxies I: Narrow Line Region Outflows in NGC 4151

The origin of narrow line region (NLR) outflows remains unknown. In this paper, we explore the scenario in which these outflows are circumnuclear clouds driven by energetic accretion disk winds. We choose the well-studied nearby Seyfert galaxy NGC 4151 as an example. By performing 3D hydrodynamical simulations, we are able to reproduce the radial distributions of velocity, mass outflow rate and kinetic luminosity of NLR outflows in the inner 100 pc deduced from spatial resolved spectroscopic observations. The demanded kinetic luminosity of disk winds is about two orders of magnitude higher than that inferred from the NLR outflows, but is close to the ultrafast outflows (UFO) detected in X-ray spectrum and a few times lower than the bolometric luminosity of the Seyfert. Our simulations imply that the scenario is viable for NGC 4151. The existence of the underlying disk winds can be confirmed by their impacts on higher density ISM, e.g., shock excitation signs, and the pressure in NLR.Comment: 10 pages, 5 figures, accepted by Ap

Valley detection using a graphene gradual pn junction with spin-orbit coupling: an analytical conductance calculation

Graphene pn junction is the brick to build up variety of graphene nano-structures. The analytical formula of the conductance of graphene gradual pn junctions in the whole bipolar region has been absent up to now. In this paper, we analytically calculated that pn conductance with the spin-orbit coupling and stagger potential taken into account. Our analytical expression indicates that the energy gap causes the conductance to drop a constant value with respect to that without gap in a certain parameter region, and manifests that the curve of the conductance versus the stagger potential consists of two Gaussian peaks -- one valley contributes one peak. The latter feature allows one to detect the valley polarization without using double-interface resonant devices.Comment: 5 pages. 3 figure

Topologically trivial and nontrivial edge bands in graphene induced by irradiation

We proposed a minimal model to describe the Floquet band structure of two-dimensional materials with light-induced resonant inter-band transition. We applied it to graphene to study the band features caused by the light irradiation. Linearly polarized light induces pseudo gaps (gaps are functions of wavevector), and circularly polarized light causes real gaps on the quasi-energy spectrum. If the polarization of light is linear and along the longitudinal direction of zigzag ribbons, flat edge bands appear in the pseudo gaps, and if is in the lateral direction of armchair ribbons, curved edge bands can be found. For the circularly polarized cases, edge bands arise and intersect in the gaps of both types of ribbons. The edge bands induced by the circularly polarized light are helical and those by linearly polarized light are topologically trivial ones. The Chern number of the Floquet band, which reflects the number of pairs of helical edge bands in graphene ribbons, can be reduced into the winding number at resonance.Comment: 7 pages, 4 figure

Transport theory for electrical detection of the spin texture and spin-momentum locking of topological surface states

The surface states of three-dimensional topological insulators exhibit a helical spin texture with spin locked to momentum. To date, however, the direct all-electrical detection of the helical spin texture has remained elusive owing to the lack of necessary spin-sensitive measurements. We here provide a general theory for spin polarized transports of helical Dirac electrons through spin-polarized scanning tunneling microscopy (STM). It is found that different from conventional magnetic materials, the tunneling conductance through the TI surface acquires an extra component determined by the in-plane spin texture, exclusively associated with spin momentum locking. Importantly, this extra conductance unconventionally depends on the spatial azimuthal angle of the magnetized STM tip, which is never carried out in previous STM theory. By magnetically doping to break the symmetry of rotation and time reversal of the TI surface, we find that the measurement of the spatial resolved conductance can reconstruct the helical structure of spin texture. Furthermore, one can extract the SML angle if the in-plane magnetization is induced purely by the spin-orbit coupling of surface Dirac elections. Our theory offers an alternative way, rather than using angle resolved photoemission spectroscopy, to electrical identify the helical spin texture on TI surfaces.Comment: 6 pages, 4 figure

Spin helical states and spin transport of the line defect in silicene lattice

We investigated the electronic structure of a silicene-like lattice with a line defect under the consideration of spin-orbit coupling. In the bulk energy gap, there are defect related bands corresponding to spin helical states localized beside the defect line: spin-up electrons flow forward on one side near to the line defect and move backward on the other side, and vice verse for spin-down electrons. When the system is subjected to random distribution of spin-flipping scatterers, electrons suffer much less spin-flipped scattering when they transport along the line defect than in the bulk. An electric gate above the line defect can tune the spin-flipped transmission, which makes the line defect as a spin-controllable waveguide.Comment: 13 pages, 5 figure

Electronic transmission of a nanowire partly irradiated under terahertz electromagnetic field

We theoretically study the electronic transport of a nanowire partly irradiated under an external terahertz (THz) electromagnetic field. Although the electrons in the ballistic nanowires only suffer lateral collision with photons the reflection of electrons also takes place in this partly irradiated case. Using free-electron model and scattering matrix approach we showed that at resonance there exists a step decrement of 50 percent for the transmission probability as the amplitude of field increases to a certain volume. And the coherent structure of transmission for the system apparently appears when the field irradiate the middle part of nanowire only. This sensitive transmission property of the system may be used in the THz detection.Comment: Latex, 7 pages and 4 figure