721 research outputs found
Zigzag graphene nanoribbons without inversion symmetry
Graphene on a substrate will suffer an inversion-symmetry-breaking (ISB)
lattice potential. Taking electron-electron interaction into account, we study
in this paper the possibility of half-metallicity and noncollinear (NC)
magnetic phase for graphene zigzag nanoribbons without inversion symmetry. At
half-filling it is found that half-metallic(HM) state can be achieved at an
intermediate value of the ISB potential due to its competition with the
electron-electron interaction. Away from half-filling, the phase diagrams of
doping versus ISB potential for different ribbon width are given, where the
regimes for the HM states and NC magnetic state are clearly indicated and
discussed. For ribbons with perfect edges, we predict a topological transition
between two HM states with different magnetic structures, which is accompanied
by an abrupt transition of electrical conductance along the ribbon from
to .Comment: 7 pages, 7 figure
Independent director network, agency costs and stock price crash risk
It is of great significance to improve the corporate governance structure
to study whether independent directors play the role of ‘vase’ in
the governance of listed companies. Based on the social network
theory, this article constructs the social network formed by interlocking
independent directors and examines the influence of independent
director network on stock price crash risk. The mechanism test
analyses the mediating effect of principal–agent problem and large
shareholder’s tunnelling on stock price crash risk. The empirical
research shows that the higher the network centrality of the company’s
independent directors, the lower the stock price crash risk.
The independent director network can restrain the company’s stock
price crash risk by reducing two types of agency costs. Further
research finds that the influence of independent director network on
stock price crash risk is more pronounced in companies with unreasonable
ownership structure, poor internal governance and weak
external supervision. The research conclusions have important implications
for listed companies to reduce the risk of stock price crash
and maintain the stability of the capitalmarket
Close-in Exoplanets as Candidates of Strange Quark Matter Objects
Since the true ground state of the hadrons may be strange quark matter (SQM),
pulsars may actually be strange stars rather than neutron stars. According to
this SQM hypothesis, strange planets can also stably exist. The density of
normal matter planets can hardly be higher than 30 g cm. As a result,
they will be tidally disrupted when its orbital radius is less than , or when the orbital period () is
less than . On the contrary, a strange planet can safely
survive even when it is very close to the host, due to its high density. The
feature can help us identify SQM objects. In this study, we have tried to
search for SQM objects among close-in exoplanets orbiting around pulsars.
Encouragingly, it is found that four pulsar planets (XTE J1807-294 b, XTE
J1751-305 b, PSR 0636 b, PSR J1807-2459A b) completely meet the criteria of
, and are thus good candidates for SQM planets.
The orbital periods of two other planets (PSR J1719+14 b and PSR J2051-0827 b)
are only slightly higher than the criteria. They could be regarded as potential
candidates. Additionally, we find that the periods of five white dwarf planets
(GP Com b, V396 Hya b, J1433 b, WD 0137-349 b, and SDSS J1411+2009 b) are less
than 0.1 days. We argue that they might also be SQM planets. It is further
found that the persistent gravitational wave emissions from at least three of
these close-in planetary systems are detectable to LISA. More encouragingly,
the advanced LIGO and Einstein Telescope are able to detect the gravitational
wave bursts produced by the merger events of such SQM planetary systems, which
will provide a unique test for the SQM hypothesis.Comment: 13 pages, 5 figures, 4 tables, comments and suggestions are welcom
Renal infarction resulting from traumatic renal artery dissection
Renal artery dissection may be caused by iatrogenic injury, trauma, underlying arterial diseases such as fibromuscular disease, atherosclerotic disease, or connective tissue disease. Radiological imaging may be helpful in detecting renal artery pathology, such as renal artery dissection. For patients with acute, isolated renal artery dissection, surgical treatment, endovascular management, or medical treatment have been considered effective measures to preserve renal function. We report a case of renal infarction that came about as a consequence of renal artery dissection
Low-energy Scattering of System and the Resonance-like Structure
In this paper, low-energy scattering of the meson
system is studied within L\"uscher's finite-size formalism using
twisted mass gauge field configurations. With three different pion mass values,
the -wave threshold scattering parameters, namely the scattering length
and the effective range , are extracted in channel. Our
results indicate that, in this particular channel, the interaction between the
two vector charmed mesons is weakly repulsive in nature hence do not support
the possibility of a shallow bound state for the two mesons, at least for the
pion mass values being studied. This study provides some useful information on
the nature of the newly discovered resonance-like structure
observed in various experiments.Comment: 11 pages, 6 figures. arXiv admin note: substantial text overlap with
arXiv:1403.131
CT-based lung motion differences in patients with usual interstitial pneumonia and nonspecific interstitial pneumonia
We applied quantitative CT image matching to assess the degree of motion in the idiopathic ILD such as usual interstitial pneumonia (UIP) and nonspecific interstitial pneumonia (NSIP). Twenty-one normal subjects and 42 idiopathic ILD (31 UIP and 11 NSIP) patients were retrospectively included. Inspiratory and expiratory CT images, reviewed by two experienced radiologists, were used to compute displacement vectors at local lung regions matched by image registration. Normalized three-dimensional and two-dimensional (dorsal-basal) displacements were computed at a sub-acinar scale. Displacements, volume changes, and tissue fractions in the whole lung and the lobes were compared between normal, UIP, and NSIP subjects. The dorsal-basal displacement in lower lobes was smaller in UIP patients than in NSIP or normal subjects (p = 0.03, p = 0.04). UIP and NSIP were not differentiated by volume changes in the whole lung or upper and lower lobes (p = 0.53, p = 0.12, p = 0.97), whereas the lower lobe air volume change was smaller in both UIP and NSIP than normal subjects (p = 0.02, p = 0.001). Regional expiratory tissue fractions and displacements showed positive correlations in normal and UIP subjects but not in NSIP subjects. In summary, lung motionography quantified by image registration-based lower lobe dorsal-basal displacement may be used to assess the degree of motion, reflecting limited motion due to fibrosis in the ILD such as UIP and NSIP
Efficient hybrid density functional calculation by deep learning
Hybrid density functional calculation is indispensable to accurate
description of electronic structure, whereas the formidable computational cost
restricts its broad application. Here we develop a deep equivariant neural
network method (named DeepH-hybrid) to learn the hybrid-functional Hamiltonian
from self-consistent field calculations of small structures, and apply the
trained neural networks for efficient electronic-structure calculation by
passing the self-consistent iterations. The method is systematically checked to
show high efficiency and accuracy, making the study of large-scale materials
with hybrid-functional accuracy feasible. As an important application, the
DeepH-hybrid method is applied to study large-supercell Moir\'{e} twisted
materials, offering the first case study on how the inclusion of exact exchange
affects flat bands in the magic-angle twisted bilayer graphene
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