74,918 research outputs found
Graphene-like Dirac states and Quantum Spin Hall Insulators in the square-octagonal MX2 (M=Mo, W; X=S, Se, Te) Isomers
We studied the square-octagonal lattice of the transition metal
dichalcogenide MX (with =Mo, W; =S, Se and Te), as an isomer of the
normal hexagonal compound of MX. By band structure calculations, we observe
the graphene-like Dirac band structure in a rectangular lattice of MX with
nonsymmorphic space group symmetry. Two bands with van Hove singularity points
cross each at the Fermi energy, leading to two Dirac cones that locates at
opposite momenta. Spin-orbit coupling can open a nontrivial gap at these Dirac
points and induce the quantum spin Hall (QSH) phase, the 2D topological
insulator. Here, square-octagonal MX structures realize the interesting
graphene physics, such as Dirac bands and QSH effect, in the transition metal
dichalcogenides.Comment: 4 pages, 3 figures, 1 Tabl
The Berry curvature dipole in Weyl semimetal materials: an ab initio study
Noncentrosymmetric metals are anticipated to exhibit a photocurrent in
the nonlinear optical response caused by the Berry curvature dipole in momentum
space. Weyl semimetals (WSMs) are expected to be excellent candidates for
observing these nonlinear effects because they carry a large Berry curvature
concentrated in small regions, i.e., near the Weyl points. We have implemented
the semiclassical Berry curvature dipole formalism into an scheme
and investigated the second-order nonlinear response for two representative
groups of materials: the TaAs-family type-I WSMs and MoTe-family type-II
WSMs. Both types of WSMs exhibited a Berry curvature dipole, in which type-II
Weyl points are usually superior to the type-I because of the strong tilt.
Corresponding nonlinear susceptibilities in several materials promise a
nonlinear Hall effect in the field limit, which is within the
experimentally detectable range.Comment: 6 pages, 4 figures and 1 tabl
Topological surface states and Fermi arcs of the noncentrosymmetric Weyl semimetals TaAs, TaP, NbAs, and NbP
Very recently the topological Weyl semimetal (WSM) state was predicted in the
noncentrosymmetric compounds TaAs, TaP, NbAs, and NbP and soon led to
photoemission and transport experiments to verify the presumed topological
properties such as Fermi arcs (unclosed Fermi surfaces) and the chiral anomaly.
In this work, we have performed fully \textit{ab initio} calculations of the
surface band structures of these four WSM materials and revealed the Fermi arcs
with spin-momentum-locked spin texture. On the (001) polar surface, the shape
of the Fermi surface depends sensitively on the surface terminations (cations
or anions), although they exhibit the same topology with arcs. The anion (P or
As) terminated surfaces are found to fit recent photoemission measurements
well. Such surface potential dependence indicates that the shape of the Fermi
surface can be manipulated by depositing guest species (such as K atoms), as we
demonstrate. On the polar surface of a WSM without inversion symmetry,
Rashba-type spin polarization naturally exists in the surface states and leads
to strong spin texture. By tracing the spin polarization of the Fermi surface,
we can also distinguish Fermi arcs from trivial Fermi circles. The four
compounds NbP, NbAs, TaP, and TaAs present an increasing amplitude of
spin-orbit coupling (SOC) in the band structure. By comparing their surface
states, we reveal the evolution of topological Fermi arcs from the
spin-degenerate Fermi circle to spin-split arcs when the SOC increases from
zero to a finite value. Our work will help us understand the complicated
surface states of WSMs and allow us to manipulate them, especially for future
spin-revolved photoemission and transport experiments.Comment: This manuscript has been submitted to Physical Review B on 22 Jul.
201
DTD level authorization in XML documents with usage control
[Summary]: In recent years an increasing amount of semi-structured data has become important to humans and programs. XML promoted by the World Wide Web Consortium (W3C) is rapidly emerging as the new standard language for semi-structured data representation and exchange on the Internet. XML documents may contain private information that cannot be shared by all user communities. So securing XML data is becoming increasingly important and several approaches have been designed to protect information in a website. However, these approaches typically are used at file system level, rather than for the data in XML documents. Usage control has been considered as the next generation access control model with distinguishing properties of decision continuity. Usage control enables finer-grained control
over usage of digital objects than that of traditional access control policies and models.
In this paper, we present a usage control model to protect
information distributed on the web, which allows the access
restrictions directly at DTD-level and XML document-level.
Finally, comparisons with related works are analysed
Topological invariants for holographic semimetals
We study the behavior of fermion spectral functions for the holographic
topological Weyl and nodal line semimetals. We calculate the topological
invariants from the Green functions of both holographic semimetals using the
topological Hamiltonian method, which calculates topological invariants of
strongly interacting systems from an effective Hamiltonian system with the same
topological structure. Nontrivial topological invariants for both systems have
been obtained and the presence of nontrivial topological invariants further
supports the topological nature of the holographic semimetals.Comment: 39 pages, 11 figures, 1 table; v2: match published versio
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