3,448 research outputs found
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.
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Prediction of Near-Room-Temperature Quantum Anomalous Hall Effect on Honeycomb Materials
Recently, this long-sought quantum anomalous Hall effect was realized in the
magnetic topological insulator. However, the requirement of an extremely low
temperature (approximately 30 mK) hinders realistic applications. Based on
\textit{ab-initio} band structure calculations, we propose a quantum anomalous
Hall platform with a large energy gap of 0.34 and 0.06 eV on honeycomb lattices
comprised of Sn and Ge, respectively. The ferromagnetic order forms in one
sublattice of the honeycomb structure by controlling the surface
functionalization rather than dilute magnetic doping, which is expected to be
visualized by spin polarized STM in experiment. Strong coupling between the
inherent QSH state and ferromagnetism results in considerable exchange
splitting and consequently an FM insulator with a large energy gap. The
estimated mean-field Curie temperature is 243 and 509 K for Sn and Ge lattices,
respectively. The large energy gap and high Curie temperature indicate the
feasibility of the QAH effect in the near-room-temperature and even
room-temperature regions.Comment: 6 pages, 4 figures and 1 tabl
Ab initio study of topological surface states of strained HgTe
The topological surface states of mercury telluride (HgTe) are studied by ab
initio calculations assuming different strains and surface terminations. For
the Te-terminated surface, a single Dirac cone exists at the point.
The Dirac point shifts up from the bulk valence bands into the energy gap when
the substrate-induced strain increases. At the experimental strain value
(0.3%), the Dirac point lies slightly below the bulk valence band maximum. A
left-handed spin texture was observed in the upper Dirac cone, similar to that
of the BiSe-type topological insulator. For the Hg-terminated surface,
three Dirac cones appear at three time-reversal-invariant momenta, excluding
the point, with nontrivial spin textures.Comment: 4 pages, 3 figure
Hidden type-II Weyl points in the Weyl semimetal NbP
As one of Weyl semimetals discovered recently, NbP exhibits two groups of
Weyl points with one group lying inside the plane and the other group
staying away from this plane. All Weyl points have been assumed to be type-I,
for which the Fermi surface shrinks into a point as the Fermi energy crosses
the Weyl point. In this work, we have revealed that the second group of Weyl
points are actually type-II, which are found to be touching points between the
electron and hole pockets in the Fermi surface. Corresponding Weyl cones are
strongly tilted along a line approximately off the axis in the
(or ) plane, violating the Lorentz symmetry but still
giving rise to Fermi arcs on the surface. Therefore, NbP exhibits both type-I
( plane) and type-II ( plane) Weyl points.Comment: 5 pages and 4 figure
Developing a SCORM-based U-Learning LMS System
An integrated content and learning management system (LMSs) which has the characteristic of being ubiquitous is the most essential component of U-Learning. However, most modern learning management systems have different architectures, which makes it’s difficult to integrate the numerous learning resources, and reusability is hard to achieve. Otherwise, most learning resources read on mobile platforms are still restricted to electric books or digital learning materials. So, it’s not easy to manage the learning progress and immediately information providing or interactions between learners and instructors are impossible. A research is proposed in this paper to develop a SCORM compliant blended U-Learning LMS system, which emphasizes the content compilation, content packaging and the implement of SCORM run-time environment to have learning materials being reusable and interoperable
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