556 research outputs found
Topological Materials: Weyl Semimetals
Topological insulators and topological semimetals are both new classes of
quantum materials, which are characterized by surface states induced by the
topology of the bulk band structure. Topological Dirac or Weyl semimetals show
linear dispersion round nodes, termed the Dirac or Weyl points, as the
three-dimensional analogue of graphene. We review the basic concepts and
compare these topological states of matter from the materials perspective with
a special focus on Weyl semimetals. The TaAs family is the ideal materials
class to introduce the signatures of Weyl points in a pedagogical way, from
Fermi arcs to the chiral magneto-transport properties, followed by the hunting
for the type-II Weyl semimetals in WTe2, MoTe2 and related compounds. Many
materials are members of big families and topological properties can be tuned.
As one example, we introduce the multifuntional topological materials, Heusler
compounds, in which both topological insulators and magnetic Weyl semimetals
can be found. Instead of a comprehensive review, this article is expected to
serve as a helpful introduction and summary by taking a snapshot of the quickly
expanding field.Comment: 19 pages, 7 figures, an invited review article for Annual Review of
Condensed Matter Physic
Berry-curvatures and anomalous Hall effect in Heusler compounds
Berry curvatures are computed for a set of Heusler compounds using density
functional (DF) calculations and the wave functions that DF provide. The
anomalous Hall conductivity is obtained from the Berry curvatures. It is
compared with experimental values in the case of CoCrAl and CoMnAl. A
notable trend cannot be seen but the range of values is quite enormous. The
results for the anomalous Hall conductivities and their large variations can be
qualitatively understood by means of the band structure and the Fermi-surface
topology
Non-collinear Antiferromagnets and the Anomalous Hall Effect
The anomalous Hall effect is investigated theoretically by means of density
functional calculations for the non-collinear antiferromagnetic order of the
hexagonal compounds MnGe and MnSn using various planar triangular
magnetic configurations as well as unexpected non-planar configurations. The
former give rise to anomalous Hall conductivities (AHC) that are found to be
extremely anisotropic. For the planar cases the AHC is connected with
Weyl-points in the energy-band structure, which are described in detail. If
this case were observable in MnGe, a large AHC of about 900 S/cm should be
expected. However, in MnGe it is the non-planar configuration that is
energetically favored, in which case it gives rise to an AHC of 100 S/cm. The
non-planar configuration allows a quantitative evaluation of the topological
Hall effect that is seen to determine this value of the conductivity to a large
extent. For MnSn it is the planar configurations that are predicted to be
observable. In this case the AHC can be as large as 250 S/cm.Comment: 5 pages, 7 figure
Language Processing Evidence for Linguistic Structure
This article illustrates how data from language processing
experiments might bear on theoretical linguistic issues and controversies.
Based on the results from real-time processing studies of subject raising vs.
subject control structures and successive-cyclic wh-movement in English,
and of long-distance scrambling in Japanese, this article examines how
language processing data can help shed light on the nature of the linguistic
representations of different types of non-canonically ordered sentences.Este artículo ilustra cómo datos provenientes de experimentos de
procesamiento del lenguaje pueden arrojar luz sobre algunos temas (algunos
controvertidos) relacionados con la teoría lingüística. Basado en los
resultados de estudios de procesamiento a tiempo real de estructuras de
ascenso de sujeto vs. estructuras de control de sujeto y movimiento qusucesivamente
cíclico en inglés, así como desplazamiento libre (scrambling)
de larga distancia en japonés, este artículo examina cómo los datos de
procesamiento del lenguaje pueden ayudar a dilucidar la naturaleza de las
representaciones lingüísticas de distintos tipos de oraciones ordenadas de
forma no canónica.Este artigo ilustra de que forma dados experimentais do
processamento de linguagem podem ser suportados por questões
linguísticas teóricas e por controvérsias. Com base nos resultados de
processamento em tempo real em estudos de subida de sujeito vs. estruturas
de controlo de sujeito e movimento wh- sucessivo cíclico em inglês, e de
scrambling de longa distância em japonês, este artigo analisa a forma como os
dados de processamento de linguagem podem ajudar a esclarecer a natureza
das representações linguísticas dos diferentes tipos de orações nãocanónicas
A large energy-gap oxide topological insulator based on the superconductor BaBiO3
Mixed-valent perovskite oxides based on BaBiO3 (BBO) are, like cuperates,
well-known high-Tc superconductors. Recent ab inito calculations have assigned
the high-Tc superconductivity to a correlation-enhanced electron--phonon
coupling mechanism, stimulating the prediction and synthesis of new
superconductor candidates among mixed-valent thallium perovskites. Existing
superconductivity has meant that research has mainly focused on hole-doped
compounds, leaving electron-doped compounds relatively unexplored. Here we
demonstrate through ab inito calculations that BBO emerges as a topological
insulator (TI) in the electron-doped region, where the spin-orbit coupling
(SOC) effect is significant. BBO exhibits the largest topological energy gap of
0.7 eV among currently known TI materials, inside which Dirac-type topological
surface states (TSSs) exit. As the first oxide TI, BBO is naturally stable
against surface oxidization and degrading, different from chalcoginide TIs. An
extra advantage of BBO lies in its ability to serve an interface between the
TSSs and the superconductor for the realization of Majorana Fermions
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
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