187 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
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.
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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
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
Distinct magnetic gaps between antiferromagnetic and ferromagnetic orders driven by surface defects in the topological magnet MnBi2Te4
The magnetic topological insulator, MnBiTe, shows metallic behavior
at zero magnetic fields (antiferromagnetic phase, AFM) in thin film transport,
which coincides with gapless surface states observed by angle-resolved
photoemission spectroscopy, while it can become a Chern insulator at field
larger than 6 T (ferromagnetic phase, FM). Thus, the zero-field surface
magnetism was once speculated to be different from the bulk AFM phase. However,
recent magnetic force microscopy refutes this assumption by detecting
persistent AFM order on the surface. In this work, we propose a mechanism
related to surface defects that can rationalize these contradicting
observations in different experiments. We find that co-antisites (exchanging Mn
and Bi atoms in the surface van der Waals layer) can strongly suppress the
magnetic gap down to several meV in the AFM phase without violating the
magnetic order but preserve the magnetic gap in the FM phase. The different gap
sizes between AFM and FM phases are caused by the defect-induced surface charge
redistribution among top two van der Waals layers. This theory can be validated
by the position- and field-dependent gap in future surface spectroscopy
measurements. Our work suggests suppressing related defects in samples to
realize the quantum anomalous Hall insulator or axion insulator at zero fields.Comment: 5 pages, 4 figures+1 table. Supplemental material is availabl
Topological superconductivity at the edge of transition metal dichalcogenides
Time-reversal breaking topological superconductors are new states of matter
which can support Majorana zero modes at the edge. In this paper, we propose a
new realization of one-dimensional topological superconductivity and Majorana
zero modes. The proposed system consists of a monolayer of transition metal
dichalcogenides MX2 (M=Mo, W; X=S, Se) on top of a superconducting substrate.
Based on first-principles calculations, we show that a zigzag edge of the
monolayer MX2 terminated by metal atom M has edge states with strong spin-orbit
coupling and spontaneous magnetization. By proximity coupling with a
superconducting substrate, topological superconductivity can be induced at such
an edge. We propose NbS2 as a natural choice of substrate, and estimate the
proximity induced superconducting gap based on first-principles calculation and
low energy effective model. As an experimental consequence of our theory, we
predict that Majorana zero modes can be detected at the 120 degree corner of a
MX2 flake in proximity with a superconducting substrate
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