80 research outputs found
Upper bound of a band complex
Band structure for a crystal generally consists of connected components in
energy-momentum space, known as band complexes. Here, we explore a fundamental
aspect regarding the maximal number of bands that can be accommodated in a
single band complex. We show that in principle a band complex can have no
finite upper bound for certain space groups. It means infinitely many bands can
entangle together, forming a connected pattern stable against
symmetry-preserving perturbations. This is demonstrated by our developed
inductive construction procedure, through which a given band complex can always
be grown into a larger one by gluing a basic building block to it. As a
by-product, we demonstrate the existence of arbitrarily large accordion type
band structures containing bands, with .Comment: 6 pages, 4 figure
Observation of Symmetry-Protected Dirac States in Nonsymmorphic -Antimonene
Two-dimensional (2D) Dirac states with linear band dispersion have attracted
enormous interest since the discovery of graphene. However, to date, 2D Dirac
semimetals are still very rare due to the fact that 2D Dirac states are
generally fragile against perturbations such as spin-orbit couplings.
Nonsymmorphic crystal symmetries can enforce the formation of Dirac nodes,
providing a new route to establishing symmetry-protected Dirac states in 2D
materials. Here we report the symmetry-protected Dirac states in nonsymmorphic
alpha-antimonene. The antimonene was synthesized by the method of molecular
beam epitaxy. Two Dirac cones with large anisotropy were observed by
angle-resolved photoemission spectroscopy. The Dirac state in alpha-antimonene
is of spin-orbit type in contrast to the spinless Dirac states in graphene. The
result extends the 'graphene' physics into a new family of 2D materials where
spin-orbit coupling is present.Comment: 4 figure
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