Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Nonsymmorphic-symmetry-protected hourglass Dirac loop, nodal line, and
  Dirac point in bulk and monolayer $X_3$SiTe$_6$ ($X=$ Ta, Nb)   Nonsymmorphic space group symmetries can generate exotic band-crossings in
topological metals and semimetals. Here, based on symmetry analysis and
first-principles calculations, we reveal rich band-crossing features in the
existing layered compounds Ta$_3$SiTe$_6$ and Nb$_3$SiTe$_6$, enabled by
nonsymmorphic symmetries. We show that in the absence of spin-orbit coupling
(SOC), these three-dimensional (3D) bulk materials possess accidental Dirac
loops and essential fourfold nodal lines. In the presence of SOC, there emerges
an hourglass Dirac loop---a fourfold degenerate nodal loop, on which each point
is a neck-point of an hourglass-type dispersion. We show that this interesting
type of band-crossing is protected and dictated by the nonsymmorphic space
group symmetries, and it gives rise to drumhead-like surface states.
Furthermore, we also investigate these materials in the monolayer form. We show
that these two-dimensional (2D) monolayers host nodal lines in the absence of
SOC, and the nodal lines transform to essential spin-orbit Dirac points when
SOC is included. Our work suggests a realistic material platform for exploring
the fascinating physics associated with nonsymmorphic band-crossings in both 3D
and 2D systems.

11
The categories of this paper are: hep-lat
8
The categories of this paper are: hep-ex
27
The categories of this paper are: comp-gas
5
The categories of this paper are: gr-qc
16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
9
The categories of this paper are: nlin
33
The categories of this paper are: plasm-ph
14
The categories of this paper are: math-ph
35
The categories of this paper are: mtrl-th
20
The categories of this paper are: acc-phys
1
The categories of this paper are: cs
32
The categories of this paper are: atom-ph
21
The categories of this paper are: adap-org
6
The categories of this paper are: astro-ph
0
The categories of this paper are: hep-ph