Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Experimental observation of bulk nodal lines and electronic surface
  states in ZrB$_2$   Topological nodal-line semimetals are characterized by the line-contact bulk
band crossings and the topological surface states. Breaking certain protecting
symmetry turns this system into a Dirac semimetal or Weyl semimetal that hosts
zero-dimensional isolated nodal points. Recent advances in band theory
predicted a topological nodal-line semimetal state possessing a new type of
nodal line in AlB$_2$-type diborides. Here, we report an experimental
realization of nodal-line fermions and associated surface states near the Fermi
energy in ZrB$_2$ by angle-resolved photoemission spectroscopy combined with
first-principles calculations. The Dirac nodal lines in ZrB$_2$ wind into two
groups of nodal rings, which are linked together along the $\Gamma$-$K$
direction. We further observe a distinct surface state connecting to each nodal
line, indicative of the nontrivial topological nature of the bulk nodal lines.
Therefore, our results provide convincing experimental evidence of the
nodal-line semimetal states in ZrB$_2$ both in the bulk and on the surface,
suggesting ZrB$_2$ as a remarkable platform for discovering unique phenomena
induced by nodal-line fermions.

16
The categories of this paper are: stat
14
The categories of this paper are: math-ph
36
The categories of this paper are: cmp-lg
5
The categories of this paper are: gr-qc
19
The categories of this paper are: eess
26
The categories of this paper are: bayes-an
23
The categories of this paper are: solv-int
31
The categories of this paper are: ao-sci
11
The categories of this paper are: hep-lat
30
The categories of this paper are: q-alg
10
The categories of this paper are: q-bio
20
The categories of this paper are: acc-phys
18
The categories of this paper are: econ
8
The categories of this paper are: hep-ex
1
The categories of this paper are: cs
35
The categories of this paper are: mtrl-th