Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Discovery of a Three-dimensional Topological Dirac Semimetal, Na3Bi   Three-dimensional (3D) topological Dirac semimetals (TDSs) represent a novel
state of quantum matter that can be viewed as '3D graphene'. In contrast to
two-dimensional (2D) Dirac fermions in graphene or on the surface of 3D
topological insulators, TDSs possess 3D Dirac fermions in the bulk. The TDS is
also an important boundary state mediating numerous novel quantum states, such
as topological insulators, Weyl semi-metals, Axion insulators and topological
superconductors. By investigating the electronic structure of Na3Bi with angle
resolved photoemission spectroscopy, we discovered 3D Dirac fermions with
linear dispersions along all momentum directions for the first time.
Furthermore, we demonstrated that the 3D Dirac fermions in Na3Bi were protected
by the bulk crystal symmetry. Our results establish that Na3Bi is the first
model system of 3D TDSs, which can also serve as an ideal platform for the
systematic study of quantum phase transitions between rich novel topological
quantum states.

7
The categories of this paper are: hep-th
0
The categories of this paper are: hep-ph
16
The categories of this paper are: stat
33
The categories of this paper are: plasm-ph
22
The categories of this paper are: chao-dyn
30
The categories of this paper are: q-alg
29
The categories of this paper are: funct-an
5
The categories of this paper are: gr-qc
6
The categories of this paper are: astro-ph
23
The categories of this paper are: solv-int
34
The categories of this paper are: chem-ph
25
The categories of this paper are: patt-sol
13
The categories of this paper are: quant-ph
14
The categories of this paper are: math-ph
8
The categories of this paper are: hep-ex
3
The categories of this paper are: physics