Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Natural Negative-Refractive-Index Materials   Our calculation shows that negative refractive index (NRI), which was known
to exist only in metamaterials in the past, can be found in Dirac semimetals
(DSM). Electrons in DSM have zero effective mass and hence the system carries
no nominal energy scale. Therefore, unlike those of ordinary materials, the
electromagnetic responses of the electrons in DSM will not be overwhelmed by
the physical effects related to electron mass. NRI is induced by the
combination of the quantum effect of vacuum polarization and its finite
temperature correction which is proportional to $T^4$ at low temperature. It is
a phenomenon of resonance between the incident light and the unique structure
of Dirac cones, which allows numerous states to participate in electron-hole
pair production excited by the incident light with similar dispersion relation
to that of Dirac cones. NRI phenomenon of DSM manifests in an extensive range
of photon frequency and wave number and can be observed around giga Hertz range
at temperature slightly higher than room temperature.

11
The categories of this paper are: hep-lat
17
The categories of this paper are: q-fin
36
The categories of this paper are: cmp-lg
30
The categories of this paper are: q-alg
35
The categories of this paper are: mtrl-th
18
The categories of this paper are: econ
22
The categories of this paper are: chao-dyn
9
The categories of this paper are: nlin
28
The categories of this paper are: alg-geom
23
The categories of this paper are: solv-int
25
The categories of this paper are: patt-sol
21
The categories of this paper are: adap-org
27
The categories of this paper are: comp-gas
0
The categories of this paper are: hep-ph
34
The categories of this paper are: chem-ph
1
The categories of this paper are: cs