Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Universal linear and nonlinear electrodynamics of the Dirac fluid   A general relation is derived between the linear and second-order nonlinear
ac conductivities of an electron system in the hydrodynamic regime of
frequencies below the interparticle scattering rate. The magnitude and
tensorial structure of the hydrodynamic nonlinear conductivity are shown to
differ from their counterparts in the more familiar kinetic regime of higher
frequencies. Due to universality of the hydrodynamic equations, the obtained
formulas are valid for systems with an arbitrary Dirac-like dispersion, ranging
from solid-state electron gases to free-space plasmas, either massive or
massless, at any temperature, chemical potential or space dimension.
Predictions for photon drag and second-harmonic generation in graphene are
presented as one application of this theory.

9
The categories of this paper are: nlin
14
The categories of this paper are: math-ph
30
The categories of this paper are: q-alg
16
The categories of this paper are: stat
24
The categories of this paper are: dg-ga
0
The categories of this paper are: hep-ph
13
The categories of this paper are: quant-ph
1
The categories of this paper are: cs
31
The categories of this paper are: ao-sci
27
The categories of this paper are: comp-gas
18
The categories of this paper are: econ
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex
5
The categories of this paper are: gr-qc
32
The categories of this paper are: atom-ph
20
The categories of this paper are: acc-phys