Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Physical Principles of the Amplification of Electromagnetic Radiation
  Due to Negative Electron Masses in a Semiconductor Superlattice   In a superlattice placed in crossed electric and magnetic fields, under
certain conditions, the inversion of electron population can appear at which
the average energy of electrons is above the middle of the miniband and the
effective mass of the electron is negative. This is the implementation of the
negative effective mass amplifier and generator (NEMAG) in the superlattice. It
can result in the amplification and generation of terahertz radiation even in
the absence of negative differential conductivity.

20
The categories of this paper are: acc-phys
2
The categories of this paper are: math
29
The categories of this paper are: funct-an
3
The categories of this paper are: physics
24
The categories of this paper are: dg-ga
22
The categories of this paper are: chao-dyn
12
The categories of this paper are: nucl-th
34
The categories of this paper are: chem-ph
27
The categories of this paper are: comp-gas
28
The categories of this paper are: alg-geom
1
The categories of this paper are: cs
36
The categories of this paper are: cmp-lg
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The categories of this paper are: econ
25
The categories of this paper are: patt-sol
11
The categories of this paper are: hep-lat
5
The categories of this paper are: gr-qc