Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Zero field line in the magnetic spectra of negatively charged
  nitrogen-vacancy centers in diamond   The dependence of the luminescence of diamonds with negatively charged
nitrogen-vacancy centers (NV-) vs. applied magnetic field (magnetic spectrum)
was studied. A narrow line in zero magnetic field was discovered. The
properties of this line are considerably different from those of other narrow
magnetic spectrum lines. Its magnitude is weakly dependent of the orientation
of the single-crystal sample to the external magnetic field. This line is also
observed in a powdered sample. The shape of the line changes greatly when
excitation light polarization is varied. The magnitude of the line has a
non-linear relation to excitation light intensity. For low intensities this
dependence is close to a square law. To explain the mechanism giving rise to
this line in the magnetic spectrum, we suggest a model based on the
dipole-dipole interaction between different NV- centers.

7
The categories of this paper are: hep-th
8
The categories of this paper are: hep-ex
29
The categories of this paper are: funct-an
36
The categories of this paper are: cmp-lg
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
1
The categories of this paper are: cs
5
The categories of this paper are: gr-qc
16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
0
The categories of this paper are: hep-ph
27
The categories of this paper are: comp-gas
25
The categories of this paper are: patt-sol
31
The categories of this paper are: ao-sci
35
The categories of this paper are: mtrl-th
6
The categories of this paper are: astro-ph