Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Similarity of the 3.42 eV and near-band-edge 3.47 eV luminescence bands
  in GaN   We demonstrate that the 3.42 eV photoluminescence (PL) band in GaN is of the
same intrinsic origin as the near-edge $\sim$3.47 eV band, but arises from
regions of inversed polarity characterized by different strain and growth rate.
Two absorption edges are thermally detected at 0.35 K in nanocolumn structures,
exhibiting both bands. Micro-PL studies have shown similar temperature/power
behavior of these bands, with a competition in intensity in closely spaced
spots accompanied by alterations of exciton level ordering. Strain-induced
one-dimensional carrier confinement in small inversion domains likely explains
the discrete narrow lines observed between the bands.

31
The categories of this paper are: ao-sci
29
The categories of this paper are: funct-an
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The categories of this paper are: alg-geom
16
The categories of this paper are: stat
32
The categories of this paper are: atom-ph
7
The categories of this paper are: hep-th
21
The categories of this paper are: adap-org
3
The categories of this paper are: physics
17
The categories of this paper are: q-fin
9
The categories of this paper are: nlin
24
The categories of this paper are: dg-ga
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The categories of this paper are: supr-con
22
The categories of this paper are: chao-dyn
1
The categories of this paper are: cs
14
The categories of this paper are: math-ph
35
The categories of this paper are: mtrl-th