Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Optical probing of spin dynamics of two-dimensional and bulk electrons
  in a GaAs/AlGaAs heterojunction system
7
The categories of this paper are: hep-th
32
The categories of this paper are: atom-ph
11
The categories of this paper are: hep-lat
33
The categories of this paper are: plasm-ph
2
The categories of this paper are: math
19
The categories of this paper are: eess
6
The categories of this paper are: astro-ph
37
The categories of this paper are: supr-con
12
The categories of this paper are: nucl-th
26
The categories of this paper are: bayes-an
3
The categories of this paper are: physics
31
The categories of this paper are: ao-sci
18
The categories of this paper are: econ
17
The categories of this paper are: q-fin
22
The categories of this paper are: chao-dyn
16
The categories of this paper are: stat