Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Spin pseudogap in the $S=\frac{1}{2}$ chain material Sr$_2$CuO$_3$ with
  impurities   The low energy magnetic excitation spectrum of the Heisenberg
antiferromagnetic $S = 1/2$ chain system Sr$_2$CuO$_3$ with Ni- and
Ca-impurities is studied by neutron spectroscopy. In all cases, a
defect-induced spectral pseudogap is observed and shown to scale
proportionately to the number of scattering centers in the spin chains.

6
The categories of this paper are: astro-ph
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The categories of this paper are: alg-geom
3
The categories of this paper are: physics
7
The categories of this paper are: hep-th
5
The categories of this paper are: gr-qc
18
The categories of this paper are: econ
1
The categories of this paper are: cs
32
The categories of this paper are: atom-ph
23
The categories of this paper are: solv-int
31
The categories of this paper are: ao-sci
37
The categories of this paper are: supr-con
15
The categories of this paper are: nucl-ex
27
The categories of this paper are: comp-gas
24
The categories of this paper are: dg-ga
26
The categories of this paper are: bayes-an
36
The categories of this paper are: cmp-lg