Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Effect of strain and oxygen vacancies on the structure of 180^o
  ferroelectric domain walls in PbTiO_3
31
The categories of this paper are: ao-sci
30
The categories of this paper are: q-alg
23
The categories of this paper are: solv-int
15
The categories of this paper are: nucl-ex
36
The categories of this paper are: cmp-lg
11
The categories of this paper are: hep-lat
13
The categories of this paper are: quant-ph
3
The categories of this paper are: physics
1
The categories of this paper are: cs
22
The categories of this paper are: chao-dyn
14
The categories of this paper are: math-ph
21
The categories of this paper are: adap-org
9
The categories of this paper are: nlin
20
The categories of this paper are: acc-phys
33
The categories of this paper are: plasm-ph
2
The categories of this paper are: math