Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Effect of pressure on the structural phase transition and
  superconductivity in (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ ($x = 0$ and 0.45) single
  crystals   The effects of pressure up to $\sim 20$ kbar, on the structural phase
transition of lightly Sn-doped BaFe$_2$As$_2$, as well as on the
superconducting transition temperature and upper critical field of
(Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ single crystals have been studied. Both
transitions decrease with pressure in an almost linear fashion. Under pressure,
the upper critical field curve, $H_{c2}(T)$, shifts down in temperature to
follow the zero field $T_c$ with very little change in slope.

13
The categories of this paper are: quant-ph
21
The categories of this paper are: adap-org
6
The categories of this paper are: astro-ph
23
The categories of this paper are: solv-int
16
The categories of this paper are: stat
0
The categories of this paper are: hep-ph
36
The categories of this paper are: cmp-lg
18
The categories of this paper are: econ
5
The categories of this paper are: gr-qc
11
The categories of this paper are: hep-lat
2
The categories of this paper are: math
29
The categories of this paper are: funct-an
30
The categories of this paper are: q-alg
37
The categories of this paper are: supr-con
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The categories of this paper are: patt-sol
20
The categories of this paper are: acc-phys