Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: EPR study of superconductors   Historical review on the studies of the electron paramagnetic resonance in
superconductors performed in the period from 1970 to 1990 at the Kazan Physical
Technical Institute of the Russian Academy of Sciences (group of Dr. E. G.
Kharakhash'yan) in collaboration with Kazan State University (group of Prof. B.
I. Kochelaev) and with the Institute for Physical Problems of Russian Academy
of Sciences (group of Prof. N. E. Alekseevskii) is presented. We have observed
for first time electron paramagnetic resonance of impurities in a type II
superconductor; found indication for a long-range exchange interaction between
magnetic impurities arising due to the superconducting correlations; observed
the magnetic ordering of impurities in the superconducting state; and, finally,
we found one of the first evidences for heterogeneity of the 1:2:3 high-$T_c$
superconductor which is its natural property

18
The categories of this paper are: econ
22
The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
7
The categories of this paper are: hep-th
8
The categories of this paper are: hep-ex
26
The categories of this paper are: bayes-an
29
The categories of this paper are: funct-an
20
The categories of this paper are: acc-phys
34
The categories of this paper are: chem-ph
15
The categories of this paper are: nucl-ex
36
The categories of this paper are: cmp-lg
27
The categories of this paper are: comp-gas
11
The categories of this paper are: hep-lat
30
The categories of this paper are: q-alg
25
The categories of this paper are: patt-sol
16
The categories of this paper are: stat