Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Infrared study of the spin reorientation transition and its reversal in
  the superconducting state in underdoped Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$
  _{2} $   With infrared spectroscopy we investigated the spin-reorientation transition
from an orthorhombic antiferromagnetic (o-AF) to a tetragonal AF (t-AF) phase
and the reentrance of the o-AF phase in the superconducting state of underdoped
Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $. In agreement with the predicted
transition from a single-$\mathbf{Q}$ to a double-$\mathbf{Q}$ AF structure, we
found that a distinct spin density wave (SDW) develops in the t-AF phase. The
pair breaking peak of this SDW acquires much more low-energy spectral weight
than the one in the o-AF state which indicates that it competes more strongly
with superconductivity. We also observed additional phonon modes in the t-AF
phase which likely arise from a Brillouin-zone folding that is induced by the
double-$\mathbf{Q}$ magnetic structure with two Fe sublattices exhibiting
different magnitudes of the magnetic moment.

31
The categories of this paper are: ao-sci
12
The categories of this paper are: nucl-th
29
The categories of this paper are: funct-an
25
The categories of this paper are: patt-sol
6
The categories of this paper are: astro-ph
10
The categories of this paper are: q-bio
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex
35
The categories of this paper are: mtrl-th
26
The categories of this paper are: bayes-an
13
The categories of this paper are: quant-ph
33
The categories of this paper are: plasm-ph
14
The categories of this paper are: math-ph
2
The categories of this paper are: math
3
The categories of this paper are: physics
7
The categories of this paper are: hep-th