Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Selection rules and dynamic magnetoelectric effect of the spin waves in
  multiferroic BiFeO$_3$   We report the magnetic field dependence of the THz absorption and
non-reciprocal directional dichroism spectra of BiFeO$_3$ measured on the three
principal crystal cuts for fields applied along the three principal directions
of each cut. From the systematic study of the light polarization dependence we
deduced the optical selection rules of the spin-wave excitations. Our THz data,
combined with small-angle neutron scattering results showed that i) an in-plane
magnetic field rotates the $\mathbf{q}$ vectors of the cycloids perpendicular
to the magnetic field, and ii) the selection rules are mostly determined by the
orientation of the $\mathbf{q}$ vector with respect to the electromagnetic
fields. We observed a magnetic field history dependent change in the strength
and the frequency of the spin-wave modes, which we attributed to the change of
the orientation and the length of the cycloidal $\mathbf{q}$ vector,
respectively. Finally, we compared our experimental data with the results of
linear spin-wave theory that reproduces the magnetic field dependence of the
spin-wave frequencies and most of the selection rules, from which we identified
the spin-polarization coupling terms relevant for the optical magnetoelectric
effect.

29
The categories of this paper are: funct-an
5
The categories of this paper are: gr-qc
28
The categories of this paper are: alg-geom
10
The categories of this paper are: q-bio
34
The categories of this paper are: chem-ph
7
The categories of this paper are: hep-th
23
The categories of this paper are: solv-int
3
The categories of this paper are: physics
12
The categories of this paper are: nucl-th
18
The categories of this paper are: econ
1
The categories of this paper are: cs
26
The categories of this paper are: bayes-an
19
The categories of this paper are: eess
8
The categories of this paper are: hep-ex
6
The categories of this paper are: astro-ph
13
The categories of this paper are: quant-ph