Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On orbital angular momentum conservation in Brillouin light scattering
  within a ferromagnetic sphere   Magnetostatic modes supported by a ferromagnetic sphere have been known as
the Walker modes, each of which possesses an orbital angular momentum as well
as a spin angular momentum along a static magnetic field. The Walker modes with
non-zero orbital angular momenta exhibit topologically non-trivial spin
textures, which we call \textit{magnetic quasi-vortices}. Photons in optical
whispering gallery modes supported by a dielectric sphere possess orbital and
spin angular momenta forming \textit{optical vortices}. Within a ferromagnetic,
as well as dielectric, sphere, two forms of vortices interact in the process of
Brillouin light scattering. We argue that in the scattering there is a
selection rule that dictates the exchange of orbital angular momenta between
the vortices. The selection rule is shown to be responsible for the
experimentally observed nonreciprocal Brillouin light scattering.

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The categories of this paper are: q-fin
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The categories of this paper are: adap-org
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The categories of this paper are: solv-int
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The categories of this paper are: math
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The categories of this paper are: mtrl-th
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The categories of this paper are: funct-an
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The categories of this paper are: q-alg
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The categories of this paper are: eess
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The categories of this paper are: atom-ph
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The categories of this paper are: stat
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The categories of this paper are: hep-th
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The categories of this paper are: chem-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: nlin
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The categories of this paper are: supr-con
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The categories of this paper are: comp-gas