Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Identifying optical signatures of momentum-dark excitons in transition
  metal dichalcogenide monolayers   Transition metal dichalcogenide (TMD) monolayers (MLs) exhibit rich
photoluminescence spectra associated with interband optical transitions of
direct-gap semiconductors. Upon absorption of photons, direct excitons with
zero center-of-mass momentum are formed by photo-excited electrons in the
conduction band and the respective unoccupied states in the valence band of the
same valley. Different spin configurations of such momentum-direct excitons as
well as their charged counterparts provide a powerful platform for spin-valley
and microcavity physics in two-dimensional materials. The corresponding
spectral signatures, however, are insufficient to explain the main
characteristic peaks observed in the photoluminescence spectra of ML TMDs on
the basis of momentum-\textit{direct} excitons alone. Here, we show that the
notion of momentum-\textit{indirect} excitons is important for the
understanding of the versatile photoluminescence features. Taking into account
phonon-assisted radiative recombination pathways for electrons and holes from
dissimilar valleys, we interpret unidentified peaks in the emission spectra as
acoustic and optical phonon sidebands of momentum-dark excitons. Our approach
will facilitate the interpretation of optical, valley and spin phenomena in
TMDs arising from bright and dark exciton manifolds.

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The categories of this paper are: hep-ph
12
The categories of this paper are: nucl-th
17
The categories of this paper are: q-fin
33
The categories of this paper are: plasm-ph
34
The categories of this paper are: chem-ph
3
The categories of this paper are: physics
18
The categories of this paper are: econ
15
The categories of this paper are: nucl-ex
29
The categories of this paper are: funct-an
30
The categories of this paper are: q-alg
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The categories of this paper are: gr-qc
2
The categories of this paper are: math
28
The categories of this paper are: alg-geom
36
The categories of this paper are: cmp-lg
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The categories of this paper are: nlin
23
The categories of this paper are: solv-int