Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Exciton formation assisted by longitudinal optical phonons in monolayer
  transition metal dichalcogenides   We examine a mechanism by which excitons are generated via the LO
(longitudinal optical) phonon-assisted scattering process after optical
excitation of monolayer transition metal dichalcogenides. The exciton formation
time is computed as a function of the exciton center-of-mass wavevector,
electron and hole temperatures, and carrier densities for known values of the
Fr\"ohlich coupling constant, LO phonon energy, lattice temperature, and the
exciton binding energy in layered structures. For the monolayer MoS$_2$, we
obtain ultrafast exciton formation times on the sub-picosecond time scale at
charge densities of 5 $\times$ 10$^{11}$ cm$^{-2}$ and carrier temperatures
less than 300 K, in good agreement with recent experimental findings ($\approx$
0.3 ps). While excitons are dominantly created at zero center-of-mass
wavevectors at low charge carrier temperatures ($\approx$ 30 K), the exciton
formation time is most rapid at non-zero wavevectors at higher temperatures
($\ge $ 120 K) of charge carriers. The results show the inverse square-law
dependence of the exciton formation times on the carrier density, consistent
with a square-law dependence of photoluminescence on the excitation density.
Our results show that excitons are formed more rapidly in exemplary monolayer
selenide-based dichalcogenides (MoSe$_2$ and WSe$_2$) than sulphide-based
dichalcogenides (MoS$_2$ and WS$_2$).

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The categories of this paper are: quant-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: patt-sol
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The categories of this paper are: nlin
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The categories of this paper are: funct-an
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The categories of this paper are: stat
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The categories of this paper are: q-alg
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The categories of this paper are: hep-lat
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The categories of this paper are: atom-ph
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The categories of this paper are: astro-ph
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The categories of this paper are: math
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The categories of this paper are: supr-con
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The categories of this paper are: cmp-lg
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The categories of this paper are: nucl-th
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The categories of this paper are: eess
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The categories of this paper are: nucl-ex