Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Tuning valley polarization in a WSe2 monolayer with a tiny magnetic
  field   In monolayers of semiconducting transition metal dichalcogenides, the light
helicity ($\sigma^+$ or $\sigma^-$) is locked to the valley degree of freedom,
leading to the possibility of optical initialization of distinct valley
populations. However, an extremely rapid valley pseudospin relaxation (at the
time scale of picoseconds) occurring for optically bright (electric-dipole
active) excitons imposes some limitations on the development of
opto-valleytronics. Here we show that inter-valley scattering of excitons can
be significantly suppressed in a $\mathrm{WSe}_2$ monolayer, a direct-gap
two-dimensional semiconductor with the exciton ground state being optically
dark. We demonstrate that the already inefficient relaxation of the exciton
pseudospin in such system can be suppressed even further by the application of
a tiny magnetic field of $\sim$100 mT. Time-resolved spectroscopy reveals the
pseudospin dynamics to be a two-step relaxation process. An initial decay of
the pseudospin occurs at the level of dark excitons on a time scale of 100 ps,
which is tunable with a magnetic field. This decay is followed by even longer
decay ($>1$ ns), once the dark excitons form more complex objects allowing for
their radiative recombination. Our finding of slow valley pseudospin relaxation
easily manipulated by the magnetic field open new prospects for engineering the
dynamics of the valley pseudospin in transition metal dichalcogenides.

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The categories of this paper are: q-bio
1
The categories of this paper are: cs
8
The categories of this paper are: hep-ex
28
The categories of this paper are: alg-geom
0
The categories of this paper are: hep-ph
26
The categories of this paper are: bayes-an
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The categories of this paper are: hep-th
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The categories of this paper are: hep-lat
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The categories of this paper are: adap-org
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The categories of this paper are: nucl-ex
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The categories of this paper are: plasm-ph
35
The categories of this paper are: mtrl-th
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The categories of this paper are: astro-ph
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The categories of this paper are: eess
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The categories of this paper are: q-alg
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The categories of this paper are: funct-an