Nanophysics: Spectral classification of excitons

Date:
September 10, 2020
Source:
Ludwig-Maximilians-Universita"t Mu"nchen
Summary:
Ultrathin layers of tungsten diselenide have potential applications
in opto-electronics and quantum technologies. Researchers have now
explored how this material interacts with light in the presence
of strong magnetic fields.



FULL STORY ========================================================================== Ultrathin layers of tungsten diselenide have potential applications
in opto- electronics and quantum technologies. Researchers have now
explored how this material interacts with light in the presence of strong magnetic fields.


========================================================================== Owing to their astonishing and versatile properties, atomically
thin monolayer and bilayer forms of semiconducting transition metal dichalcogenides have aroused great interest in recent years. Most
attention has so far been paid to the optical properties of these
materials, such as molybdenum sulfide (MoS) and tungsten diselenide
(WSe2). These compounds show great promise as nanoscale elements for applications in opto-electronic and quantum technologies. In a new study,
LMU physicists led by Alexander Ho"gele have now developed a theoretical
model, which describes the effects of magnetic fields on the behavior of 'excitons' in two-dimensional ultrathin transition metal dichalcogenides.

Excitons are strongly bound 'quasiparticles', composed of an electron in
the conduction band and its positively charged counterpart in the valence
band referred to as a 'hole'. In the presence of strong magnetic fields,
the energy states of such quasiparticles (i.e. the frequencies at which
they emit and absorb light) split up. This spectral splitting can be experimentally measured and -- more importantly in the present context --
it can also be theoretically predicted.

In the new study, the team cooled monolayer and bilayer samples of
WSe2 to the temperature of liquid helium of a few degrees Kelvin. The researchers then used optical spectroscopy to measure the emission
spectra as a fucntion of magnetic field up to 9 Tesla and determined
the field-induced splitting. "Measurements like this are useful to
study excitons, which in turn determine the light- matter interaction
of semiconductors," Ho"gele explains.

It was already known that excitons can form in different
configurations. In addition to bright excitons, which couple directly
to light, the pairing of electrons and holes can produce 'spin-dark'
and 'momentum-dark' excitons. Up to now, it has not been possible to conclusively assign the signatures observed in emission spectra to
these different exciton species. In the presence of magnetic field,
however, individual emission peaks exhibit characteristic spectral
splittings. "This splitting can be used to discriminate between the
various types of excitons," says Ho"gele, "but only if we have the
according theoretical model." The LMU team developed theory to calculate
from first principles the spectral splitting for the different types
of excitons in monolayer and bilayer WSe2 subjected to magnetic field,
and compared their theoretical predictions with the experimental data.

The results provide a better understanding of the opto-electronic
properties of WSe2 and related transition-metal dichalcogenides where
excitons represent the primary interface for light to interact with
nanoscale matter. Ultrathin layers of WSe2 serve as a testbed for
technological exploitations of light-matter coupling in opto-electronic
devices including photodetectors and emitters or photovoltaic
devices. "These ultrathin materials are mechanically flexible and
extremely compact," says Ho"gele. They are also potentially viable for
quantum technologies as they host 'valleys' as quantum degrees of freedom
that can serve as qubits, the basic units of information processing in
quantum computers.


========================================================================== Story Source: Materials provided by
Ludwig-Maximilians-Universita"t_Mu"nchen. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Jonathan Fo"rste, Nikita V. Tepliakov, Stanislav Yu. Kruchinin,
Jessica
Lindlau, Victor Funk, Michael Fo"rg, Kenji Watanabe, Takashi
Taniguchi, Anvar S. Baimuratov, Alexander Ho"gele. Exciton g-factors
in monolayer and bilayer WSe2 from experiment and theory. Nature
Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-18019-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200910110854.htm

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