Strongly interacting Rydberg excitons in Cu2O
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Date
2020
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Abstract
Excitons are fundamental electronic excitations in a semiconductor. They are bound states
consisting of an electron in the conduction band and a positively charged hole in the valence
band. They have hydrogen-like properties and are therefore referred to as the hydrogen
analogues in solid states. In the semiconductor Cuprous Oxide, Cu2O, highly excited states
of excitons can be observed with principal quantum numbers up to n = 25. These states
are called Rydberg excitons in analogy to their atomic counterparts, the Rydberg atoms.
The intention of this thesis is to gain new insights into fundamental properties of these
Rydberg excitons by optical spectroscopy. The focus lies on one-photon absorption spectroscopy
with high spectral resolution up to 5 nanoelectronvolts. The physics addressed in
this thesis can be divided into three main topics.
First, the interaction of Rydberg excitons with high principal quantum numbers n with
both external electric and magnetic fields is studied. In the regime of high n, the density of
states becomes so large that it becomes unfeasible to use a microscopic theory that explicitly
considers every single state. Instead, general n-dependent scaling laws for various fundamental
properties of Rydberg excitons are derived theoretically and proven experimentally.
These scaling laws provide an efficient description of Rydberg excitons in the high-n regime
and give fundamental insights into similarities and differences between Rydberg excitons
and Rydberg atoms.
Second, the behavior of Rydberg excitons surrounded by an electron-hole plasma is investigated.
For this purpose, absorption spectra are presented, recorded at different densities
of free carriers injected into the crystal by an off-resonant pump laser. Carrier densities as
low as 0.01 µm−3 are found to lead to a lowering of the band gap and the disappearance
of the highest exciton lines. A model based on screening of the Coulomb interaction by
free charge carriers is presented that allows for a phenomenological description of the data.
In this context, the experimental parameter space spanned by excitation power and temperature
is investigated to determine the limiting factors for the observation of Rydberg
excitons with principal quantum numbers higher than n = 25, the highest Rydberg exciton
state observed so far. Indeed, at nanowatt laser powers and millikelvin temperatures,
the extension of the observable exciton series to n = 28 is possible.
The third part of this thesis addresses mutual interactions between Rydberg excitons.
Combining data from pump-probe experiments with a detailed theoretical model for the
shape of these spectra for several exciton interaction mechanisms clearly shows that long-range
van der Waals-type interactions are the dominant contribution to interactions between
Rydberg excitons.
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Keywords
Rydberg excitons, Semiconductor physics
Subjects based on RSWK
Exziton, Absorptionsspektroskopie, Elektron-Loch-Plasma, Halbleiterphysik