77 research outputs found
Unique signatures for Bose-Einstein condensation in the decay luminescence lineshape of weakly interacting excitons in a potential trap
We calculate the spatially resolved optical emission spectrum of a weakly
interacting Bose gas of excitons confined in a three dimensional potential trap
due to interband transitions involving weak direct and phonon mediated
exciton-photon interactions. Applying the local density approximation, we show
that for a non-condensed system the spatio-spectral lineshape of the direct
process reflects directly the shape of the potential. The existence of a
Bose-Einstein condensate changes the spectrum in a characteristic way so that
it directly reflects the constant chemical potential of the excitons and the
renormalization of the quasiparticle excitation spectrum. Typical examples are
given for parameters of the lowest yellow excitons in cuprous oxide.Comment: 5 pages, 2 figure
Interaction of Rydberg Excitons in Cuprous Oxide with Phonons and Photons: Optical Linewidth and Polariton Effect
We demonstrate that the optical linewidth of Rydberg excitons in Cu2O can be
completely explained by scattering with acoustical and optical phonons, whereby
the dominant contributions stems from the non-polar optical modes. The
consequences for the observation of polariton effects are discussed. We find
that an anti-crossing of photon and exciton dispersions exists only for states
with main quantum numbers n>28, so polariton effects do not play any role in
the experiments reported up to now
The phonon assisted absorption of excitons in CuO
The basic theoretical foundation for the modelling of phonon assisted
absorption spectra in direct bandgap semiconductors, introduced by Elliott 60
years ago using second order perturbation theory, results in a square root
shaped dependency close to the absorption edge. A careful analysis of the
experiments reveals that for the yellow S excitons in CuO the lineshape
does not follow that square root dependence. The reexamination of the theory
shows that the basic assumptions of constant matrix elements and constant
energy denominators is invalid for semiconductors with dominant exciton effects
like CuO, where the phonon assisted absorption proceeds via intermediate
exciton states. The overlap between these and the final exciton states strongly
determines the dependence of the absorption on the photon energy. To describe
the experimental observed line shape of the indirect absorption of the yellow S
exciton states we find it necessary to assume a momentum dependent deformation
potential for the optical phonons
Phase separation of multicomponent excitonic Bose-Einstein condensates
For the observation of Bose-Einstein condensation, excitons in cuprous oxide
are regarded as promising candidates due to their large binding energy and long
lifetime. High particle densities may be achieved by entrapment in a stress
induced potential. We consider a multi-component gas of interacting para- and
orthoexcitons in cuprous oxide confined in a three-dimensional potential trap.
Based on the Hartree-Fock-Bogoliubov theory, we calculate density profiles as
well as decay luminescence spectra which exhibit signatures of the separation
of the Bose-condensed phases.Comment: 4 pages, 1 figure, presented at NOEKS 10, Paderborn, August 201
Beyond conventional photon-number detection with click detectors
Photon-number measurements are a fundamental technique for the discrimination
and characterization of quantum states of light. Beyond the abilities of
state-of-the-art devices, we present measurements with an array of 100
avalanche photodiodes exposed to photon-numbers ranging from well below to
significantly above one photon per diode. Despite each single diode only
discriminating between zero and non-zero photon-numbers we are able to extract
characteristic information about the quantum state. We demonstrate a vast
enhancement of the applicable intensity range by two orders of magnitude
relative to the standard application of such devices. It turns out that the
probabilistic mapping of arbitrary photon-numbers on a finite number of
registered clicks is not per se a disadvantage compared with true photon
counters. Such detector arrays can bridge the gap between single-photon and
linear detection, by directly using the recorded data, without the need of
elaborate data reconstruction methods.Comment: 4 figure
Multicomponent exciton gas in cuprous oxide: cooling behaviour and the role of Auger decay
In this paper we present a hydrodynamic model to describe the dynamics of
para- and orthoexcitons in cuprous oxide at ultralow temperatures inside a
stress induced potential trap. We take into account the finite lifetime of the
excitons, the excitation process and exciton-phonon as well as exciton-exciton
interaction. Furthermore, we model the two-body loss mechanism assuming an
Auger-like effect and compare it to an alternative explanation which relies on
the formation of biexcitons. We discuss in detail the influence on the
numerical results and compare the predictions to experimental data.Comment: 10 pages, 8 figures, submitted to J. Phys.
A universal generic description of the dynamics of the current COVID-19 pandemic
The ongoing COVID-19 pandemic is challenging every part of society. From a
scientific point of view the first major task is to predict the dynamics of the
pandemic, allowing governments to allocate proper resources and measures to
fight it, as well as gauging the success of these measures by comparison with
the predictions in hindsight. The vast majority of pandemic models are based on
extensive models with large numbers of fit parameters, leading to individual
descriptions for every hot spot on the world. This makes predictions and
comparisons cumbersome, if not impossible. We here propose a different
approach, by moving away from a description over time, and instead choosing the
total number of infected people in an enclosed area as the independent
variable. Analyzing a few hot spots data, we derive an empirical formula for
the dynamics, dependent only on three variables. The final number of infections
is strictly connected to one fit parameter we call mitigation factor, which in
turn is mostly dependent only on the enclosed population. Despite its
simpleness, this description applies to every of the around 50 countries we
have analyzed, allows to separate different waves of the pandemic, provides a
figure of merit for the overall usefulness of government measures, and shows
when a pandemic is ending. Our model is robust against undetected cases, and
allows all nations, in particular those with fewer resources, to reasonably
predict the outcome of the pandemic in their country
Influence of electron-hole plasma on Rydberg excitons in cuprous oxide
We develop a many-body approach to the behavior of exciton bound states and
the conduction electron band edge in a surrounding electron-hole plasma with a
focus on the absorption spectrum of Rydberg excitons in cuprous oxide. The
interplay of band edge and exciton levels is analyzed numerically, whereby the
self-consistent solution is compared to the semiclassical Debye approximation.
Our results provide criteria which allow to verify or rule out the different
band edge models against future experimental data.Comment: 7 pages, 9 figure
Interaction of charged impurities and Rydberg excitons in cuprous oxide
We investigate the influence of a static, uncorrelated distribution of
charged impurities on the spectrum of bound excitons in the copper oxide
CuO. We show that the statistical distribution of Stark shifts and
ionisation rates leads to the vanishing of Rydberg resonances into an apparent
continuum. The appearance of additional absorption lines due to the broken
rotational symmetry, together with spatially inhomogeneous Stark shifts, leads
to a modification of the observed line shapes that agree qualitatively with the
changes observed in the experiment.Comment: 6 pages, 4 figure
Giant Rydberg Excitons in Cuprous Oxide
Highly excited atoms with an electron moved into a level with large principal
quantum number are fascinating hydrogen-like objects. The giant extension of
these Rydberg atoms leads to huge interaction effects. Monitoring these
interactions has provided novel insights into molecular and condensed matter
physics problems on a single quantum level. Excitons, the fundamental optical
excitations in semiconductors consisting of a negatively charged electron and a
positively charged hole, are the condensed matter analogues of hydrogen. Highly
excited excitons with extensions similar to Rydberg atoms are attractive
because they may be placed and moved in a crystal with high precision using
microscopic potential landscapes. Their interaction may allow formation of
ordered exciton phases or sensing of elementary excitations in the surrounding,
also on a quantum level. Here we demonstrate the existence of Rydberg excitons
in cuprous oxide, Cu2O, with principal quantum numbers as large as n=25 . These
states have giant wave function extensions of more than 2 micrometers, compared
to about a nanometer for the ground state. The strong dipole-dipole interaction
is evidenced by a blockade effect, where the presence of an exciton prevents
excitation of a further exciton in its vicinity
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