88 research outputs found
Influence of the charge carrier tunneling processes on the recombination dynamics in single lateral quantum dot molecules
We report on the charge carrier dynamics in single lateral quantum dot
molecules and the effect of an applied electric field on the molecular states.
Controllable electron tunneling manifests itself in a deviation from the
typical excitonic decay behavior which is strongly influenced by the tuning
electric field and inter-molecular Coulomb energies. A rate equation model is
developed to gain more insight into the charge transfer and tunneling
mechanisms. Non-resonant (phonon-mediated) electron tunneling which changes the
molecular exciton character from direct to indirect, and vice versa, is found
to be the dominant tunable decay mechanism of excitons besides radiative
recombination.Comment: 4 pages, 4 figure
Polarization fine-structure and enhanced single-photon emission of self-assembled lateral InGaAs quantum dot molecules embedded in a planar micro-cavity
Single lateral InGaAs quantum dot molecules have been embedded in a planar
micro-cavity in order to increase the luminescence extraction efficiency. Using
a combination of metal-organic vapor phase and molecular beam epitaxy samples
could be produced that exhibit a 30 times enhanced single-photon emission rate.
We also show that the single-photon emission is fully switchable between two
different molecular excitonic recombination energies by applying a lateral
electric field. Furthermore, the presence of a polarization fine-structure
splitting of the molecular neutral excitonic states is reported which leads to
two polarization-split classically correlated biexciton exciton cascades. The
fine-structure splitting is found to be on the order of 10 micro-eV.Comment: 14 pages, 4 figures; the following article has been submitted to
Journal of Applied Physics (29th ICPS - invited paper); after it is
published, it will be found at http://jap.aip.org
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Towards deterministically controlled InGaAs/GaAs lateral quantum dot molecules
We report on the fabrication, detailed characterization and modeling of lateral InGaAs quantum dot molecules (QDMs) embedded in a GaAs matrix and we discuss strategies to fully control their spatial configuration and electronic properties. The three-dimensional morphology of encapsulated QDMs was revealed by selective wet chemical etching of the GaAs top capping layer and subsequent imaging by atomic force microscopy (AFM). The AFM investigation showed that different overgrowth procedures have a profound consequence on the QDM height and shape. QDMs partially capped and annealed in situ for micro- photoluminescence spectroscopy consist of shallow but well-defined quantum dots (QDs) in contrast to misleading results usually provided by surface morphology measurements when they are buried by a thin GaAs layer. This uncapping approach is crucial for determining the QDM structural parameters, which are required for modeling the system. A single-band effective-mass approximation is employed to calculate the confined electron and heavy-hole energy levels, taking the geometry and structural information extracted from the uncapping experiments as inputs. The calculated transition energy of the single QDM shows good agreement with the experimentally observed values. By decreasing the edge-to-edge distance between the two QDs within a QDM, a splitting of the electron (hole) wavefunction into symmetric and antisymmetric states is observed, indicating the presence of lateral coupling. Site control of such lateral QDMs obtained by growth on a pre-patterned substrate, combined with a technology to fabricate gate structures at well-defined positions with respect to the QDMs, could lead to deterministically controlled devices based on QDMs. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
Science with a small two-band UV-photometry mission III: Active Galactic Nuclei and nuclear transients
In this review (the third in the series focused on a small two-band
UV-photometry mission), we assess possibilities for a small UV two-band
photometry mission in studying accreting supermassive black holes (SMBHs; mass
range -). We focus on the following
observational concepts: (i) dedicated monitoring of selected type-I Active
Galactic Nuclei (AGN) in order to measure the time delay between the far-UV,
the near-UV, and other wavebands (X-ray and optical), (ii) nuclear transients
including (partial) tidal disruption events and repetitive nuclear transients,
and (iii) the study of peculiar sources, such as changing-look AGN, hollows and
gaps in accretion disks, low-luminosity AGN, and candidates for
Intermediate-Mass Black Holes (IMBHs; mass range -)
in galactic nuclei. For tidal disruption events (TDEs), high-cadence UV
monitoring is crucial for distinguishing among different scenarios for the
origin of the UV emission. The small two-band UV space telescope will also
provide the information about the near- and far-UV continuum variability for
rare transients, such as repetitive partial TDEs and jetted TDEs. We also
discuss the possibilities to study and analyze sources with non-standard
accretion flows, such as AGN with gappy disks, low-luminosity active galactic
nuclei with intermittent accretion, and SMBH binaries potentially involving
intermediate-mass black holes.Comment: Submitted to Space Science Review
Alive but Barely Kicking: News from 3+ yr of Swift and XMM-Newton X-Ray Monitoring of Quasiperiodic Eruptions from eRO-QPE1
Quasiperiodic eruptions (QPEs) represent a novel class of extragalactic X-ray transients that are known to repeat at roughly regular intervals of a few hours to days. Their underlying physical mechanism is a topic of heated debate, with most models proposing that they originate either from instabilities within the inner accretion flow or from orbiting objects. At present, our knowledge of how QPEs evolve over an extended timescale of multiple years is limited, except for the unique QPE source GSN 069. In this study, we present results from strategically designed Swift observing programs spanning the past 3 yr, aimed at tracking eruptions from eRO-QPE1. Our main results are as follows: (1) the recurrence time of eruptions can vary from flare to flare and is in the range of 0.6–1.2 days; (2) there is no detectable secular trend in evolution of the recurrence times; (3) consistent with prior studies, their eruption profiles can have complex shapes; and (4) the peak flux of the eruptions has been declining over the past 3 yr, with the eruptions barely detected in the most recent Swift data set taken in 2023 June. This trend of weakening eruptions has been reported recently in GSN 069. However, because the background luminosity of eRO-QPE1 is below our detection limit, we cannot verify whether the weakening is correlated with the background luminosity (as is claimed to be the case for GSN 069). We discuss these findings within the context of various proposed QPE models
Interdependencies between Leverage and Capital Ratios in the Banking Sector of the Czech Republic
In this paper we discuss the implications of the Basel III requirements on the leverage ratio for the banking sector in the Czech Republic. We identify the potential binding constraints from regulatory limits and analyze the interactions among leverage and capital ratios over the country’s economic cycle (during the period 2007-2014).
The historical data confirm stronger capital ratios of the banks and an overall solid leverage level with only 5% of the total historical observations being lower than the regulatory recommendations. By analyzing the components of ratios, we conclude that the banks are focusing more on the optimization of risk weighted assets. Strong co-movement patterns between leverage and assets point to the active management of leverage as a means of expanding and contracting the size of balance sheets and maximizing the utility of the capital.
The analysis of correlation patterns among the variables indicates that the total assets (and exposure) in contrast to Tier 1 capital are the main contributors to the cyclical movements in the leverage. The leverage and the total assets also demonstrate a weak correlation with GDP, but a strong co-movement with loans to the private sector
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