Exciton-phonon-scattering: A competition between bosonic and fermionic nature of bound electron-hole pairs

The question of macroscopic occupation and spontaneous emergence of coherence for exciton ensembles has gained renewed attention due to the rise of van der Waals heterostructures made of atomically thin semiconductors. The hosted interlayer excitons exhibit nanosecond lifetimes, long enough to allow for excitonic thermalization in time. Several experimental studies reported signatures of macroscopic occupation effects at elevated exciton densities. With respect to theory, excitons are composite particles formed by fermionic constituents, and a general theoretical argument for a bosonic thermalization of an exciton gas beyond the linear regime is still missing. Here, we derive an equation for the phonon mediated thermalization at densities above the classical limit, and identify which conditions favor the thermalization of fermionic or bosonic character, respectively. In cases where acoustic, quasielastic phonon scattering dominates the dynamics, our theory suggests that transition metal dichalcogenide (TMDC) excitons might be bosonic enough to show bosonic thermalization behaviour and decreasing dephasing for increasing exciton densities. This can be interpreted as a signature of an emerging coherence in the exciton ground state, and agrees well with the experimentally observed features, such as a decreasing linewidth for increasing densities

Photo-physics and electronic structure of lateral graphene/MoS2 and metal/MoS2 junctions

Integration of semiconducting transition metal dichalcogenides (TMDs) into functional optoelectronic circuitries requires an understanding of the charge transfer across the interface between the TMD and the contacting material. Here, we use spatially resolved photocurrent microscopy to demonstrate electronic uniformity at the epitaxial graphene/molybdenum disulfide (EG/MoS2) interface. A 10x larger photocurrent is extracted at the EG/MoS2 interface when compared to metal (Ti/Au) /MoS2 interface. This is supported by semi-local density-functional theory (DFT), which predicts the Schottky barrier at the EG/MoS2 interface to be ~2x lower than Ti/MoS2. We provide a direct visualization of a 2D material Schottky barrier through combination of angle resolved photoemission spectroscopy with spatial resolution selected to be ~300 nm (nano-ARPES) and DFT calculations. A bending of ~500 meV over a length scale of ~2-3 micrometer in the valence band maximum of MoS2 is observed via nano-ARPES. We explicate a correlation between experimental demonstration and theoretical predictions of barriers at graphene/TMD interfaces. Spatially resolved photocurrent mapping allows for directly visualizing the uniformity of built-in electric fields at heterostructure interfaces, providing a guide for microscopic engineering of charge transport across heterointerfaces. This simple probe-based technique also speaks directly to the 2D synthesis community to elucidate electronic uniformity at domain boundaries alongside morphological uniformity over large areas

Exploring the atmosphere composition with innovative online data analysis services integrating novel level-3 products from Copernicus Sentinel mission

For more than 10 years, the EOC Geoservice (https://geoservice.dlr.de) is operational and provides access to all its hosted data collections and products via the OGC-compliant interfaces WMS and WCS. ISO metadata on data collections and products are exposed via compliant catalogue services. The EOC Geoservice operationally provides access to a multitude of operational EO and EO related produtcts. Among these are specific Sentinel-2-products, value-added land-surface products such as the World Settlement Footprint and a variety of atmosphere-relateted products from GOME-2/MetOp-A/B/C Level-3 as well as innovative Level 3 trace gas, cloud and radiation products derived from Sentinel-5P/TROPOMI observations. The EOC Geoservice faces the problem that it publishes its extremely heterogenious data collections and product though a number of different catalogue services. The challenge for EOC Geoservice is to provide harmonized and standardized metadata for its complete variety of products to support state-of-the-art data discovery. Moreover, state-of-the-art data access to support scientific data analysis by data cube approaches is required. The solution which is implemented by the EOC Geoservice extends the existing OGC Web Service APIs with an additional STAC (SpatioTemporal Asset Cataloguem https://stacspec.org) EO product catalog. This interface allows to easily access published datasets via data cube concepts, supporting direct integration in operational processing environments or into interactive Jupyter notebooks. The aforementioned additional STAC EO product catalog is based on the existing catalog entries in the database. By using templates, they are converted into one of the following five standards: OpenSearch XML EO (to support existing applicatons), OpenSearch JSON, STAC (JSON) and STAC (HMTL). This conversion is performed on-the-fly, all required information for collection and product metadata is stored in an internal Postgres-database. In order to facilitate the uptake of new products (such as individual L3 atmosphere products from S5P/TROPOMI) by the EOC Geoservice, configurable tools for the automated extraction of relevant metadata information from CF-compliant NetCDF files are currently in development. As other software components, this tools is going to be published as open source. For all technologies, the GeoServer software is the technical backbone. Throughout the above mentioned 10-year period, DLR has significantly supported its further-development as open-source software. Its most recent improvements (integration of STAC) have been funded in the framework of the ESA GSTP-project Technologies for the Management of Long EO Data Time Series (LOOSE). Integration of all innovative interfaces into an operational data discovery, access and analysis service (EOC Geoservice and DataCube) for the Copernicus atmospheric composition missions Sentinel-5P, Sentinel-4 and Sentinel-5 is supported by the DLR programmatic project Innovative Produktentwicklung zur Analyse der Atmosphärenzusammensetzung (INPULS)

The Anomalous Photo‐Nernst Effect of Massive Dirac Fermions In HfTe5

Abstract The quantum geometric Berry curvature results in an anomalous correction to the band velocity of crystal electrons with a corresponding transverse (thermo)electric conductivity. However, time‐reversal symmetry typically constrains the direct observation and exploitation of anomalous transport to magnetic compounds. Here, it is demonstrated the anomalous Hall and Nernst conductivities are essential for describing the optoelectronic transport in thin films of the non‐magnetic, weakly gapped semimetal HfTe5 subject to an external magnetic field. A focused photoexcitation adresses the symmetries of the local Nernst conductivity, which unveils a hitherto hidden, anomalous photo‐Nernst effect of three‐dimensional (3D) massive Dirac fermions. The experimental temperature and density dependencies are compared with a semiclassical Boltzmann transport model. For HfTe5 thin films with the Fermi level close to the gap, the model suggests that the anomalous photo‐Nernst currents originate from an intrinsic Berry curvature mechanism, where the Zeeman interaction effectively breaks time‐reversal symmetry of the massive Dirac fermions already at moderate external magnetic fields

Defect-Engineered Magnetic Field Dependent Optoelectronics of Vanadium Doped Tungsten Diselenide Monolayers

© 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH.The ability to dope transition metal dichalcogenides such as tungsten diselenide (WSe2) with magnetic transition metal atoms in a controlled manner has motivated intense research with the aim of generating dilute magnetic semiconductors. In this work, semiconducting WSe2 monolayers, substitutionally doped with vanadium atoms, are investigated using low-temperature luminescence and optoelectronic spectroscopy. V-dopants lead to a p-type doping character and an impurity-related emission ≈160 meV below the neutral exciton, both of which scale with the nominal percentage of V-dopants. Measurements using field-effect devices of 0.3% V-doped WSe2 demonstrate bipolar carrier tunability. The doped monolayers display a clear magnetic hysteresis in transport measurements both under illumination and without illumination, whereas the valley polarization of the excitons reveals a nonlinear g-factor without a magnetic hysteresis within the experimental uncertainty. Hence, this work on V-doped WSe2 provides crucial insights concerning suitable characterization methods on magnetic properties of doped 2D materials.11Nsciescopu

Optical dipole orientation of interlayer excitons in MoSe₂-WSe₂ heterostacks

We report on the far-field photoluminescence intensity distribution of interlayer excitons in MoSe$_{2}$-WSe$_{2}$ heterostacks as measured by back focal plane imaging in the temperature range between 1.7 K and 20 K. By comparing the data with an analytical model describing the dipolar emission pattern in a dielectric environment, we are able to obtain the relative contributions of the in- and out-of-plane transition dipole moments associated to the interlayer exciton photon emission. We determine the transition dipole moments for all observed interlayer exciton transitions to be (99 $\pm$ 1)% in-plane for R- and H-type stacking, independent of the excitation power and therefore the density of the exciton ensemble in the experimentally examined range. Finally, we discuss the limitations of the presented measurement technique to observe correlation effects in exciton ensembles