20 research outputs found
Dark excitons in transition metal dichalcogenides
Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong
Coulomb interaction that manifests in tightly bound excitons. Due to the
complex electronic band structure exhibiting several spin-split valleys in the
conduction and valence band, dark excitonic states can be formed. They are
inaccessibly by light due to the required spin-flip and/or momentum transfer.
The relative position of these dark states with respect to the optically
accessible bright excitons has a crucial impact on the emission efficiency of
these materials and thus on their technological potential. Based on the
solution of the Wannier equation, we present the excitonic landscape of the
most studied TMD materials including the spectral position of momentum- and
spin-forbidden excitonic states. We show that the knowledge of the electronic
dispersion does not allow to conclude about the nature of the material's band
gap, since excitonic effects can give rise to significant changes. Furthermore,
we reveal that an exponentially reduced photoluminescence yield does not
necessarily reflect a transition from a direct to a non-direct gap material,
but can be ascribed in most cases to a change of the relative spectral distance
between bright and dark excitonic states
Phonon-assisted Photoluminescence from Dark Excitons in Monolayers of Transition Metal Dichalcogenides
The photoluminescence (PL) spectrum of transition metal dichalcogenides
(TMDs) shows a multitude of emission peaks below the bright exciton line and
not all of them have been explained yet. Here, we study the emission traces of
phonon-assisted recombinations of momentum-dark excitons. To this end, we
develop a microscopic theory describing simultaneous exciton, phonon and photon
interaction and including consistent many-particle dephasing. We explain the
drastically different PL below the bright exciton in tungsten- and
molybdenum-based materials as result of different configurations of bright and
dark states. In good agreement with experiments, we show that WSe exhibits
clearly visible low-temperature PL signals stemming from the phonon-assisted
recombination of momentum-dark excitons
Phonon-Assisted Photoluminescence from Indirect Excitons in Monolayers of Transition-Metal Dichalcogenides
The photoluminescence (PL) spectrum of transition-metal dichalcogenides (TMDs) shows a multitude of emission peaks below the bright exciton line, and not all of them have been explained yet. Here, we study the emission traces of phonon-assisted recombinations of indirect excitons. To this end, we develop a microscopic theory describing simultaneous exciton, phonon, and photon interaction and including consistent many-particle dephasing. We explain the drastically different PL below the bright exciton in tungsten- and molybdenum-based materials as the result of different configurations of bright and momentum-dark states. In good agreement with experiments, our calculations predict that WSe2 exhibits clearly visible low-temperature PL signals stemming from the phonon-assisted recombination of momentum-dark K-K′ excitons
Human subcortical brain asymmetries in 15,847 people worldwide reveal effects of age and sex
The two hemispheres of the human brain differ functionally and structurally. Despite over a century of research, the extent to which brain asymmetry is influenced by sex, handedness, age, and genetic factors is still controversial. Here we present the largest ever analysis of subcortical brain asymmetries, in a harmonized multi-site study using meta-analysis methods. Volumetric asymmetry of seven subcortical structures was assessed in 15,847 MRI scans from 52 datasets worldwide. There were sex differences in the asymmetry of the globus pallidus and putamen. Heritability estimates, derived from 1170 subjects belonging to 71 extended pedigrees, revealed that additive genetic factors influenced the asymmetry of these two structures and that of the hippocampus and thalamus. Handedness had no detectable effect on subcortical asymmetries, even in this unprecedented sample size, but the asymmetry of the putamen varied with age. Genetic drivers of asymmetry in the hippocampus, thalamus and basal ganglia may affect variability in human cognition, including susceptibility to psychiatric disorders
6. Vortrag (27.09.2012): CO2-Reduktionsstrategie der ETH Zürich
Inhalt: Infrastruktur und CO2-Bilanz der ETH Zürich; Facts zu den Dienstreisenden; Umfrage und Resultate; Fazi
Phonon-assisted Photoluminescence from Dark Excitons in Monolayers of Transition Metal Dichalcogenides
The photoluminescence (PL) spectrum of transition metal dichalcogenides (TMDs) shows a multitude of emission peaks below the bright exciton line and not all of them have been explained yet. Here, we study the emission traces of phonon-assisted recombinations of momentum-dark excitons. To this end, we develop a microscopic theory describing simultaneous exciton, phonon and photon interaction and including consistent many-particle dephasing. We explain the drastically different PL below the bright exciton in tungsten- and molybdenum-based materials as result of different configurations of bright and dark states. In good agreement with experiments, we show that WSe2 exhibits clearly visible low-temperature PL signals stemming from the phonon-assisted recombination of momentum-dark excitons