34 research outputs found
Probing and manipulating valley coherence of dark excitons in monolayer WSe
Monolayers of semiconducting transition metal dichalcogenides are
two-dimensional direct-gap systems which host tightly-bound excitons with an
internal degree of freedom corresponding to the valley of the constituting
carriers. Strong spin-orbit interaction and the resulting ordering of the
spin-split subbands in the valence and conduction bands makes the lowest-lying
excitons in WX (X~being S or Se) spin-forbidden and optically dark. With
polarization-resolved photoluminescence experiments performed on a WSe
monolayer encapsulated in a hexagonal boron nitride, we show how the intrinsic
exchange interaction in combination with the applied in-plane and/or
out-of-plane magnetic fields enables one to probe and manipulate the valley
degree of freedom of the dark excitons.Comment: Manuscript: 6 pages, 3 figures; SM: 6 pages, 5 figure
Neutral and charged dark excitons in monolayer WS
Low temperature and polarization resolved magneto-photoluminescence
experiments are used to investigate the properties of dark excitons and dark
trions in a monolayer of WS encapsulated in hexagonal BN (hBN). We find
that this system is an -type doped semiconductor and that dark trions
dominate the emission spectrum. In line with previous studies on WSe, we
identify the Coulomb exchange interaction coupled neutral dark and grey
excitons through their polarization properties, while an analogous effect is
not observed for dark trions. Applying the magnetic field in both perpendicular
and parallel configurations with respect to the monolayer plane, we determine
the g-factor of dark trions to be -8.6. Their decay rate is close to 0.5
ns, more than 2 orders of magnitude longer than that of bright excitons.Comment: 6 pages, 6 figures, supplemental materia
Valley polarization of singlet and triplet trions in WS monolayer in magnetic fields
The spectral signatures associated with different negatively charged exciton
complexes (trions) in a WS monolayer encapsulated in hBN, are analyzed from
low temperature and polarization resolved reflectance contrast (RC) and
photoluminescence (PL) experiments, with an applied magnetic field. Based on
results obtained from the RC experiment, we show that the valley Zeeman effect
affects the optical response of both the singlet and the triplet trion species
through the evolution of their energy and of their relative intensity, when
applying an external magnetic field. Our analysis allows us to estimate a free
electron concentration of cm. The observed
evolutions based on PL experiments on the same sample are different and can
hardly be understood within the same simple frame highlighting the complexity
of relaxation processes involved in the PL response.Comment: 7 pages, 4 figures; source file correcte
Excitons and trions in WSSe monolayers
The possibility of almost linear tuning of the band gap and of the electrical
and optical properties in monolayers (MLs) of semiconducting transition metal
dichalcogenide (S-TMD) alloys opens up the way to fabricate materials with
on-demand characteristics. By making use of photoluminescence spectroscopy, we
investigate optical properties of WSSe MLs with a S/Se ratio of 57/43 deposited
on SiO/Si substrate and encapsulated in hexagonal BN flakes. Similarly to
the WS and WSe MLs, we assign the WSSe MLs to the ML family
with the dark ground exciton state. We find that, in addition to the neutral
bright A exciton line, three observed emission lines are associated with
negatively charged excitons. The application of in-plane and out-of-plane
magnetic fields allows us to assign undeniably the bright and dark (spin- and
momentum-forbidden) negative trions as well as the phonon replica of the dark
spin-forbidden complex. Furthermore, the existence of the single photon
emitters in the WSSe ML is also demonstrated, thus prompting the opportunity to
enlarge the wavelength range for potential future quantum applications of
S-TMDs.Comment: 6 pages, 5 figures, +ES
Karyotype differentiation in 19 species of river loach fishes (Nemacheilidae, Teleostei): extensive variability associated with rDNA and heterochromatin distribution and its phylogenetic and ecological interpretation
Simulation of a portable container hybrid energy source
Hybrid energy generation incorporating the renewable energy sources is gaining recognition and importance in the world. Institute of Aviation has thus decided to open a project: construction of a portable energy source container, incorporating modern hybrid solutions. In this article, the basic approach to creating a model of such a hybrid energy source is described: an initial scheme and composition of a portable container hybrid energy source, consisting of a wind turbine, photovoltaic array, battery bank and a backup diesel generator is proposed. A preliminary model of the portable hybrid energy source container, with two separate control system schemes, was created in TRNSYS
Simulation Studio software and is presented in this article. The data acquired from the TRNSYS simulations of the hybrid source container are gathered and organized, and then presented and analysed. From a theoretical point of view, it is interesting to see how the renewable based system characteristics differ with changing weather conditions, and how supporting the load is being switched between the renewable system and the diesel generator. From a practical point of view, it will be interesting to see the diesel fuel savings, and related to them lowered exhaust gases emission, coming from reduced diesel generator operation times. A problem of reducing emissions and fuel consumption, and matching the operation of energy generation to consumer need more flexibly that is offered by a hybrid source is particularly important