The optical selection rules for interband transitions in WSe_{2}, WS_{2}, and MoSe_{2} transition metal dichalcogenide monolayers are investigated by polarization-resolved photoluminescence ...experiments with a signal collection from the sample edge. These measurements reveal a strong polarization dependence of the emission lines. We see clear signatures of the emitted light with the electric field oriented perpendicular to the monolayer plane, corresponding to an interband optical transition forbidden at normal incidence used in standard optical spectroscopy measurements. The experimental results are in agreement with the optical selection rules deduced from group theory analysis, highlighting the key role played by the different symmetries of the conduction and valence bands split by the spin-orbit interaction. These studies yield a direct determination of the bright-dark exciton splitting, for which we measure 40±1 meV and 55±2 meV in WSe_{2} and WS_{2} monolayer, respectively.
We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For MoSe sub(2) ...monolayer, we measure (ProQuest: Formulae and/or non-USASCII text omitted) = 1.8+ or -0.2 ps at T= 7K that we interpret as the intrinsic radiative recombination time. Similar values are found for WSe sub(2) monolayers. Our detailed analysis suggests the following scenario: at low temperature (T< ~ 50 K), the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the photoluminescence intensity drops drastically with a decay time in the range of the picosecond driven by the escape of excitons from the radiative window due to exciton-phonon interactions. Following this first nonthermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and nonradiative recombination channels including exciton-exciton annihilation control the latter. Finally the temperature dependence of the measured exciton and trion dynamics indicates that the two populations are not in thermodynamical equilibrium.
The direct gap interband transitions in transition metal dichalcogenide monolayers are governed by chiral optical selection rules. Determined by laser helicity, optical transitions in either the ...K^{+} or K^{-} valley in momentum space are induced. Linearly polarized laser excitation prepares a coherent superposition of valley states. Here, we demonstrate the control of the exciton valley coherence in monolayer WSe_{2} by tuning the applied magnetic field perpendicular to the monolayer plane. We show rotation of this coherent superposition of valley states by angles as large as 30° in applied fields up to 9 T. This exciton valley coherence control on the ps time scale could be an important step towards complete control of qubits based on the valley degree of freedom.
Abstract
Excitons, Coulomb bound electron–hole pairs, are composite bosons and their interactions in traditional semiconductors lead to condensation and light amplification. The much stronger Coulomb ...interaction in transition metal dichalcogenides such as WSe
2
monolayers combined with the presence of the valley degree of freedom is expected to provide new opportunities for controlling excitonic effects. But so far the bosonic character of exciton scattering processes remains largely unexplored in these two-dimensional materials. Here we show that scattering between B-excitons and A-excitons preferably happens within the same valley in momentum space. This leads to power dependent, negative polarization of the hot B-exciton emission. We use a selective upconversion technique for efficient generation of B-excitons in the presence of resonantly excited A-excitons at lower energy; we also observe the excited A-excitons state 2
s
. Detuning of the continuous wave, low-power laser excitation outside the A-exciton resonance (with a full width at half maximum of 4 meV) results in vanishing upconversion signal.
The strong light-matter interaction and the valley selective optical selection rules make monolayer (ML) MoS2 an exciting 2D material for fundamental physics and optoelectronics applications. But, so ...far, optical transition linewidths even at low temperature are typically as large as a few tens of meV and contain homogeneous and inhomogeneous contributions. This prevented in-depth studies, in contrast to the better-characterized ML materials MoSe2 and WSe2 . In this work, we show that encapsulation of ML MoS2 in hexagonal boron nitride can efficiently suppress the inhomogeneous contribution to the exciton linewidth, as we measure in photoluminescence and reflectivity a FWHM down to 2 meV at T=4K . Narrow optical transition linewidths are also observed in encapsulated WS2 , WSe2 , and MoSe2 MLs. This indicates that surface protection and substrate flatness are key ingredients for obtaining stable, high-quality samples. Among the new possibilities offered by the well-defined optical transitions, we measure the homogeneous broadening induced by the interaction with phonons in temperature-dependent experiments. We uncover new information on spin and valley physics and present the rotation of valley coherence in applied magnetic fields perpendicular to the ML.
The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of ...the “spin-forbidden” dark excitons in WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field Bz. In agreement with the theoretical predictions deduced from group theory analysis, magnetophotoluminescence experiments reveal a zero-field splitting δ=0.6±0.1meV between two dark exciton states. The low-energy state is strictly dipole forbidden (perfectly dark) at Bz=0, while the upper state is partially coupled to light with z polarization (“gray” exciton). The first determination of the dark neutral exciton lifetime τD in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure τD∼110±10ps for the gray exciton state, i.e., two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at T=12K.
Excitons, Coulomb bound electron-hole pairs, are composite bosons and their interactions in traditional semiconductors lead to condensation and light amplification. The much stronger Coulomb ...interaction in transition metal dichalcogenides such as WSe
monolayers combined with the presence of the valley degree of freedom is expected to provide new opportunities for controlling excitonic effects. But so far the bosonic character of exciton scattering processes remains largely unexplored in these two-dimensional materials. Here we show that scattering between B-excitons and A-excitons preferably happens within the same valley in momentum space. This leads to power dependent, negative polarization of the hot B-exciton emission. We use a selective upconversion technique for efficient generation of B-excitons in the presence of resonantly excited A-excitons at lower energy; we also observe the excited A-excitons state 2s. Detuning of the continuous wave, low-power laser excitation outside the A-exciton resonance (with a full width at half maximum of 4 meV) results in vanishing upconversion signal.
Charged excitons, or X± trions, in monolayer transition-metal dichalcogenides have binding energies of several tens of meV. Together with the neutral exciton X0 they dominate the emission spectrum at ...low and elevated temperatures. We use charge-tunable devices based on WSe2 monolayers encapsulated in hexagonal boron nitride to investigate the difference in binding energy between X+ and X− and the X− fine structure. We find in the charge-neutral regime, the X0 emission accompanied at lower energy by a strong peak close to the longitudinal optical (LO) phonon energy. This peak is absent in reflectivity measurements, where only the X0 and an excited state of the X0 are visible. In the n-doped regime, we find a closer correspondence between emission and reflectivity as the trion transition with a well-resolved fine-structure splitting of 6 meV for X− is observed. We present a symmetry analysis of the different X+ and X− trion states and results of the binding energy calculations. We compare the trion binding energy for the n- and p-doped regimes with our model calculations for low carrier concentrations. We demonstrate that the splitting between the X+ and X− trions as well as the fine structure of the X− state can be related to the short-range Coulomb-exchange interaction between the charge carriers.
Context.
The census of the globular clusters (GCs) in the Milky Way is still a work in progress. The advent of new deep surveys has made it possible to discover many new star clusters both in the ...Galactic disk and bulge, but many of these new candidates have not yet been studied in detail, leaving a veil on their true physical nature.
Aims.
We explore the nature of 19 new GC candidates in the Galactic bulge by analysing their colour–magnitude diagrams (CMDs) in the near-infrared (NIR) using the VISTA Variables in the Via Láctea Survey (VVV) database. We estimate their main astrophysical parameters: reddening and extinction, distance, total luminosity, mean cluster proper motions (PMs), metallicity, and age.
Methods.
We obtain the cluster catalogues including the likely cluster members by applying a decontamination procedure on the observed CMDs based on the vector PM diagrams from VIRAC2. We adopt NIR reddening maps in order to calculate the reddening and extinction for each cluster, and then estimate the distance moduli and heliocentric distances. Metallicities and ages are evaluated by fitting theoretical stellar isochrones. We also calculate their luminosities in comparison with known Galactic GCs.
Results.
We estimate a wide reddening range of 0.25 ⩽
E
(
J
−
K
s
)⩽2.0 mag and extinction 0.11 ⩽
A
Ks
⩽ 0.86 mag for the sample clusters, as expected in the bulge regions. The range of heliocentric distances is 6.8 ⩽
D
⩽ 11.4 kpc. This allows us to place these clusters between 0.56 and 3.25 kpc from the Galactic centre, assuming
R
⊙
= 8.2 kpc. Also, their PMs are kinematically similar to the typical motion of the Galactic bulge, apart from VVV-CL160, which shows different PMs. We also derive their metallicities and ages, finding −1.40⩽ Fe/H ⩽ 0.0 dex and
t
≈ 8 − 13 Gyr respectively. The luminosities are calculated both in
K
s
- and
V
-bands, recovering −3.4 ⩽
M
V
⩽ −7.5. We also examine the possible RR Lyrae members found in the cluster fields.
Conclusions.
Based on their positions, kinematics, metallicities, and ages, and comparing our results with the literature, we conclude that nine candidates are real GCs, seven need more observations to be fully confirmed as GCs, and three candidates are discarded as GCs and appear to be younger open clusters.
PDF
