5,285 research outputs found
Temperature dependence of exciton recombination in semiconducting single-wall carbon nanotubes
We study the excitonic recombination dynamics in an ensemble of (9,4)
semiconducting single-wall carbon nanotubes by high sensitivity time-resolved
photo-luminescence experiments. Measurements from cryogenic to room temperature
allow us to identify two main contributions to the recombination dynamics. The
initial fast decay is temperature independent and is attributed to the presence
of small residual bundles that create external non-radiative relaxation
channels. The slow component shows a strong temperature dependence and is
dominated by non-radiative processes down to 40 K. We propose a quantitative
phenomenological modeling of the variations of the integrated photoluminescence
intensity over the whole temperature range. We show that the luminescence
properties of carbon nanotubes at room temperature are not affected by the
dark/bright excitonic state coupling
Splitting between Bright and Dark excitons in Transition Metal Dichalcogenide Monolayers
The optical properties of transition metal dichalcogenide monolayers such as
the two-dimensional semiconductors MoS and WSe are dominated by
excitons, Coulomb bound electron-hole pairs. The light emission yield depends
on whether the electron-hole transitions are optically allowed (bright) or
forbidden (dark). By solving the Bethe Salpeter Equation on top of wave
functions in density functional theory calculations, we determine the sign and
amplitude of the splitting between bright and dark exciton states. We evaluate
the influence of the spin-orbit coupling on the optical spectra and clearly
demonstrate the strong impact of the intra-valley Coulomb exchange term on the
dark-bright exciton fine structure splitting.Comment: 6 pages, 2 figure
Polarization and time-resolved photoluminescence spectroscopy of excitons in MoSe2 monolayers
We investigate valley exciton dynamics in MoSe2 monolayers in polarization-
and time-resolved photoluminescence (PL) spectroscopy at 4K. Following
circularly polarized laser excitation, we record a low circular polarization
degree of the PL of typically . This is about 10 times lower than the
polarization induced under comparable conditions in MoS2 and WSe2 monolayers.
The evolution of the exciton polarization as a function of excitation laser
energy and power is monitored in PL excitation (PLE) experiments. Fast PL
emission times are recorded for both the neutral exciton of ps and for
the charged exciton (trion) of 12 ps.Comment: 4 pages, 3 figure
Exciton fine structure and spin decoherence in monolayers of transition metal dichalcogenides
We study the neutral exciton energy spectrum fine structure and its spin
dephasing in transition metal dichalcogenides such as MoS. The interaction
of the mechanical exciton with its macroscopic longitudinal electric field is
taken into account. The splitting between the longitudinal and transverse
excitons is calculated by means of the both electrodynamical approach and
perturbation theory. This long-range exciton
exchange interaction can induce valley polarization decay. The estimated
exciton spin dephasing time is in the picosecond range, in agreement with
available experimental data.Comment: 5 pages, 3 figure
Exciton dynamics in WSe2 bilayers
We investigate exciton dynamics in 2H-WSe2 bilayers in time-resolved
photoluminescence (PL) spectroscopy. Fast PL emission times are recorded for
both the direct exciton with ~ 3 ps and the indirect optical
transition with ~ 25 ps. For temperatures between 4 to 150 K
remains constant. Following polarized laser excitation, we observe
for the direct exciton transition at the K point of the Brillouin zone
efficient optical orientation and alignment during the short emission time
. The evolution of the direct exciton polarization and intensity as a
function of excitation laser energy is monitored in PL excitation (PLE)
experiments.Comment: 4 pages, 3 figure
Giant spin-dependent photo-conductivity in GaAsN dilute nitride semiconductor
A theoretical and experimental study of the spin-dependent photoconductivity
in dilute Nitride GaAsN is presented. The non linear transport model we develop
here is based on the rate equations for electrons, holes, deep paramagnetic and
non paramagnetic centers both under CW and pulsed optical excitation. Emphasis
is given to the effect of the competition between paramagnetic centers and non
paramagnetic centers which allows us to reproduce the measured characteristics
of the spin-dependent recombination power dependence. Particular attention is
paid to the role of an external magnetic field in Voigt geometry. The
photoconductivity exhibits a Hanle-type curve whereas the spin polarization of
electrons shows two superimposed Lorentzian curves with different widths,
respectively related to the recombination of free and trapped electrons. The
model is capable of reproducing qualitatively and quantitatively the most
important features of photoluminescence and photocurrent experiments and is
helpful in providing insight on the various mechanisms involved in the electron
spin polarization and filtering in GaAsN semiconductors.Comment: 10 pages, 5 figure
Measurement of heavy-hole spin dephasing in (InGa)As quantum dots
We measure the spin dephasing of holes localized in self-assembled (InGa)As
quantum dots by spin noise spectroscopy. The localized holes show a distinct
hyperfine interaction with the nuclear spin bath despite the p-type symmetry of
the valence band states. The experiments reveal a short spin relaxation time
{\tau}_{fast}^{hh} of 27 ns and a second, long spin relaxation time
{\tau}_{slow}^{hh} which exceeds the latter by more than one order of
magnitude. The two times are attributed to heavy hole spins aligned
perpendicular and parallel to the stochastic nuclear magnetic field. Intensity
dependent measurements and numerical simulations reveal that the long
relaxation time is still obscured by light absorption, despite low laser
intensity and large detuning. Off-resonant light absorption causes a
suppression of the spin noise signal due to the creation of a second hole
entailing a vanishing hole spin polarization.Comment: accepted to be published in AP
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