68,185 research outputs found
Direct and indirect exciton mixture in double quantum wells
The exciton system in double quantum well is considered under condition when
the ground state is the spatially indirect exciton. At high pumping growth of
the exciton concentration can lead to so significant increase of the indirect
exciton energy that becomes equal to the direct exciton energy. Then further
increase of pumping leads to formation of mixed direct - indirect exciton
phase. A rough estimate of the exciton energy in the mixed phase explains
puzzling features of some recent exciton measurements. An experiment that would
reveal main characteristic features of the mixed phase is suggested.Comment: 10 pages, 2 figure
Suppressing Diffusion-Mediated Exciton Annihilation in 2D Semiconductors Using the Dielectric Environment
Atomically thin semiconductors such as monolayer MoS2 and WS2 exhibit
nonlinear exciton-exciton annihilation at notably low excitation densities
(below ~10 excitons/um2 in MoS2). Here, we show that the density threshold at
which annihilation occurs can be tuned by changing the underlying substrate.
When the supporting substrate is changed from SiO2 to Al2O3 or SrTiO3, the rate
constant for second-order exciton-exciton annihilation, k_XX [cm2/s], is
reduced by one or two orders of magnitude, respectively. Using transient
photoluminescence microscopy, we measure the effective room-temperature exciton
diffusion coefficient in chemical-treated MoS2 to be D = 0.06 +/- 0.01 cm2/s,
corresponding to a diffusion length of LD = 350 nm for an exciton lifetime of
{\tau} = 20 ns, which is independent of the substrate. These results, together
with numerical simulations, suggest that the effective exciton-exciton
annihilation radius monotonically decreases with increasing refractive index of
the underlying substrate. Exciton-exciton annihilation limits the overall
efficiency of 2D semiconductor devices operating at high exciton densities; the
ability to tune these interactions via the dielectric environment is an
important step toward more efficient optoelectronic technologies featuring
atomically thin materials
Device-spectroscopy of magnetic field effects in a polyfluorene organic light-emitting diode
We perform charge-induced absorption and electroluminescence spectroscopy in
a polyfluorene organic magnetoresistive device. Our experiments allow us to
measure the singlet exciton, triplet exciton and polaron densities in a live
device under an applied magnetic field, and to distinguish between three
different models that were proposed to explain organic magnetoresistance. These
models are based on different spin-dependent interactions, namely exciton
formation, triplet exciton-polaron quenching and bipolaron formation. We show
that the singlet exciton, triplet exciton and polaron densities and
conductivity all increase with increasing magnetic field. Our data are
inconsistent with the exciton formation and triplet-exciton polaron quenching
models.Comment: 4 pages, two figure
Exciton-exciton interaction in transition-metal dichalcogenide monolayers
We study theoretically the Coulomb interaction between excitons in transition
metal dichalcogenide (TMD) monolayers. We calculate direct and exchange
interaction for both ground and excited states of excitons. The screening of
the Coulomb interaction, specific to monolayer structures, leads to the unique
behavior of the exciton-exciton scattering for excited states, characterized by
the non-monotonic dependence of the interaction as function of the transferred
momentum. We find that the nontrivial screening enables the description of TMD
exciton interaction strength by approximate formula which includes exciton
binding parameters. The influence of screening and dielectric environment on
the exciton-exciton interaction was studied, showing qualitatively different
behavior for ground state and excited states of excitons. Furthermore, we
consider exciton-electron interaction, which for the excited states is governed
by the dominant attractive contribution of the exchange component, which
increases with the excitation number. The results provide a quantitative
description of the exciton-exciton and exciton-electron scattering in
transition metal dichalcogenides, and are of interest for the design of
perspective nonlinear optical devices based on TMD monolayers.Comment: 10 pages, 6 figure
First-principles method of propagation of tightly bound excitons: exciton band structure of LiF and verification with inelastic x-ray scattering
We propose a simple first-principles method to describe propagation of
tightly bound excitons. By viewing the exciton as a composite object (an
effective Frenkel exciton in Wannier orbitals), we define an exciton kinetic
kernel to encapsulate the exciton propagation and decay for all binding energy.
Applied to prototypical LiF, our approach produces three exciton bands, which
we verified quantitatively via inelastic x-ray scattering. The proposed
real-space picture is computationally inexpensive and thus enables study of the
full exciton dynamics, even in the presence of surfaces and impurity
scattering. It also provides intuitive understanding to facilitate practical
exciton engineering in semiconductors, strongly correlated oxides, and their
nanostructures.Comment: 5 pages, 4 figures. Accepted by PR
- …