251 research outputs found
Three-particle states and brightening of intervalley excitons in a doped MoS monolayer
Optical spectra of two-dimensional transition-metal dichalcogenides (TMDC)
are influenced by complex multi-particle excitonic states. Their theoretical
analysis requires solving the many-body problem, which in most cases, is
prohibitively complicated. In this work, we calculate the optical spectra by
exact diagonalization of the three-particle Hamiltonian within the Tamm-Dancoff
approximation where the doping effects are accounted for via the Pauli blocking
mechanism, modelled by a discretized mesh in the momentum space. The
single-particle basis is extracted from the {\it ab initio} calculations.
Obtained three-particle eigenstates and the corresponding transition dipole
matrix elements are used to calculate the linear absorption spectra as a
function of the doping level. Results for negatively doped MoS monolayer
(ML) are in an excellent quantitative agreement with the available experimental
data, validating our approach. The results predict additional spectral features
due to the intervalley exciton that is optically dark in an undoped ML but is
brightened by the doping. Our approach can be applied to a plethora of other
atomically thin semiconductors, where the doping induced brightening of the
many-particle states is also anticipated
Polaronic Signatures in Mid-Infrared Spectra: Prediction for LaMnO3 and CaMnO3
Hole-doped LaMnO3 and electron-doped CaMnO3 form self-trapped electronic
states. The spectra of these states have been calculated using a two orbital
(Mn eg Jahn-Teller) model, from which the non-adiabatic optical conductivity
spectra are obtained. In both cases the optical spectrum contains weight in the
gap region, whose observation will indicate the self-trapped nature of the
carrier states. The predicted spectra are proportional to the concentration of
the doped carriers in the dilute regime, with coefficients calculated with no
further model parameters.Comment: 6 pages with 3 figures imbedde
Intersubband decay of 1-D exciton resonances in carbon nanotubes
We have studied intersubband decay of E22 excitons in semiconducting carbon
nanotubes experimentally and theoretically. Photoluminescence excitation line
widths of semiconducting nanotubes with chiral indicess (n, m) can be mapped
onto a connectivity grid with curves of constant (n-m) and (2n+m). Moreover,
the global behavior of E22 linewidths is best characterized by a strong
increase with energy irrespective of their (n-m) mod(3)= \pm 1 family
affiliation. Solution of the Bethe-Salpeter equations shows that the E22
linewidths are dominated by phonon assisted coupling to higher momentum states
of the E11 and E12 exciton bands. The calculations also suggest that the
branching ratio for decay into exciton bands vs free carrier bands,
respectively is about 10:1.Comment: 4 pages, 4 figure
How does the substrate affect the Raman and excited state spectra of a carbon nanotube?
We study the optical properties of a single, semiconducting single-walled
carbon nanotube (CNT) that is partially suspended across a trench and partially
supported by a SiO2-substrate. By tuning the laser excitation energy across the
E33 excitonic resonance of the suspended CNT segment, the scattering
intensities of the principal Raman transitions, the radial breathing mode
(RBM), the G-mode and the D-mode show strong resonance enhancement of up to
three orders of magnitude. In the supported part of the CNT, despite a loss of
Raman scattering intensity of up to two orders of magnitude, we recover the E33
excitonic resonance suffering a substrate-induced red shift of 50 meV. The peak
intensity ratio between G-band and D-band is highly sensitive to the presence
of the substrate and varies by one order of magnitude, demonstrating the much
higher defect density in the supported CNT segments. By comparing the E33
resonance spectra measured by Raman excitation spectroscopy and
photoluminescence (PL) excitation spectroscopy in the suspended CNT segment, we
observe that the peak energy in the PL excitation spectrum is red-shifted by 40
meV. This shift is associated with the energy difference between the localized
exciton dominating the PL excitation spectrum and the free exciton giving rise
to the Raman excitation spectrum. High-resolution Raman spectra reveal
substrate-induced symmetry breaking, as evidenced by the appearance of
additional peaks in the strongly broadened Raman G band. Laser-induced line
shifts of RBM and G band measured on the suspended CNT segment are both linear
as a function of the laser excitation power. Stokes/anti-Stokes measurements,
however, reveal an increase of the G phonon population while the RBM phonon
population is rather independent of the laser excitation power.Comment: Revised manuscript, 20 pages, 8 figure
Self-trapped Exciton and Franck-Condon Spectra Predicted in LaMnO
Because the ground state has cooperative Jahn-Teller order, electronic
excitations in LaMnO are predicted to self-trap by local rearrangement of
the lattice. The optical spectrum should show a Franck-Condon series, that is,
a Gaussian envelope of vibrational sidebands. Existing data are reinterpreted
in this way. The Raman spectrum is predicted to have strong multiphonon
features.Comment: 5 pages with two embedded postscript figure
Nonlinear spectroscopy of excitonic states in transition metal dichalcogenides
Second-harmonic generation (SHG) is a well-known nonlinear spectroscopy
method to probe electronic structure, specifically, in transition metal
dichalcogenide (TMDC) monolayers. This work investigates the nonlinear dynamics
of a strongly excited TMDC monolayer by solving the time evolution equations
for the density matrix. It is shown that the presence of excitons qualitatively
changes the nonlinear dynamics leading, in particular, to a huge enhancement of
the nonlinear signal as a function of the dielectric environment. It is also
shown that the SHG polarization angular diagram and its dependence on the
driving strength are very sensitive to the type of exciton state. This
sensitivity suggests that SHG spectroscopy is a convenient tool for analyzing
the fine structure of excitonic states.Comment: 13 pages, 5 figure
Magnetic Brightening of Carbon Nanotube Photoluminescence through Symmetry Breaking
Often a modification of microscopic symmetry in a system can result in a
dramatic change in its macroscopic properties. Here we report that symmetry
breaking by a tube-threading magnetic field can drastically increase the
photoluminescence quantum yield of semiconducting single-walled carbon
nanotubes, by as much as a factor of six, at low temperatures. To explain this
striking connection between seemingly unrelated properties, we have developed a
comprehensive theoretical model based on magnetic-field-dependent
one-dimensional exciton band structure and the interplay of strong Coulomb
interactions and the Aharonov-Bohm effect. This conclusively explains our data
as the first experimental observation of dark excitons 5-10 meV below the
bright excitons in single-walled carbon nanotubes. We predict that this quantum
yield increase can be made much larger in disorder-free samples
Can impact excitation explain efficient carrier multiplication in carbon nanotube photodiodes?
We address recent experiments (Science 325, 1367 (2009)) reporting on highly
efficient multiplication of electron-hole pairs in carbon nanotube photodiodes
at photon energies near the carrier multiplication threshold (twice the
quasi-particle band gap). This result is surprising in light of recent
experimental and theoretical work on multiexciton generation in other confined
materials, such as semiconducting nanocrystals. We propose a detailed mechanism
based on carrier dynamics and impact excitation resulting in highly efficient
multiplication of electron-hole pairs. We discuss the important time and energy
scales of the problem and provide analysis of the role of temperature and the
length of the diode
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