1,009 research outputs found
Phonon Sidebands in Transition Metal Dichalcogenides
Excitons dominate the optical properties of monolayer transition metal
dichalcogenides (TMDs). Besides optically accessible bright exciton states,
TMDs exhibit also a multitude of optically forbidden dark excitons. Here, we
show that efficient exciton-phonon scattering couples bright and dark states
and gives rise to an asymmetric excitonic line shape. The observed asymmetry
can be traced back to phonon-induced sidebands that are accompanied by a
polaron redshift. We present a joint theory-experiment study investigating the
microscopic origin of these sidebands in different TMD materials taking into
account intra- and intervalley scattering channels opened by optical and
acoustic phonons. The gained insights contribute to a better understanding of
the optical fingerprint of these technologically promising nanomaterials
Phonon-mediated negative differential conductance in molecular quantum dots
Transport through a single molecular conductor is considered, showing
negative differential conductance behavior associated with phonon-mediated
electron tunneling processes. This theoretical work is motivated by a recent
experiment by Leroy et al. using a carbon nanotube contacted by an STM tip
[Nature {\bf 432}, 371 (2004)], where negative differential conductance of the
breathing mode phonon side peaks could be observed. A peculiarity of this
system is that the tunneling couplings which inject electrons and those which
collect them on the substrate are highly asymmetrical. A quantum dot model is
used, coupling a single electronic level to a local phonon, forming polaron
levels. A "half-shuttle" mechanism is also introduced. A quantum kinetic
formulation allows to derive rate equations. Assuming asymmetric tunneling
rates, and in the absence of the half-shuttle coupling, negative differential
conductance is obtained for a wide range of parameters. A detailed explanation
of this phenomenon is provided, showing that NDC is maximal for intermediate
electron-phonon coupling. In addition, in absence of a gate, the "floating"
level results in two distinct lengths for the current plateaus, related to the
capacitive couplings at the two junctions. It is shown that the "half-shuttle"
mechanism tends to reinforce the negative differential regions, but it cannot
trigger this behavior on its own
Intersublevel Polaron Dephasing in Self-Assembled Quantum Dots
Polaron dephasing processes are investigated in InAs/GaAs dots using
far-infrared transient four wave mixing (FWM) spectroscopy. We observe an
oscillatory behaviour in the FWM signal shortly (< 5 ps) after resonant
excitation of the lowest energy conduction band transition due to coherent
acoustic phonon generation. The subsequent single exponential decay yields long
intraband dephasing times of 90 ps. We find excellent agreement between our
measured and calculated FWM dynamics, and show that both real and virtual
acoustic phonon processes are necessary to explain the temperature dependence
of the polarization decay.Comment: 10 pages, 4 figures, submitted to Phys Rev Let
Phonon Coherence and New Set of Sidebands in Phonon-Assisted Photoluminescence
We investigate excitonic polaron states comprising a local exciton and
phonons in the longitudinal optical (LO) mode by solving the Schr\"{o}dinger
equation. We derive an exact expression for the ground state (GS), which
includes multi-phonon components with coefficients satisfying the Huang-Rhys
factors. The recombination of GS and excited polaron states gives one set of
sidebands in photoluminescence (PL): the multi-phonon components in the GS
produce the Stokes lines and the zero-phonon components in the excited states
produce the anti-Stokes lines. By introducing the mixing of the LO mode and
environal phonon modes, the exciton will also couple with the latter, and the
resultant polaron states result in another set of phonon sidebands. This set
has a zero-phonon line higher and wider than that of the first set due to the
tremendous number of the environal modes. The energy spacing between the
zero-phonon lines of the first and second sets is proved to be the binding
energy of the GS state. The common exciton origin of these two sets can be
further verified by a characteristic Fano lineshape induced by the coherence in
the mixing of the LO and the environal modes.Comment: 5 pages, 3 figures 1 figure (fig. 1) replaced 1 figure (fig. 2)
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Chirality Dependence of the -Momentum Dark Excitons in Carbon Nanotubes
Using a collection of twelve semiconducting carbon nanotube samples, each
highly enriched in a single chirality, we study the chirality dependence of the
-momentum dark singlet exciton using phonon sideband optical spectroscopy.
Measurements of bright absorptive and emissive sidebands of this finite
momentum exciton identify its energy as 20 - 38 meV above the bright singlet
exciton, a separation that exhibits systematic dependencies on tube diameter,
family, and semiconducting type. We present calculations that explain
how chiral angle dependence in this energy separation relates to the Coulomb
exchange interaction, and elaborate the dominance of the phonon
sidebands over the zone-center phonon sidebands over a wide range of
chiralities. The Kataura plot arising from these data is qualitatively well
described by theory, but the energy separation between the sidebands shows a
larger chiral dependence than predicted. This latter observation may indicate a
larger dispersion for the associated phonon near the point than expected
from finite distance force modeling.Comment: 24 pages, 12 figures, 1 table; slight title change, Figures 1 and 11
added, reference added, presentation improved throughout documen
Local density of states on a vibrational quantum dot out of equilibrium
We calculate the nonequilibrium local density of states on a vibrational
quantum dot coupled to two electrodes at T=0 using a numerically exact
diagrammatic Monte Carlo method. Our focus is on the interplay between the
electron-phonon interaction strength and the bias voltage. We find that the
spectral density exhibits a significant voltage dependence if the voltage
window includes one or more phonon sidebands. A comparison with
well-established approximate approaches indicates that this effect could be
attributed to the nonequilibrium distribution of the phonons. Moreover, we
discuss the long transient dynamics caused by the electron-phonon coupling.Comment: 9 pages, 11 figure
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