1,180 research outputs found
Kondo Resonance Decoherence by an External Potential
The Kondo problem, for a quantum dot (QD), subjected to an external bias, is
analyzed in the limit of infinite Coulomb repulsion by using a consistent
equations of motion method based on a slave-boson Hamiltonian. Utilizing a
strict perturbative solution in the leads-dot coupling, T, to T^4 and T^6
orders, we calculate the QD spectral density and conductance, as well as the
decoherent rate that drive the systemm from the strong to the weak coupling
regime. Our results indicate thet the weak coupling regime is reached for
voltages larger than a few units of the Kondo temperature.Comment: 5 figure
Diffuse Surface Scattering in the Plasmonic Resonances of Ultra-Low Electron Density Nanospheres
Localized surface plasmon resonances (LSPRs) have recently been identified in
extremely diluted electron systems obtained by doping semiconductor quantum
dots. Here we investigate the role that different surface effects, namely
electronic spill-out and diffuse surface scattering, play in the optical
properties of these ultra-low electron density nanosystems. Diffuse scattering
originates from imperfections or roughness at a microscopic scale on the
surface. Using an electromagnetic theory that describes this mechanism in
conjunction with a dielectric function including the quantum size effect, we
find that the LSPRs show an oscillatory behavior both in position and width for
large particles and a strong blueshift in energy and an increased width for
smaller radii, consistent with recent experimental results for photodoped ZnO
nanocrystals. We thus show that the commonly ignored process of diffuse surface
scattering is a more important mechanism affecting the plasmonic properties of
ultra-low electron density nanoparticles than the spill-out effect.Comment: 19 pages, 5 figures. Accepted for publication in The Journal of
Physical Chemistry Letter
Surface scattering contribution to the plasmon width in embedded Ag nanospheres
Nanometer-sized metal particles exhibit broadening of the localized surface
plasmon resonance (LSPR) in comparison to its value predicted by the classical
Mie theory. Using our model for the LSPR dependence on non-local surface
screening and size quantization, we quantitatively relate the observed plasmon
width to the nanoparticle radius and the permittivity of the surrounding
medium . For Ag nanospheres larger than 8 nm only the non-local
dynamical effects occurring at the surface are important and, up to a diameter
of 25 nm, dominate over the bulk scattering mechanism. Qualitatively, the LSPR
width is inversely proportional to the particle size and has a nonmonotonic
dependence on the permittivity of the host medium, exhibiting for Ag a maximum
at . Our calculated LSPR width is compared with recent
experimental data.Comment: 11 pages, 4 figures. Accepted for publication in Optics Expres
Dressed tunneling approximation for electronic transport through molecular transistors
A theoretical approach for the non-equilibrium transport properties of
nanoscale systems coupled to metallic electrodes with strong electron-phonon
interactions is presented. It consists in a resummation of the dominant Feynman
diagrams from the perturbative expansion in the coupling to the leads. We show
that this scheme eliminates the main pathologies found in previous simple
analytical approaches for the polaronic regime. The results for the spectral
and transport properties are compared with those from several other approaches
for a wide range of parameters. The method can be formulated in a simple way to
obtain the full counting statistics. Results for the shot and thermal noise are
presented.Comment: 11 pages, 11 figures. Accepted for publication in Physical Review
Electron-Electron and Electron-Phonon Interactions in the Dynamics of Trap-Filling in Charged Quantum Dots
We analyze theoretically the effects of electron-electron and electron-phonon
interactions in the dynamics of a system of a few electrons that can be trapped
to a localized state and detrapped to an extended band state of a small quantum
dot (QD) using a simple model. In our model the QD is described by one or two
single-particle energy levels while the trap is described by one
single-particle level connected to the QD by a hopping Hamiltonian.
Electron-electron Coulomb repulsion and electron-phonon interactions are
included in the localized trap state. In spite of its simplicity the time
dependent model has no analytical solution but a numerically exact one can be
found at a relatively low computational cost. Using values of the parameters
appropriate for defects in semiconductor QDs, we find that the electronic
motion is quasi-periodic in time, with oscillations around mean values that are
set in timescales of typically a few tenths of picoseconds to picoseconds. We
increase the number of electrons initially in the QD from one to four and find
that one electron is transferred to the trap state for three electrons in the
QD. At the more efficient values of the electron-phonon coupling, these
characteristics are quite independent of the value of the electron-electron
Coulomb repulsion in the trap, up to the value above which its infinite limit
is reached. We conclude that strong electron-phonon interaction is an efficient
mechanism that can provide the complete filling of a deep trap state on a sub
to picoseconds timescale, faster than radiative exciton decay and Auger
recombination processes. This leads to the complete suppression of the
luminescence to the deep trap state and to the accumulation of electrons in the
QD.Comment: Accepted for publication in The Journal of Physical Chemistry
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