662 research outputs found

### Excited-state relaxation in PbSe quantum dots

In solids the phonon-assisted, nonradiative decay from high-energy electronic excited states to low-energy electronic excited states is picosecond fast. It was hoped that electron and hole relaxation could be slowed down in quantum dots, due to the unavailability of phonons energy matched to the large energy-level spacings (“phonon-bottleneck”). However, excited-state relaxation was observed to be rather fast (1 ps) in InP, CdSe, and ZnO dots, and explained by an efficient Auger mechanism, whereby the excess energy of electrons is nonradiatively transferred to holes, which can then rapidly decay by phonon emission, by virtue of the densely spaced valence-band levels. The recent emergence of PbSe as a novel quantum-dot material has rekindled the hope for a slow down of excited-state relaxation because hole relaxation was deemed to be ineffective on account of the widely spaced hole levels. The assumption of sparse hole energy levels in PbSe was based on an effective-mass argument based on the light effective mass of the hole. Surprisingly, fast intraband relaxation times of 1–7 ps were observed in PbSe quantum dots and have been considered contradictory with the Auger cooling mechanism because of the assumed sparsity of the hole energy levels. Our pseudopotential calculations, however, do not support the scenario of sparse hole levels in PbSe: Because of the existence of three valence-band maxima in the bulk PbSe band structure, hole energy levels are densely spaced, in contradiction with simple effective-mass models. The remaining question is whether the Auger decay channel is sufficiently fast to account for the fast intraband relaxation. Using the atomistic pseudopotential wave functions of Pb2046Se2117 and Pb260Se249 quantum dots, we explicitly calculated the electron-hole Coulomb integrals and the PS electron Auger relaxation rate. We find that the Auger mechanism can explain the experimentally observed PS intraband decay time scale without the need to invoke any exotic relaxation mechanisms

### Localization and band-gap pinning in semiconductor superlattices with layer-thickness fluctuations

We consider (AlAs)_n/(GaAs)_n superlattices with random thickness
fluctuations Delta-n around the nominal period n. Using three-dimensional
pseudopotential plane-wave band theory, we show that (i) any amount Delta-n/n
of thickness fluctuations leads to band-edge wavefunction localization, (ii)
for small Delta-n/n the SL band gap is pinned at the gap level produced by a
single layer with ``wrong'' thickness n + Delta-n, (iii) the bound states due
to monolayer thickness fluctuations lead to significant band-gap reductions,
(iv) AlAs/GaAs SL's with monolayer thickness fluctuations have a direct
band gap, while the ideal SL's are indirect for n<4.Comment: 10 pages, Revtex. 3 figures available at
http://www.cecam.fr/~mader/elstruc.html . Published in Europhys. Lett. 31,
107 (95

### Dependence of the electronic structure of self-assembled InGaAs/GaAs quantum dots on height and composition

While electronic and spectroscopic properties of self-assembled
In_{1-x}Ga_{x}As/GaAs dots depend on their shape, height and alloy
compositions, these characteristics are often not known accurately from
experiment. This creates a difficulty in comparing measured electronic and
spectroscopic properties with calculated ones. Since simplified theoretical
models (effective mass, k.p, parabolic models) do not fully convey the effects
of shape, size and composition on the electronic and spectroscopic properties,
we offer to bridge the gap by providing accurately calculated results as a
function of the dot height and composition. Prominent results are the
following. (i) Regardless of height and composition, the electron levels form
shells of nearly degenerate states. In contrast, the hole levels form shells
only in flat dots and near the highest hole level (HOMO). (ii) In alloy dots,
the electrons' ``s-p'' splitting depends weakly on height, while the ``p-p''
splitting depends non-monotonically. In non-alloyed InAs/GaAs dots, both these
splittings depend weakly on height. For holes in alloy dots, the ``s-p''
splitting decreases with increasing height, whereas the ``p-p'' splitting
remains nearly unchaged. Shallow, non-alloyed dots have a ``s-p'' splitting of
nearly the same magnitude, whereas the ``p-p'' splitting is larger. (iii) As
height increases, the ``s'' and ``p'' character of the wavefunction of the HOMO
becomes mixed, and so does the heavy- and light-hole character. (iv) In alloy
dots, low-lying hole states are localized inside the dot. Remarkably, in
non-alloyed InAs/GaAs dots these states become localized at the interface as
height increases. This localization is driven by the biaxial strain present in
the nanostructure.Comment: 14 pages, 12 figure

### Multi-excitons in self-assembled InAs/GaAs quantum dots: A pseudopotential, many-body approach

We use a many-body, atomistic empirical pseudopotential approach to predict
the multi-exciton emission spectrum of a lens shaped InAs/GaAs self-assembled
quantum dot. We discuss the effects of (i) The direct Coulomb energies,
including the differences of electron and hole wavefunctions, (ii) the exchange
Coulomb energies and (iii) correlation energies given by a configuration
interaction calculation. Emission from the groundstate of the $N$ exciton
system to the $N-1$ exciton system involving $e_0\to h_0$ and $e_1\to h_1$
recombinations are discussed. A comparison with a simpler single-band,
effective mass approach is presented

### Scattering in Noncommutative Quantum Mechanics

We derive the correction due to noncommutativity of space on Born
approximation, then the correction for the case of Yukawa potential is
explicitly calculated. The correction depends on the angle of scattering. Using
partial wave method it is shown that the conservation of the number of
particles in elastic scattering is also valid in noncommutative spaces which
means that the unitarity relation is held in noncommutative spaces. We also
show that the noncommutativity of space has no effect on the optical theorem.
Finally we study Gaussian function potential in noncommutative spaces which
generates delta function potential as $\theta \to 0$.Comment: 7 Pages, no figure, accepted for publication in Modern Physics
Letters

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