224 research outputs found
Phase lapses in scattering through multi-electron quantum dots: Mean-field and few-particle regimes
We show that the observed evolution of the transmission phase through
multi-electron quantum dots with more than approximately ten electrons, which
shows a universal (i.e., independent of N) as yet unexplained behavior, is
consistent with an electrostatic model, where electron-electron interaction is
described by a mean-field approach. Moreover, we perform exact calculations for
an open 1D quantum dot and show that carrier correlations may give rise to a
non-universal (i.e., N-dependent) behavior of the transmission phase, ensuing
from Fano resonances, which is consistent with experiments with a few (N < 10)
carriers. Our results suggest that in the universal regime the coherent
transmission takes place through a single level while in the few-particle
regime the correlated scattering state is determined by the number of bound
particles.Comment: 14 pages, 3 figures, RevTex4 preprint format, to appear in Phys. Rev.
Landau levels, edge states and magneto-conductance in GaAs/AlGaAs core-shell nanowires
Magnetic states of the electron gas confined in modulation-doped core-shell
nanowires are calculated for a transverse field of arbitrary strength and
orientation. Magneto-conductance is predicted within the Landauer approach. The
modeling takes fully into account the radial material modulation, the prismatic
symmetry and the doping profile of realistic GaAs/AlGaAs devices within an
envelope-function approach, and electron-electron interaction is included in a
mean-field self-consistent approach. Calculations show that in the low
free-carrier density regime, magnetic states can be described in terms of
Landau levels and edge states, similar to planar two-dimensional electron gases
in a Hall bar. However, at higher carrier density the dominating
electron-electron interaction leads to a strongly inhomogeneous localization at
the prismatic heterointerface. This gives rise to a complex band dispersion,
with local minima at finite values of the longitudinal wave vector, and a
region of negative magneto-resistance. The predicted marked anisotropy of the
magneto-conductance with field direction is a direct probe of the inhomogeneous
electron gas localization of the conductive channel induced by the prismatic
geometry
Exact two-body quantum dynamics of an electron-hole pair in semiconductor coupled quantum wells: a time-dependent approach
We simulate the time-dependent coherent dynamics of a spatially indirect
exciton (an electron-hole pair with the two particles confined in different
layers) in a GaAs coupled quantum well system. We use a unitary wave-packet
propagation method taking into account in full the four degrees of freedom of
the two particles in a two-dimensional system, including both the long-range
Coulomb attraction and arbitrary two-dimensional electrostatic potentials
affecting the electron and/or the hole separately. The method has been
implemented for massively parallel architectures to cope with the huge
numerical problem, showing good scaling properties and allowing evolution for
tens of picoseconds. We have investigated both transient time phenomena and
asymptotic time transmission and reflection coefficients for potential profiles
consisting of i) extended barriers and wells and ii) a single-slit geometry. We
found clear signatures of the internal two-body dynamics, with transient
phenomena in the picosecond time-scale which might be revealed by optical
spectroscopy. Exact results have been compared with mean-field approaches
which, neglecting dynamical correlations by construction, turn out to be
inadequate to describe the electron-hole pair evolution in realistic
experimental conditions.Comment: 12 two-column pages + 3 supplemental material pages, 9 figures, to
appear on Phys.Rev.
Space- and time-dependent quantum dynamics of spatially indirect excitons in semiconductor heterostructures
We study the unitary propagation of a two-particle one-dimensional
Schr\"odinger equation by means of the Split-Step Fourier method, to study the
coherent evolution of a spatially indirect exciton (IX) in semiconductor
heterostructures. The mutual Coulomb interaction of the electron-hole pair and
the electrostatic potentials generated by external gates and acting on the two
particles separately are taken into account exactly in the two-particle
dynamics. As relevant examples, step/downhill and barrier/well potential
profiles are considered. The space- and time-dependent evolution during the
scattering event as well as the asymptotic time behavior are analyzed. For
typical parameters of GaAs-based devices the transmission or reflection of the
pair turns out to be a complex two-particle process, due to comparable and
competing Coulomb, electrostatic and kinetic energy scales. Depending on the
intensity and anisotropy of the scattering potentials, the quantum evolution
may result in excitation of the IX internal degrees of freedom, dissociation of
the pair, or transmission in small periodic IX wavepackets due to dwelling of
one particle in the barrier region. We discuss the occurrence of each process
in the full parameter space of the scattering potentials and the relevance of
our results for current excitronic technologies.Comment: 28 pages, 10 figures, preprint forma
Symmetries in the collective excitations of an electron gas in core-shell nanowires
We study the collective excitations and inelastic light scattering
cross-section of an electron gas confined in a GaAs/AlGaAs coaxial quantum
well. These system can be engineered in a core-multi-shell nanowire and inherit
the hexagonal symmetry of the underlying nanowire substrate. As a result, the
electron gas forms both quasi 1D channels and quasi 2D channels at the quantum
well bents and facets, respectively. Calculations are performed within the RPA
and TDDFT approaches. We derive symmetry arguments which allow to enumerate and
classify charge and spin excitations and determine whether excitations may
survive to Landau damping. We also derive inelastic light scattering selection
rules for different scattering geometries. Computational issues stemming from
the need to use a symmetry compliant grid are also investigated systematically
Magneto-photoluminescence in GaAs/AlAs core-multishell nanowires: a theoretical investigation
The magneto-photoluminescence in modulation doped core-multishell nanowires
is predicted as a function of photo-excitation intensity in non-perturbative
transverse magnetic fields. We use a self-consistent field approach within the
effective mass approximation to determine the photoexcited electron and hole
populations, including the complex composition and anisotropic geometry of the
nano-material. The evolution of the photoluminescence is analyzed as a function
of i) photo-excitation power, ii) magnetic field intensity, iii) type of
doping, and iv) anisotropy with respect to field orientation.Comment: 11 pages, 11 figures, accepted for publication in Physical Review
Field-controlled suppression of phonon-induced transitions in coupled quantum dots
We calculate the longitudinal-acoustic phonon scattering rate for a vertical
double quantum dot system with weak lateral confinement and show that a strong
modulation of the single-electron excited states lifetime can be induced by an
external magnetic or electric field. The results are obtained for typical
realistic devices using a Fermi golden rule approach and a three-dimensional
description of the electronic quantum states.Comment: REVTex4 class, 6 pages, 3 figures, to be published in Applied Physics
Letter
Predicting signatures of anisotropic resonance energy transfer in dye-functionalized nanoparticles
Resonance energy transfer (RET) is an inherently anisotropic process. Even
the simplest, well-known F\"orster theory, based on the transition
dipole-dipole coupling, implicitly incorporates the anisotropic character of
RET. In this theoretical work, we study possible signatures of the fundamental
anisotropic character of RET in hybrid nanomaterials composed of a
semiconductor nanoparticle (NP) decorated with molecular dyes. In particular,
by means of a realistic kinetic model, we show that the analysis of the dye
photoluminescence difference for orthogonal input polarizations reveals the
anisotropic character of the dye-NP RET which arises from the intrinsic
anisotropy of the NP lattice. In a prototypical core/shell wurtzite CdSe/ZnS NP
functionalized with cyanine dyes (Cy3B), this difference is predicted to be as
large as 75\% and it is strongly dependent in amplitude and sign on the dye-NP
distance. We account for all the possible RET processes within the system,
together with competing decay pathways in the separate segments. In addition,
we show that the anisotropic signature of RET is persistent up to a large
number of dyes per NP.Comment: 9 pages, 5 figures. Supplementary information available at
http://pubs.rsc.org/en/content/articlelanding/2016/ra/c6ra22433d/unauth#!divAbstrac
Aharonov-Bohm oscillations and electron gas transitions in hexagonal core-shell nanowires with an axial magnetic field
We use spin-density-functional theory within an envelope function approach to
calculate electronic states in a GaAs/InAs core-shell nanowire pierced by an
axial magnetic field. Our fully 3D quantum modeling includes explicitly the
description of the realistic cross-section and composition of the sample, and
the electrostatic field induced by external gates in two different device
geometries, gate-all-around and back-gate. At low magnetic fields, we
investigate Aharonov-Bohm oscillations and signatures therein of the discrete
symmetry of the electronic system, and we critically analyze recent
magnetoconductance observations. At high magnetic fields we find that several
charge and spin transitions occur. We discuss the origin of these transitions
in terms of different localization and Coulomb regimes and predict their
signatures in magnetoconductance experiments
Signatures of molecular correlations in the few-electron dynamics of coupled quantum dots
We study the effect of Coulomb interaction on the few-electron dynamics in
coupled semiconductor quantum dots by exact diagonalization of the few-body
Hamiltonian. The oscillation of carriers is strongly affected by the number of
confined electrons and by the strength of the interdot correlations.
Single-frequency oscillations are found for either uncorrelated or highly
correlated states, while multi-frequency oscillations take place in the
intermediate regime. Moreover, Coulomb interaction renders few-particle
oscillations sensitive to perturbations in spatial directions other than that
of the tunneling, contrary to the single-particle case. The inclusion of
acoustic phonon scattering does not modify the carrier dynamics substantially
at short times, but can damp oscillation modes selectively at long times.Comment: 4 pages, 5 figures, RevTex4 two-column format, to appear in Phys.
Rev.
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