158 research outputs found
Quantum interference in nanometric devices: ballistic transport across arrays of T-shaped quantum wires
We propose that the recently realized T-shaped semiconductor quantum wires
(T-wires) could be exploited as three-terminal quantum interference devices.
T-wires are formed by intersecting two quantum wells (QWs). By use of a
scattering matrix approach and the Landauer-B\"uttiker theory, we calculate the
conductance for ballistic transport in the parent QWs and across the wire
region as a function of the injection energy. We show that different
conductance profiles can be selected by tailoring the widths of the QWs and/or
combining more wires on the scale of the Fermi wavelength. Finally, we discuss
the possibility of obtaining spin-dependent conductance of ballistic holes in
the same structures.Comment: To appear in the 09/15/97 issue of Appl. Phys. Lett. (9 pages in
REVTEX + 2 figures in postscript
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
Optical near-field mapping of excitons and biexcitons in naturally occurring semiconductor quantum dots
We calculate the near-field optical spectra of excitons and biexcitons in
semiconductor quantum dots naturally occurring at interface fluctuations in
GaAs-based quantum wells, using a non-local description of the response
function to a spatially modulated electro-magnetic field. The relative
intensity of the lowest, far-field forbidden excitonic states is predicted; the
spatial extension of the ground biexciton state is found in agreement with
recently published experiments
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
Semiconductor quantum tubes: dielectric modulation and excitonic response
We study theoretically the optical properties of quantum tubes,
one-dimensional semiconductor nanostructures where electrons and holes are
confined to a cylindrical shell. In these structures, which bridge between 2D
and 1D systems, the electron-hole interaction may be modulated by a dielectric
substance outside the quantum tube and possibly inside its core. We use the
exact Green's function for the appropriate dielectric configuration and exact
diagonalization of the electron-hole interaction within an effective mass
description to predict the evolution of the exciton binding energy and
oscillator strength. Contrary to the homogeneous case, in dielectrically
modulated tubes the exciton binding is a function of the tube diameter and can
be tuned to a large extent by structure design and proper choice of the
dielectric media.Comment: 9 pages, 6 figures, in print for Phys. Rev.
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
A new quasiparticle in carbon nanotubes
Trions—one electron bound to two holes via Coulomb forces—can be observed in the optical spectra of doped carbon nanotube
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