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

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.

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

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