Archivio della ricerca dell'Università di Modena e Reggio Emiliahttps://iris.unimore.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Wed, 13 Nov 2019 03:31:38 GMT2019-11-13T03:31:38Z101201Alla scoperta delle nanotecnologiehttp://hdl.handle.net/11380/1007714Titolo: Alla scoperta delle nanotecnologie
Abstract: La fisica alla nanoscala
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/11380/10077142014-01-01T00:00:00ZCylindrical two-dimensional electron gas in a transverse magnetic fieldhttp://hdl.handle.net/11380/597863Titolo: Cylindrical two-dimensional electron gas in a transverse magnetic field
Abstract: We compute the single-particle states of a two-dimensional (2D) electron gas confined to the surface of a cylinder immersed in a magnetic field. The envelope-function equation is solved exactly for both a homogeneous and a periodically modulated magnetic field perpendicular to the cylinder axis. The nature and energy dispersion of the quantum states reflects the interplay between different length scales, namely, the cylinder diameter, the magnetic length, and, possibly, the wavelength of the field modulation. We show that a transverse homogeneous magnetic field drives carrier states from a quasi-2D (cylindrical) regime to a quasi-one-dimensional regime where carriers form channels along the cylinder surface. Furthermore, a magnetic field which is periodically modulated along the cylinder axis may confine the carriers to tunnel-coupled stripes, rings, and dots on the cylinder surface depending on the ratio between the field periodicity and the cylinder radius. Results in different regimes are traced to either incipient Landau-level formation or Aharonov-Bohm behavior.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11380/5978632008-01-01T00:00:00ZEffect of electron-electron interaction on the phonon-mediated spin relaxation in quantum dotshttp://hdl.handle.net/11380/597862Titolo: Effect of electron-electron interaction on the phonon-mediated spin relaxation in quantum dots
Abstract: We estimate the spin relaxation rate due to spin-orbit coupling and acoustic phonon scattering in weakly confined quantum dots with up to five interacting electrons. The full configuration interaction approach is used to account for the interelectron repulsion, and Rashba and Dresselhaus spin-orbit couplings are exactly diagonalized. We show that electron-electron interaction strongly affects spin-orbit admixture in the sample. Consequently, relaxation rates strongly depend on the number of carriers confined in the dot. We identify the mechanisms which may lead to improved spin stability in few electron (> 2) quantum dots as compared to the usual one and two electron devices. Finally, we discuss recent experiments on triplet-singlet transitions in GaAs dots subject to external magnetic fields. Our simulations are in good agreement with the experimental findings, and support the interpretation of the observed spin relaxation as being due to spin-orbit coupling assisted by acoustic phonon emission.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/11380/5978622007-01-01T00:00:00ZTHEORETICAL-ANALYSIS OF RESONANT MAGNETOTUNNELING SPECTROSCOPY OF HOLEShttp://hdl.handle.net/11380/10484Titolo: THEORETICAL-ANALYSIS OF RESONANT MAGNETOTUNNELING SPECTROSCOPY OF HOLES
Abstract: We calculate the hole energy band structure in GaAs-AlAs Quantum Wells (QWs) in high in-plane magnetic fields and simulate Resonant Magnetotunneling Spectroscopy (RMS) experiments, recently proposed as a probe of the hole energy dispersion. We find that the high magnetic fields used in RMS result in small modifications of the QW hole subbands in thin QWs. However, for wide QWs or very large magnetic fields, the field strongly affects the hole dispersion by changing the heavy hole-light hole mixing. We give a simple expression to evaluate to which extent the mixing is modified as a function of the QW width and field strength.
Sat, 01 Jan 1994 00:00:00 GMThttp://hdl.handle.net/11380/104841994-01-01T00:00:00ZCURRENT-INDUCED SPIN SEPARATION IN P-DOPED ASYMMETRIC DOUBLE-QUANTUM WELLShttp://hdl.handle.net/11380/10723Titolo: CURRENT-INDUCED SPIN SEPARATION IN P-DOPED ASYMMETRIC DOUBLE-QUANTUM WELLS
Abstract: In asymmetric double quantum wells, the ''spin'' splitting of the hole subbands at finite parallel momentum, due to the asymmetric potential, is related to a ''spin'' dependent delocalization of the wavefunction over the two wells. We show that the application of an electric field parallel to the interfaces in a p-doped asymmetric double quantum well can exploit such spin-dependent tunneling probability and separate carriers of opposite spins into the two wells, yielding a macroscopic magnetic moment of different magnitude and either the same or opposite direction in the two wells.
Sat, 01 Jan 1994 00:00:00 GMThttp://hdl.handle.net/11380/107231994-01-01T00:00:00ZPhotoluminescence spectroscopy of trions in quantum dots: A theoretical descriptionhttp://hdl.handle.net/11380/597011Titolo: Photoluminescence spectroscopy of trions in quantum dots: A theoretical description
Abstract: We present a full configuration-interaction study of the spontaneous recombination of neutral and singly charged excitons (trions) in semiconductor quantum dots from weak- to strong-coupling regimes. We find that the enhancement of the recombination rate of neutral excitons with increasing dot size is suppressed for negative trions and even reversed for positive trions. Our findings agree with recent comprehensive photoluminescence experiments in self-assembled quantum dots [P. Dalgarno , Phys. Rev. B 77, 245311 (2008)] and confirm the major role played by correlations in the valence band. The effect of the temperature on the photoluminescence spectrum and that of the ratio between the electron and hole wave-function length scales are also described.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11380/5970112008-01-01T00:00:00ZSpectral diffusion and line broadening in single self-assembled GaAs/AlGaAs quantum dot photoluminescencehttp://hdl.handle.net/11380/597007Titolo: Spectral diffusion and line broadening in single self-assembled GaAs/AlGaAs quantum dot photoluminescence
Abstract: We experimentally and theoretically investigate the photoluminescence broadening of different excitonic complexes in single self-assembled GaAs/AlGaAs quantum dots. We demonstrate that the excitonic fine-structure splitting leads to a sizable line broadening whenever the detection is not resolved in polarization. The residual broadening in polarized measurements is systematically larger for the exciton with respect to both the trion and the biexciton recombination. The experimental data agree with calculations of the quantum confined Stark effect induced by charge defects in the quantum dot (QD) environment, denoting the role of the QD spectator carrier rearrangement in reducing the perturbation of the fluctuating environment. (C) 2008 American Institute of Physics.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11380/5970072008-01-01T00:00:00ZFew-particle Electron Dynamics in Coupled Quantum Dots with Phonon Interactionhttp://hdl.handle.net/11380/597435Titolo: Few-particle Electron Dynamics in Coupled Quantum Dots with Phonon Interaction
Abstract: The time evolution of a correlated multi-electron state ina semiconductor coupled quantum dot, interacting withthe acoustic phonon bath, is calculated within the masterequation formalism, including electron-electron interactionexactly through the configuration-interactionapproach. The system is evolved after an initializationphase, with the system under strong bias, leading to thecharge density mainly localized in one of the dots. Whenthe system is evolved (in unbiased condition), dampedmulti-frequency charge oscillations are found, whichstrongly depend on the 3D system geometry and initialization.We describe the approach used to obtain themulti-electron states and to include the phonon-inducedtransitions between them in the time evolution. Furthermore,the effect of a magnetic field applied in the axialdirection of the vertical double-dot cylindrical structureis discussed.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11380/5974352008-01-01T00:00:00ZTheory of valence-band holes as Luttinger spinors in vertically coupled quantum dotshttp://hdl.handle.net/11380/596916Titolo: Theory of valence-band holes as Luttinger spinors in vertically coupled quantum dots
Abstract: We describe the valence-band holes of quantum dot molecules formed by two vertically coupled disks, using a four-band k . p Hamiltonian. It is shown that the strong spin-orbit coupling of the valence band introduces characteristic features in the hole tunneling, which are not captured by the usual single-band heavy-hole approximation. Therefore, a treatment of hole states as multiband Luttinger spinors is required. Within this description the parity symmetry in the vertical direction is lost, and chirality symmetry must be used instead. Effects of spin-orbit coupling on the hole and exciton states, as well as on the optical transitions are discussed. We show that, with increasing interdot distance, the spin-orbit interaction leads to a bonding-antibonding ground-state transition and to quenching of the excitonic emission. These results are relevant to recent experiments.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11380/5969162008-01-01T00:00:00ZLandau levels, edge states, and magnetoconductance in GaAs/AlGaAs core-shell nanowireshttp://hdl.handle.net/11380/936889Titolo: Landau levels, edge states, and magnetoconductance in GaAs/AlGaAs core-shell nanowires
Abstract: Magnetic states of the electron gas confined in modulation-doped core-shell nanowires are calculated for
a transverse field of arbitrary strength and orientation. Magnetoconductance 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 magnetoresistance. The predicted marked anisotropy of the magnetoconductance with field direction is
a direct probe of the inhomogeneous electron gas localization of the conductive channel induced by the prismatic
geometry.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/11380/9368892013-01-01T00:00:00Z