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- Beyer, Jan, 1980-
(författare)
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Spin Properties in InAs/GaAs Quantum Dot based Nanostructures
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semiconductor quantum dots (QDs) are a promising building block of future spin-functional devices for applications in spintronics and quantum information processing. Essential to the realization of such devices is our ability to create a desired spin orientation of charge carriers (electrons and holes), typically via injection of spin polarized carriers from other parts of the QD structures. In this thesis, the optical orientation technique has been used to characterize spin generation, relaxation and detection in self-assembled single and multi-QD structures in the InAs/GaAs system prepared by modern molecular beam epitaxy technique.Optical generation of spin-oriented carriers in the wetting layer (WL) and GaAs barrier was carried out via circularly polarized excitation of uncorrelated electron-hole pairs from band-to-band transitions or via resonant excitation of correlated electron-hole pairs, i.e. excitons. It was shown that the generation and injection of uncorrelated electron-hole pairs is advantageous for spin-preserving injection into the QDs. The lower spin injection efficiency of excitons was attributed to an enhanced spin relaxation caused by the mutual electron-hole Coulomb exchange interaction. This correlation affects the spin injection efficiency up to elevated temperatures of around 150 K.Optical orientation at the energy of the WL light-hole (lh) exciton (XL) is accompanied by simultaneous excitation from the heavy-hole (hh) valence band at high ~k-vectors. Quantum interference of the two excitation pathways in the spectral vicinity of the XL energy resulted in occurrence of an asymmetric absorption peak, a Fano resonance. Complete quenching of spin generation efficiency at the resonance was observed and attributed to enhanced spin scattering between the hh and lh valence bands in conjunction with the Coulomb exchange interaction in the XL. This mechanism remains effective up to temperatures exceeding 100 K.In longitudinal magnetic fields up to 2 T, the spin detection efficiency in the QD ensemble was observed to increase by a factor of up to 2.5 in the investigated structures. This is due to the suppression of two spin depolarization mechanisms of the QD electron: the hyperfine interaction with the randomly oriented nuclear spins and the anisotropic exchange interaction with the hole. At higher magnetic fields, when these spin depolarization processes are quenched, only anisotropic QD structures (such as double QDs, aligned along a specific crystallographic axis) still exhibit a rather strong field dependence of the QD electron spin polarization under non-resonant excitation. Here, an increased spin relaxation in the spin injector, i.e. the WL or GaAs barrier, is suggested to lead to more efficient thermalization of the spins to the lower Zeeman-split spin state before capture to the QD.Finally, the influence of elevated temperatures on the spin properties of the QD structures was studied. The temperature dependence of dynamic nuclear polarization (DNP) of the host lattice atoms in the QDs and its effect on the QD electron spin relaxation and dephasing were investigated for temperatures up to 85 K. An increase in DNP efficiency with temperature was found, accompanied by a decrease in the extent of spin dephasing. Both effects are attributed to an accelerating electron spin relaxation, suggested to be due to phonon-assisted electronnuclear spin flip-flops driven by the hyperfine interaction. At even higher temperatures, reaching up to room temperature, a surprising, sharp rise in the QD polarization degree has been found. Experiments in a transverse magnetic field showed a rather constant QD spin lifetime, which could be governed by the spin dephasing time T*2. The observed rising in QD spin polarization degree could be likely attributed to a combined effect of shortening of trion lifetime and increasing spin injection efficiency from the WL. The latter may be caused by thermal activation of non-radiative carrier relaxation channels.
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| 2. |
- Ouchterlony, Thomas
(författare)
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Electron Transport and Chaos in Model Mesoscopic Systems
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis mesoscopic structures intermediate in size between classical macroscopic objects and quantum mechanical objects as atoms are treated. The size of mesoscopic systems are of the same order as the wavelength of the electrons, which makes it necessary to take quantum mechanics into account. However the systems are much larger than atoms and molecules and provide a link between classical and quantum physics. These systems are very interesting both for understanding fundamental physics and technologically, since semiconductor components in about a decade will be small enough to make quantum mechanical effects important.The mesoscopic systems, or nanostructures as they are also called due to their size of about 10 - 1000 nm (nanometer), are fabricated from GaAs/ A1GaAs heterostructures. The A1GaAs is n-doped in order to create a two-dimensional electrongas (2DEG) between the GaAs and A1GaAs. Further constriction in the movement of the electrons are made by applying a gate voltage to the gate at the top of the heterostructure.Several different kinds of models for have been used to investigate different aspects of mesoscopic systems. The models are ranging from a basically onedimensional model calculating the conductance over a quantum point contact with an assumed linear potential drop to a realistic model for calculating the conductance through an arbitrary mesoscopic system knowing the gate structure and the physical structure of the heterostructure. In the latter model the potential in the 2DEG is calculated using a self-consistent Thomas-Fermi method and a hybrid recursive Green's function method uses this potential to yield the conductance.Other investigations made in this thesis are electron transport in quantum dots coupled in a deterministic aperiodic order and studies of chaos in mesoscopic systems. One interesting aspect of studying chaos in mesoscopic systems is that classical and quantum mechanical measures of chaos can be studied for the same system, which have been done here for several different smooth potentials. The weak localization peak for the conductance at low magnetic field has been suggested to be related to the underlying classical dynamics and here this is investigated and partially questioned. A recently suggested nearest neighbor energy level distribution for mixed regular and chaotic system, called semi-Poisson distribution, intermediate between the Poisson (regular) and Wigner ( chaotic) distributions have been found in fundamentally different systems. These systems are for a quantum dot with soft potential and in a quantum dot with spin-orbit coupling.
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