Zusammenfassung: | |
The spin degree of freedom of charge carrier spins and the host’s
nuclear spins in semiconductors are potential sources for the next
generation of spintronics applications which motivate the deliberate
investigation of the spin dynamics of well-controlled model systems
like n-GaAs. Conduction electron spins are mobile in semiconductors and can be initialized, manipulated, and read out optically.
Optical pumping with circularly polarized light can for example
create a non-equilibrium electron spin polarization close to 100%.
Nuclear spins are practically appealing as well due to their very long
spin relaxation times. The mutual interaction between the electron
and nuclear spin system is mediated via the hyperfine interaction.
Indeed, through this interaction, a non-equilibrium spin polarization of electrons is transferred to the nuclear spins and results in
dynamic nuclear polarization, which inter alia has an intricate dependence on the doping density. The main objectives of this thesis
are measuring most accurately (i) the temperature dependence of
the electron spin relaxation rate and (ii) the magnetic field, doping,
and temperature dependence of the nuclear spin relaxation rate in
a set of high quality n-GaAs samples from quasi-insulating over the
metal-to-insulator transition up to the quasimetallic regime.
The temperature dependence of the electron spin relaxation time
is measured very accurately for three of the above-mentioned samples with the optical Hanle depolarization method. The measurements yield, in combination with a theoretical model, a quantitative
insight into the efficiency of the different spin relaxation mechanisms. The longest electron spin relaxation time in n-GaAs results
from an interplay of variable range hopping and hyperfine interaction for a doping concentration just below the Mott metal-to insulator transition at a finite temperature of ∼ 7 K. At higher doping densities the effect of these two mechanisms decreases such
that they are negligible in the highest doped sample. For moderate
and high temperatures, the description of the electron spin relaxation becomes unpretentious since the Dyakonov-Perel mechanism
dominates over all other electron spin relaxation mechanisms.
The Overhauser field from nuclear polarization intensifies or weakens the external magnetic field and affects the electron spin orientation. In order to measure the nuclear spin relaxation, a three-stage
time-resolved detection of the Hanle effect is used. The method includes optical pumping and measuring the difference of the nuclear
spin polarization before and after a dark (no laser light) interval
of variable length. In this way, the nuclear spin system in the absence of excitation is investigated. The magnetic field dependence
of the nuclear spin relaxation rate has a typical Lorentzian shape,
showing the spin-spin interaction’s impact at lower magnetic fields.
The strong field doping dependence of the nuclear spin relaxation
rate can be explained quantitatively, considering the effective number of localized electrons over the entire density regime. Nuclear
spin diffusion to the donor bound electrons increases the relaxation
rate of the nuclear spin measured at 6:5 K and results in a distinct
maximum at the metal-to-insulator transition. The rate in the very
high doped sample increases due to the Korringa mechanism. The
involved mechanisms explain the trend of the relaxation except for
the very low doped sample. The temperature dependence of the
lowest doped sample shows an electron spin relaxation channel affecting the nuclear spin relaxation, which is negligible at high doping
densities.
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Publikationstyp: | DoctoralThesis |
Publikationsstatus: | publishedVersion |
Erstveröffentlichung: | 2021 |
Schlagwörter (deutsch): | Galliumarsenid, Elektronenspinrelaxation, Kernspinrelaxation, Spin-Dynamik |
Schlagwörter (englisch): | gallium arsenide, electron spin relaxation, nuclear spin relaxation, spin dynamics |
Fachliche Zuordnung (DDC): | 530 | Physik |