Spin-coherence and -dephasing of donor and free conduction band electrons across the metal-insulator transition in Si:GaAs
Today, information processing and information storage are still two separate worlds. While information processing is performed by moving and manipulating electrical charges, the information is usually stored permanently as a magnetization state. The emerging field of Spintronics is trying to combine the world of semiconductor electronics and magnetism to overcome disadvantagesof each part on its own. Furthermore, new phenomena and applications associated with them may arise. Although this dissertation deals only with a sub-topic of Spintronics, namely the spin coherence and dephasing in III/V semiconductors, this topic is nevertheless fundamental to the whole field. In these systems, spins can be generated by optical excitation using circularly polarized light, while their decoherence may be observed by time-resolved pump-probe spectroscopy. This work introduces some basic experimental setups, like time-resolved Kerr/Faraday rotation spectroscopy, as well as their applicationto the model system Si:GaAs. Systematics studies of the transverse spin lifetime depending on various external parameters, such as temperature, magnetic field, laser excitation power and energy allow for the interpretation of the experimental data. Especially the doping dependence across the metal-insulator-transition gives clues to the role of the electronic states and their influence on spin coherence: Thus, delocalized donor electrons show the longestspin lifetimes exceeding 100ns (at low temperatures and for samples right at the metal-insulator transition). Over 20ns have been found for free conduction band electrons, while localized donor electrons exhibit spin lifetimes of only about 100ps. A majority of the effects found can be explained by know spindephasing mechanisms, such as D´yakonov-Perel´ or Elliott-Yafet.In addition, the influence of the nuclear magnetic moments to the electrons plays a decisive role. Using optical orientation of the electron spins, one can actually polarize the nuclear spins so high, that the resulting internal magnetic fields (about 100mT) can be on the order of the externally applied magnetic fields. Due to the weakness of the nuclear hyperfine coupling, those phenomena can be observed on the minute timescale, in contrast to the electron spins, where spin dephasing takes place on the picosecond to nanosecond timescale, as already mentioned. Due to the fundamental results presented in this work, new Spintronics devices will hopefully emerge in the near future.
info:eu-repo/classification/ddc/530, Physik, Spin Dephasing, Metal-Insulator Transition, Pump-Probe, Time-Resolved Kerr Rotation
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