||Formation dynamics of resident electron spin polarization and ensemble spin coherence in CdTe single quantum well
The resident electron spin polarization (RESP) in quantum wells (QWs) has gained considerable attention for the potential applications of spin degree of freedom in the spintronic devices and quantum information processing. This is because of the optical controllability, the robust and long spin coherence free from the recombination annihilation. Therefore, massive research has been conducted on resident electron spin relaxation, transport, control, coherence, initialization, the interaction with other excited states, donors, lattice nuclei and so on, especially in GaAs and InAs QWs. Owing to these studies, the RESP can be better manipulated within the decoherence time after its generation. Unfortunately, it is almost a blank to the important spin formation dynamics in the initial temporal region, due largely to the ultrafast internal transitions and complicated many-body interactions. This issue, specifically, unclear initial phase, has been emphasized in our study with overcoming the problems successfully.The formation dynamics of RESP and ensemble spin coherence in a naturally n-doped CdTe single QW have been studied with picosecond laser pulses by a time-resolved Kerr rotation (TRKR) method. The careful analysis of TRKR signals as compared to the theoretical fittings around time origin reveals the appealing phenomenon for the first time: a negative initial phase shift (IPS) in the precessional motion of RESP. After detailed experimental studies on the IPS of RESP, along with rate equations analysis, the theoretical model of the interplay between the excitons and RESP has been simulated. Consequently, the possibility of the negative IPS and of ensemble spin dynamics discussed gave a deep understanding of RESP generation, involving the formation and evolution of excitons, particularly in the elusive early time region.The experimental Kerr signals with high precision both in time and phase show that the magnitude of IPS as well as the amplitude of RESP precession can be affected by external parameters such as excitation power, the strength of magnetic field. Such overall trends in the experiments agree well with the model calculations. Moreover, the negative IPS is almost constant for the pulse photon energy around the transition energy in resonance with trion. Through studying nonzero IPSs, we have confirmed the RESP formation dynamical process, associated especially with a fast asymmetric hole-spin flip in trions spin dynamics.There is still a dominant remained obstacle characterized by hyperfine interaction (HFI), the coupling of electron spins to the lattice nuclear spins, although the particular benefit of RESP is the long lifetime of spin coherence without energy relaxation or forceful coupling with semiconductor environments. The HFI interaction was often ignored in the previous electron spin research of CdTe semiconductors. But the nuclear spin polarization (NSP) in an oblique magnetic field configuration has been evaluated after exploring the longitudinal and in-plane electron g factors precisely. This investigation indicates that RESP gives rise to a small effective nuclear field of a few mT.This study makes the integral RESP formation process, initial behaviors and ensemble spin coherence well-established quite fully. Moreover, the IPS is founded to provide an important tool for researching into the spin dynamics of excitons. In a word, RESP performances with the feature of low nuclear field are preferred for spintronic devices that require the long ensemble spin coherence and the manipulation at rather low magnetic fields.
Hokkaido University（北海道大学）. 博士(工学)