Investigation of the Coupled Electron-Nuclear Spin System in GaAs Under Periodic Optical Electron Spin Pumping
Macmahon, Michael
2019
Abstract
The nuclear spin system of semiconductor crystals displays a remarkable degree of environmental isolation, and in many cases a nuclear spin polarization may persist for human-perceptible durations at high temperatures. Properties like this, combined with the close relationship of the nuclear and electron spin systems via the hyperfine interaction, suggest that greater control of the polarization of nuclear spins in semiconductor crystals could provide breakthroughs in both classical and quantum information storage and processing. In this work, we examine the electron and nuclear spin systems in gallium arsenide epilayers and demonstrate that they undergo a rich and complex interaction over a timescale of seconds to minutes when the electron spin system is periodically pumped via a pulsed laser. We use optical pump-probe techniques to manipulate an electron spin polarization, whose coherence time exceeds the repetition period of the mode-locked laser. After generating spin polarization with a circularly polarized pulse, we measure the Kerr rotation angle of a reflected linearly polarized beam, as it is proportional to the degree of electron spin polarization along the optical axis at an adjustable moment in the pulse cycle. The Larmor precession of electrons in an external magnetic field leads to interference between spins excited from successive pump pulses, resulting in resonant spin amplification (RSA) of the electron spin polarization that we measure via Kerr rotation. In this work, we demonstrate our discovery of a dynamic nuclear polarization (DNP) that actively responds to the magnitude of RSA. By sweeping the magnitude of the external magnetic field, we simultaneously produce a continuously varying DNP and also detect its presence through the effect of the Overhauser field produced by polarized nuclei on the observed Larmor precession frequency of the electron spin system. Notably, the polarity of the observed DNP depends on the sweep direction of the external magnetic field. We discuss similar cases of DNP hysteresis in the existing literature, but show that these explanations do not apply to our system. This presents a mystery in regards to how to explain our results, and we perform a series of tests that rule out other initially plausible explanations. We then deepen the mystery by showing that the electron-nuclear spin system retains memory of interruptions in magnetic field sweeps. We also demonstrate a new technique to extract the Overhauser field at every timestep in the experiment and use this data to test a phenomenological model that explains many key features of these results. We conclude with a discussion of possible physical mechanisms for producing the observed DNP and highlight promising avenues for future research.Subjects
Dynamic nuclear polarization Nuclear spin physics Gallium arsenide Semiconductor spin physics Spintronics
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