Current-Induced Spin Polarization and Dynamic Nuclear Polarization: Generation and Manipulation of Electron and Nuclear Spin Polarization in Gallium Arsenide

Abstract

Spintronics would utilize the spin of the electron for information processing and storage, leading to devices that may be smaller, faster, and more energy-efficient than their electronic counterparts. Before these spin-based devices can be fully realized, we must answer the following questions: How do we effectively generate electron spin polarization? What factors impact the ability of the spins to stay polarized? How do we effectively detect spin polarization? In our materials of interest, gallium arsenide (GaAs) and its alloy indium gallium arsenide (InGaAs), the detection method is optical, via Faraday or Kerr rotation. This dissertation focuses on the electrical generation of electron spin polarization (current-induced spin polarization, or CISP) and dynamic nuclear polarization (DNP) generated by periodic optical electron spin pumping, which will impact the electron spin system.

First, we modify an electrical technique for generating CISP in InGaAs to extend the voltage range of our experiments. In previous studies, current heating limited the magnitude of the applied voltage, leading to uncertainty regarding the relationship between the voltage and the spin polarization generation rate. We modify a bipolar square wave to reduce the time spent at nonzero voltage, and by generating CISP via modified waveforms with increasing off/on ratios, we reduce heating by up to an order of magnitude on the range of 1 to 7 V applied. At off/on ratios of 5 and above, we recover the expected linear relationship between generation rate and applied voltage.

Then, we investigate DNP in GaAs under periodic optical excitation in the regime of resonant spin amplification (RSA). The measured Kerr rotation exhibits warped RSA peaks, shifted from their expected positions depending on the direction in which the external magnetic field is incremented, or swept. This points to a DNP accumulated along the external field direction, perpendicular to the direction of optical spin generation, altering electron spin precession through a sweep-direction-dependent Overhauser field, with nuclear T1 times on the order of tens of seconds. After establishing a physical framework based upon RSA and the optical Stark effect, we identify a set of experimental parameters to characterize the DNP: laser wavelength, pump power, time elapsed, and external field history. We present data exploring each of these parameter spaces, comparing to numerical simulations for the wavelength and elapsed time cases. Finally, we discuss the origin of the sweep-direction dependence.

We focus on the external field history of DNP through the steep sweep experiments, in which a field sweep is paused for two minutes to allow for DNP buildup. Certain steep fields result in post-steep RSA peaks that mimic the steeping behavior, showcasing a minutes-long precise memory of the electron-nuclear system’s magnetic field history, the steep echo. We examine the steep sweeps through the lens of nuclear-induced frequency focusing, a conceptual framework for the buildup of DNP complementing our physical model. To explain the steep echo, we propose a modification to our physical picture that would involve a distribution of electron-nuclear interactions, with preliminary simulations showing promising correspondence to our measurements.

Taken together, these studies elucidate considerations that can arise in and compromise optical spintronic experiments. Proper mitigation (for current heating) and understanding (for DNP) will aid in the future research necessary to bring about semiconductor spintronic devices.