Electron and Nuclear Spin Dynamics and Coupling in InGaAs.

Abstract

This dissertation focuses on the development of an understanding of electron spin transport, dynamics, and coupling to the nuclear spin system in gallium arsenide (GaAs), as well as the measurement techniques brought to bear in their investigation.

Current-induced electron spin polarization is shown to produce nuclear hyperpolarization through dynamic nuclear polarization. Saturated nuclear magnetic fields of several millitesla are generated upon the application of electric field over a timescale of minutes in indium gallium arsenide (InGaAs) epilayers and measured using optical Larmor magnetometry. We show that, in contrast to previous demonstrations of current-induced dynamic nuclear polarization, the direction of the current relative to the crystal axes and external magnetic field allows for control over the magnitude and direction of the saturation nuclear field. An asymmetry in saturated nuclear magnetic field for anti-parallel currents is found, and ascribed to competing electron spin alignment mechanisms which lead to nuclear polarization which is current-direction independent. The behavior of the saturated nuclear field with temperature, electric field strength, and external magnetic field strength is measured. An unexpected asymmetry in measurements of the change in nuclear field from polarization and depolarization transitions is found and determined to be the result of an unexpected phase shift in Larmor magnetometric measurements due to previous pulses. Implications for the measurements of nuclear magnetic fields resulting from the phase shift are discussed.

Time-resolved Faraday rotation (TRFR) measurements, which have proved transformative in the investigation of spin dynamics in semiconductors, are used to study nuclear polarization. We find that, in materials with spin lifetimes which are on the order of, or greater than, the laser repetition time, the collective effect of spin polarization due to the whole pump pulse train becomes important. A relative phase shift in TRFR measurements is identified which results from these spins. A closed-form expression which describes this phase shift is derived and experimentally validated. Numerical methods are used to characterize this phase shift throughout parameter space. A spin lifetime measurement based on this phase shift is described, and situations in which the model used must be augmented are discussed.