Spin States in Bismuth and Its Surfaces: Hyperfine Interaction
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The hyperfine interaction between carrier spins and nuclear spins is an important component in exploring spin-dependent properties in materials with strong spin orbit interaction.However hyperfine interaction has been less studied in bismuth (Bi), a heavy element exhibiting a strong Rashba-like spin-orbit interaction in its two-dimensional surface states due to the broken spatial inversion symmetry. In this dissertation we experimentally explore the carrier spin polarization due to transport under strong spin-orbit interaction and the nuclear polarization resulting from the relatively unexplored hyperfine interaction on Bi(111) films.The carrier and nuclear spin polarizations are expected to dynamically interact, a topic with ramifications to other materials where surface states with noteworthy properties play a role.To achieve this goal, an optimized van der Waals epitaxy growth technique for Bi(111) on mica substrates was developed and used, resulting in flat Bi surfaces with large grain sizes and a layered step height of 0.39±0.015 nm, corresponding to one Bi(111) bilayer height. A comparison between Bi(111) films grown on three different substrates (mica, InSb(111)B, and Si(111)) is discussed, for which scanning electron microscopy and atomic force microscopy are applied to obtain the structural and morphological characteristics on the film surface. Magnetotransport measurements are carried out to extract the transport properties of theBi(111) films. Using the high quality Bi(111) film deposited on mica, we develop quantum magnetotransport techniques as delicate tools to study hyperfine interaction. The approach is based on measuring quantum corrections to the conductivity due to weak antilocalization, which depend on the coherence of the spin state of the carriers. The carrier spin polarization is generated by a strong DC current in the Bi(111) surface states (here called the Edelstein effect), which then induces dynamic nuclear polarization by hyperfine interaction. Quantum transport antilocalization measurements in the Bi(111) thin-films grown on mica indicate a suppression of antilocalization by the in-plane Overhauser field from the nuclear polarization, and allow for the quantification of the Overhauser field, which is shown to depend on both polarization duration and the DC current magnitude. Various delay times between the polarization and the measurement result in an exponential decay of the Overhauser field, driven by relaxation time T1. We observe that in the Bi surface states, the appreciable electron density and strong spin-orbit interaction allow for dynamic nuclear polarization in the absence of an external magnetic field.
General Audience Abstract
This dissertation focuses on the heavy element bismuth (Bi), a semimetal with strong spin-orbit interaction at its two-dimensional surface. Given the challenge to grow high qualityBi(111) films, we present an optimized van der Waals epitaxy technique to grow Bi(111)films on mica substrates, which show a flat surface with large grain sizes and a layered step height of 0.391±0.015 nm, corresponding to one Bi(111) bilayer height. To demonstrate the high quality of the Bi(111) surface, a comparison of surface morphology was conducted among Bi(111) films deposited on three different substrates (mica, Si(111), and InSb(111)B),along with a comparison between their electronic transport properties. By applying a DC current on the high quality Bi(111) film on mica, a carrier spin polarization is established via mainly what we here call the Edelstein effect, which then induces dynamic nuclear polarization by hyperfine interaction and generates a non-equilibrium nuclear spin polarization without externally applied magnetic field. We quantified the Overhauser field from the nuclear polarization all-electrically by conducting quantum transport antilocalization experiments, which showed a suppression of antilocalization by the in-plane Overhauser field.Comparative measurements indicated that the magnitude of the Overhauser field depends onthe spin-polarizing DC current magnitude and the polarization duration. The experiments also show that antilocalization forms a sensitive probe for hyperfine interaction and nuclear polarization.
- Doctoral Dissertations