published_or_final_version / Physics / Doctoral / Doctor of Philosophy

A nanoscale Aharonov-Bohm (AB) ring with a quantum dot (QD) embedded in each arm is investigated analytically to provide electron transmission characteristics. A parallel magnetic field provides Zeeman splitting of the QD energy levels. Combined Zeeman energy level splitting and AB-effects occur with a perpendicular field. In our device, the AB-ring interferometer, Zeeman splitting of the QD energy levels creates regions of parameter space in which the electron transmission is highly spin-polarized. In addition to Zeeman splitting caused by a parallel magnetic field, combined Zeeman energy level splitting and AB-interference effects occur with a perpendicular field. The weighted spin-polarization function is calculated and presented as a function of magnetic field and electron energy. Due to a unique parameter regime in which the AB-oscillations show extreme sharpening [1], the electron transmission can be tuned to produce spin-polarized currents which can be switched and controlled by small changes of external fields. / Introduction -- AB-oscillations and resonances in a double quantum dot ring -- Results for combined Zeeman and AB effects -- Spin-polarization. / Department of Physics and Astronomy

Thesis (Ph. D.)--University of Chicago, 1929. / Lithoprinted.

published_or_final_version / abstract / Physics / Master / Master of Philosophy

[Truncated abstract.] An apparatus has been built, tested and characterised with greatly increased precision and accuracy to enable spin-polarised electron impact explorations of angular momentum effects in atoms. A combination of experimental, computational and interpretative studies has revealed new phenomena. In zinc the polarisation of the radiated photons from excited and ionised-with-excited atoms was measured in terms of integrated Stokes parameters (P1, P2 and P3) and related to electron exchange and spin-orbit interaction using normalised state multipoles.

Surface-analysis and treatment apparatus have been variously designed, manufactured, developed, and commissioned or re-commissioned, for characterising the surfaces and efficiency of positron moderators based around 3 µm thick polycrystalline-tungsten foil. These include XPS and AES, based around a CLAM2 hemispherical analyser, electron-beam heating, ion bombardment, mass spectroscopy, UHV sample mounting, UHV manipulation, gas-handling lines, and entry-lock apparatus. The CLAM2 electron spectrometer is additionally adapted for operation as a bipolar charged-particle spectrometer. All control software, and much data-analysis software, is implemented in Labview. Apparatus and techniques for safely storing, handling, transferring into vacuum, and manipulating in vacuo, a nominally 1 mCi 22Na, UHV-compatible positron source, are designed, constructed, and implemented. The efficacy of cleaning and surface-analysis apparatus are demonstrated, with some limitations and instrument malfunction identified, and solutions implemented. Methods for passivating positron-trapping states in polycrystalline tungsten are proposed, based on the current understanding of positron moderation and trapping. Improved moderator geometries have been designed and an alternative, simpler, and easier to implement solid-gas moderator proposed.

In the field of spintronics there is a strong need for an efficient spin-polarizing device. To that end, two basic devices are proposed: a series of Aharonov-Bohm (AB) rings linked in series with intermediate quantum dots (IQD) and the quantum dot spin polarizer (QDSP). In each case the system is built of quantum dots (QD), each of which can be Zeeman split with a tunable external magnetic field. Spin neutral input and output leads are also attached to each system. The Tight Binding Approximation (TBA) is used to model each system. Mathematica is used to solve the systems generated by TBA, so that the transmission or reflection of a system can be evaluated. We find that a series of AB rings provides for wide, energetically separated, spin polarized conduction bands. The QDSP provides physical separation of spin polarized electrons, making a spin polarized base current possible. / Methods of analysis -- The Aharonov-Bohm ring -- The quantum dot spin-polarizer. / Department of Physics and Astronomy