Dissertations, Theses, and Capstone Projects

Date of Degree

9-2018

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Sergey Vitkalov

Committee Members

Scott Dietrich

Pouyan Ghaemi

Alexander M. Zaitsev

Areg Ghazaryan

Sergey Vitkalov

Subject Categories

Condensed Matter Physics | Quantum Physics

Keywords

Magnetoresistance, Scattering, Temperature, Oscillations, Zeeman, Landau

Abstract

This work focuses on the study of electron transport of 2-D electron gas systems in relation to both fundamental properties of the systems such as disorder and scattering mechanisms, as well as unique magnetoresistance (MR) effects. A large portion of the discussion is built around the use of an in plane magnetic field to vary the ratio between the Zeeman energy between electrons of different spins and the Landau level spacing, creating a tool to control the quantization of the density of states (DOS).

This tool is first used to isolate Quantum Positive Magnetoresistance (QPMR), which grants insight to the level of long range disorder in a system by exposing the quantum scattering rate τq-1. Analysis of τq-1reveals the presence of both long and short rangedisorder present in our sample. It is shown that electron-electron scattering enhances thelong range disorder, and e-phonon scattering enhances the dissipative resistance.

Control of the DOS is further used to isolate any quasi-classical MR, by destroying the quantization of DOS. The result is a negative MR that behaves as Δρ/ρ0=A·Bpη where Bp is the perpendicular magnetic field, η ≈ 1.5 ± 0.1 and A(T) = [κ(τq-1) +β(τq-1) ·T2]-1. A theory based on memory effects is presented to explain the η= 1.5 power dependence. The quadratic dependence of A on temperature suggests a dependence on e-e scattering for memory effects. β(τq-1) shows a critical dependence on static disorder suggesting a strong increase of the effect of e-e scattering on memory effects with static disorder. In a broad range of the disorder the coefficient κ ~ τq-1/2, suggesting that the anomalous NMR should be significantly enhanced in systems with a long quantum lifetime.

A more in depth study of control of the quantization of the DOS shows the expected reduction of QPMR and Shubnikov–de Haas (SdH) oscillations with angle. The results follow the theory for reduction if one allows for an effective g factor with both a perpendicular magnetic field and angle dependence.

An analysis of disorder for a mutli-populated subband is investigated through Magneto Inter Sub-band Oscillations (MISO). The quantum scattering rate τq-1for each subbabnd is isolated and studied as a function of temperature. The obtained temperature dependencies are in accord with existing theory indicating e-e scattering as the dominant mechanism limiting the electron lifetime.

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