Noise Processes in Atomic-Scale Junctions and Two-Dimensional Topological Insulators
Stevens, Loah Ambrose
Doctor of Philosophy
Standard transport measurements, focusing on the first moment of the current, are crucial for understanding the behavior of a system as a function of factors such as applied voltage or current, temperature, or external fields. An even more detailed picture may be procured from the second moment of the current, the electronic noise. While the conductance derived from the first moment provides the average state of the system, the electronic noise describes how quantities such as current, voltage, or resistance fluctuate about their average values. These fluctuations respond to a variety of factors within different systems and can thus reveal information not evident by transport measurements alone. In this work, we employ noise, particularly shot noise, to study the behavior of atomic-scale gold junctions and two-dimensional topological insulators. Chapter 1 provides an introduction to quantum transport and examines how the concept of conductance evolves as the system size decreases from the macroscopic level to the few-channel limit. Chapter 2 describes the basics of electronic noise, specifically Johnson-Nyquist thermal noise, 1/f or flicker noise, and shot noise. This chapter also details the expected behavior of shot noise as relates to bias, temperature, sample size, and interaction effects. Chapter 3 introduces two-dimensional topological insulators (2DTIs), beginning with the quantum Hall effect and building to the theory of quantum spin Hall insulators. Chapter 4 outlines the methods used for the noise studies of the following chapters. Chapter 5 describes a study of shot noise in STM-style gold junctions. Shot noise was found to obey the finite temperature Landauer-Büttiker model of noise at low applied biases, but at high biases, the derived Fano factors were enhanced, which was attributed to either a bias-dependent channel-mixing mechanism or interactions between the conduction electrons and nonequilibrium phonon populations. Chapter 6 describes two studies of noise processes in InAs/GaSb quantum wells (QWs). In the first, RF noise measurements of silicon-doped InAs/GaSb bar structures revealed that the differential current noise decreases with increasing bias up to some finite bias, above which it increases linearly with increasing bias as expected. The nonmonotonic trend was suppressed by perpendicular magnetic field, increased temperatures, and applied gate voltage, leading to the belief that the trend was caused by contributions by generation-recombination noise. The second study involved both low and high frequency noise measurements in InAs/Ga0.68In0.32Sb QW Corbino structures and aimed to investigate the noise properties of the 2D bulk and the device contacts. In both frequency ranges, at high temperatures and positive gate voltages, when the 2D bulk is conductive, the measured noise is essentially flat with increasing bias, but as temperature is reduced and the bulk is gapped out, shot noise becomes detectable. The measured noise is much smaller and with broader curvature about zero bias than expected, which can be explained by a model in which the bulk and the contacts contribute to the thermal noise, but only the contacts produce shot noise. This model produces reasonable contact resistances and accurately portrays the zero bias curvature, but at the cost of anomalously large Fano factors. The large Fano factors may be due to either contributions to the shot noise by the bulk that are not included in the model or potentially by a mechanism of positive feedback between the conduction electrons and the buildup of space charge near one of the contacts. Future studies are proposed for using noise to probe the edge states in InAs/GaSb 2DTIs.
shot noise; topological insulators