| Summary: | 博士 === 國立臺灣大學 === 應用物理所 === 102 === Disorder, which can arise due to imperfection of crystal structure and Coulomb potential of ionized impurities, has led to rich physics especially in two-dimensional (2D) systems. The newly emerging materials such as graphene, where transport is govern by relativistic Dirac equation, and SrTiO3/LaAlO3 oxide interface, which may have tunable superconductivity and ferromagnetism, offer a good platform to study 2D physics and thus inspire lots of interesting experiments about the 2D charge trasnport in the presence of disorder. In this thesis, transport studies on a variety of 2D systems extending from dissipationless superconductors to highly resistive semiconductors are described, which can be categorized into the following four parts.
1. Disorder-driven enhancement of quasi-two-dimensional superconductivity in a hybrid system
A hybrid nanoelectronic system which consists of an AlGaAs/GaAs two-dimensional electron gas (2DEG) in close proximity (~ 70 nm) to an Al superconducting nanofilm (~ 60 nm) was studied . By tuning the current through the Al film, the conductance of the 2DEG can be changed and furthermore the effective disorder in the Al superconducting film is varied in a controllable way. The introduction of disorder due to the presence of nearby 2DEG layer is shown to promote superconductivity of the Al nanofilm. Such results are important and instructive since disorder is generally believed to be detrimental to superconductivity. From the viewpoint of applications, this result suggests that integration of quantum systems of diverse physical properties is promising to promote the performance of electronic devices.
2. Influence of spin-orbit coupling on quasi-two-dimensional superconductivity
Superconductivity and spin-orbit (SO) interaction have been two separate emerging fields until very recently that the correlation between them seemed to be observed. However, previous experiments concerning SO coupling are performed far beyond the superconducting state and thus a direct demonstration of how SO coupling affects superconductivity remains elusive. To this end, I investigate the SO coupling in the critical region of superconducting transition on Al nanofilms. Reducing the film thickness, which correspondingly increases the amount of disorder, is shown to enhance the strength of SO coupling. By studying magnetotransport behavior under different strength of SO coupling, the important effects of SO interaction on superconductivity is revealed. This work provides further insights into the interplay between SO coupling and superconductivity, which is of fundamental importance in realizing spintronics in a low-dimensional superconductor.
3. Evidence of carrier-carrier interactions in hydrogenated graphene
Transport properties of highly disordered hydrogenated graphene by both a high electric field and a high magnetic field were probed. By applying a high source-drain voltage Vsd, it is possible to study the current-voltage relation I-Vsd of this device. With increasing Vsd, a crossover from the linear I-Vsd regime to the non-linear one, and eventually to activationless-hopping transport occurs. In the activationless-hopping regime, the importance of Coulomb interactions between charged carriers is demonstrated. Moreover, delocalization of carriers, which are strongly localized at low T and at small Vsd, occurs in the presence of high electric field and perpendicular magnetic field.
4. Evidence of inhomogeneity of monolayer MoS2 nanoflake
Charge transport in a monolayer MoS2 nanoflake over a wide range of carrier density, temperature, and electric bias was studied. It is found that the transport is best described by a percolating picture in which the disorder breaks translational invariance, breaking the system up into a series of puddles, rather than previous pictures in which the disorder is treated as homogeneous and uniform. This work provides insight to a unified picture of charge transport in monolayer MoS2 nanoflakes and contributes to the development of next-generation MoS2–based devices.
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