無序性對二維系統電性之衝擊

無序性導致了豐富的物理，特別是在二維系統當中。無序性可以是由於晶體的不完美或是離子化雜質的庫倫位勢所產生。新興的材料，如傳輸被相對論性的Dirac 方程式所主導的石墨稀，和擁有可調控的超導性和順磁性的SrTiO3/LaAlO3 氧化物接面，提供了一個好的研究二維物理的平臺，且也因此激發了許多研究在無序性條件下關於二維電荷傳輸的有趣實驗。在這篇論文中，從無能量損耗的超導體到高阻值的半導體，各種不同的二維系統的傳輸研究被描繪。這篇論文的主要實驗結果包含以下四個部分。一、複合系統中無序性驅動之二維超導性之增強我研究由砷化鋁鎵/砷化鎵二維電子氣和鄰近(約70奈米)的鋁超導奈米膜(約60奈米)所組成的複合奈米電子系統。透過調控流經鋁膜的電流，吾人可以改變二維電子氣的電導且進一步操控性地變更鋁超導膜中等效的無序性。二維電子氣層出現所造成無序性的引入被展示出可以提升鋁奈米膜的超導性。如此的結果是重要的且具有啟發性，因為無序性普遍被相信是對超導性有破壞性的。就應用層面而言，整合具有不同物理特性的量子系統對提升電子元件的效能是有前景的。二、自旋軌道耦合對二維超導性之影響超導性和自旋軌道交互作用是兩個獨立的新興領域，直到最近這兩者的關聯似乎被觀察到。然而，過去關於自旋軌道耦合的實驗是在遠離超導態執行，因此自旋軌道耦合是如何影響超導性的論證仍是不常見的。為解決此問題，我在鋁奈米膜的超導轉變的關鍵區域中，研究自旋軌道耦合。縮減膜厚，其相對應地增加無序性，被展現出增強自旋軌道耦合的強度。經由研究在不同自旋軌道耦合強度下的磁傳輸行為，自旋軌道耦合對超導性的重要效應可被觀察。這個工作提供了對自旋軌道耦合和超導性的相互作用更進一步的理解，且對於在低維度超導體中實現自旋電子學是根本上重要的。三、在氫化石墨烯當中載子—載子交互作用之證據我利用高電場和高磁場來探測高無序性之氫化石墨烯之傳輸特性。透過施加一高源極—汲極偏壓，電流—電壓的關係可以被研究。隨著此偏壓的增加，從線性的電流—電壓關係到非線性關係，最終到activationless-hopping傳輸的轉變出現。在activationless-hopping的範疇中，帶電荷載子間的庫倫交互作用之重要性被證明。再者，隨著高電場和高垂直磁場的出現，原本在低溫低偏壓下被強力侷限的載子，其delocalization發生。四、在單層二硫化鉬奈米薄片中不均勻性之證據我研究在大範圍載子濃度，溫度，及偏壓變化下，在單層二硫化鉬奈米薄片中之電荷傳輸。我發現這種傳輸是可以很好地被percolating picture所描述。在percolating picture中，無序性破壞translational invariance使得系統解離成一系列的puddle，而不是過去無序性被處理為同種且均勻的圖像。這個工作提供了在單層二硫化鉬奈米薄片中之電荷傳輸的統一圖像的觀點且這個工作對下世代以二硫化鉬為架構的元件的發展是很有貢獻的。

關鍵字

超導性；自旋軌道耦合；石墨烯；二硫化鉬

並列摘要

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.

並列關鍵字

superconductivity ； spin-orbit coupling ； graphene ； MoS2

參考文獻

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[6] H. Q. Nguyen, S. M. Hollen, M. D. Stewart, J. Shainline, A. Yin, J. M. Xu and J. M. Valles, Phys. Rev. Lett. 103, 157001 (2009).

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無序性對二維系統電性之衝擊

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