石墨烯-鈮超導接面電性分析

A study of electric properties of graphene/niobium superconductor junction

許雅峯(Ya-Fong Syu)

指導教授 ： 陳正中

國立清華大學/理學院/物理學系/碩士(2016年)

摘要

本篇論文在研究石墨烯-鈮超導接面的電性分析並著重在探討當改變石墨烯的費米能量時其安德烈夫反射行為，石墨烯因其特殊的能帶結構，被稱為零能隙的半導體，而超導體會在其費米面上開一個超導能隙，因石墨烯的載子為無質量的相對論性狄拉克費米子，無質量的狄拉克費米子與庫柏電子對之間的交互作用令人感到興趣，本論文中使用鈮薄膜沉積在石英基板上其超導臨界溫度大約9 K，石墨烯則是使用化學氣相沉積法(CVD)所成長後轉移到鈮圖形之上接觸後形成兩個串聯接面，在此之前我們特別處理鈮的介面來確保乾淨的介面可以被達成，其中為了調控石墨烯的費米能量，我們使用液態離子凝膠覆蓋在石墨烯之上當作石墨烯的閘極，實驗中首先在室溫量測閘極偏壓對石墨烯電阻關係，決定好所調控的費米能量相對於電荷中性點(CNP)後降溫到臨界溫度以下，我們發現當費米面被調控為遠離電荷中性點時，歸一化微分電導對外加偏壓關係表現出抑制其下沉結構的行為，且其下沉結構特徵接近超導能隙，也就說當費米能量鄰近電荷中性點時其歸一化微分電導的下沉行為較為明顯，在數據分析上指出當鄰近電荷中性點時因為其安德烈夫臨界角較小所以安德烈夫反射行為被抑制，我們的實驗結果提供了第一個綜合的研究石墨烯-鈮超導接面的電學特性和拓展了一個新的辦法來更深入探討調控安德烈夫反射行為在於狄拉克費米子之上。

關鍵字

石墨烯 ； 鈮 ； 超導接面

並列摘要

We investigate the electric properties through graphene-Nb superconductor junction and aim to study Andreev reflection (AR) as a function of Fermi-level EF of graphene. The Nb thin film is deposited on quartz with superconducting temperature Tc close to 9 K. The graphene is prepared by chemical vapor deposition (CVD) and transfer on top of Nb-pattern, in which two series coupled junctions are defined. In particular, we carefully treat Nb surface so to ensure clean interface is achieved. To control EF of graphene, we employ ion-gel on graphene as a gate electrode. We first characterize the resistance of graphene with the gate voltage Vg, determine EF respected to the charge neutral point (CNP) and then cool down the sample below Tc,. We find as EF is tuned away CNP, the differential conductivity dI/dV v.s. V shows a suppressed dip structures, which width is comparable with the expected superconducting gap of Nb. Distinctly, as EF is in the vicinity of CNP, the downward feature of dI/dV becomes more pronounced. Data analysis suggest the retro-AR is enhanced near CNP due to smaller the critical angle of Andreev reflection. Our results provide the first comprehensive studies of electric properties of graphene-supercondutor interface and open a new route to further explore the tunneling processes of AR for Dirac fermions.

並列關鍵字

Graphene ； Nb ； junction

參考文獻

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