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    题名: 石墨烯/超導體/石墨烯元件之古柏電子對分裂現象探討
    作者: 謝玉玲;Hsieh,Yu-Ling
    贡献者: 物理學系
    关键词: 超導體;石墨烯;古柏電子對分裂;交叉式安德烈夫反射
    日期: 2015-08-28
    上传时间: 2015-09-23 11:35:17 (UTC+8)
    出版者: 國立中央大學
    摘要: 本論文研究超導體中電子傳輸行為,主要探討古柏電子對分裂(Cooper pair splitting, CPS)現象,古柏電子對是存在超導體內的量子糾纏態,可當作一個量子糾纏產生源應用於量子訊息傳輸。一般金屬/超導體/一般金屬的混合式元件中,一顆電子由一金屬進入超導體內,同時從另一界面反射出電洞至另一金屬內,在超導體中產生古柏電子對,這種發生在不同界面間的電子電洞轉換,稱交叉式安德烈夫反射(crossed Andreev reflection, CAR)。當元件組成為p型半導體/超導體/n型半導體(pSn),由於兩界面的傳輸載子分別為電洞與電子,此狀態可提高元件CAR效率。
    我們利用兩塊單獨的單晶石墨烯與金屬鋁製作成石墨烯/鋁/石墨烯元件,低溫下鋁為超導態,為了能個別調控兩塊石墨烯化學勢(chemical potential),製作兩個獨立上閘極,將一邊石墨烯調控為p型;另一邊則是n型,使元件處於p型石墨烯/超導體/n型石墨烯以增加發生CAR傳輸行為。我們使用電流-電流相關性量測與非局域電壓量測方法觀察兩界面電荷傳輸關聯性並判斷電子傳輸行為,超導體能隙內非局域電子傳輸有兩種方式,分別為CAR與彈性共同穿隧(elastic cotunneling, EC)。
    元件製作上,我們沒有成功使用上閘極個別調控兩塊石墨烯載子濃度與種類,只能利用下閘極同時調控兩塊石墨烯,無法將元件確實調控成pSn,因此對於元件的操控自由度大幅減小。在電流-電流相關性量測與非局域電壓量測皆觀察到EC傳輸行為,而CAR只在非局域電壓量測觀察到,並且當調控下閘極改變石墨烯濃度使元件在靠近pSn組成時CAR較明顯,與理論相符合。
    ;We study electronic transport properties in superconductor and focus on Cooper pair splitting (CPS) phenomena. Cooper pair in superconductor is a quantum entangled object and could split into two spatially-separate normal metals via crossed Andreev reflection (CAR). It can be taken as a source of entangled electrons in quantum teleportation. The efficiency of CAR can be enhanced in a system consist of p-type semiconductor/superconductor/n-type semiconductor (pSn), due to either electron or hole is missing in superconductor/semiconductor interfaces.
    We used two single crystal graphene to fabricate graphene/aluminum/graphene device and measured it below Al superconducting critical temperature. In order to tune two pieces of graphene separately to p-type and n-type, we made two independent top gates. Two kinds of electronic transport, CAR and elastic cotunneling (EC) are considered in the superconducting gap. We observed the correlation of two graphene/aluminum junctions by current-current correlation and nonlocal voltage measurements.
    In our experiments, we did not successfully tune the carrier density of graphene by top gate. Therefore, we can not make pSn devices reliably. The carrier densities of both graphene can be tuned only by bottom gate, which decreases the tuning capability of our device. We observed EC by current-current correlation and nonlocal voltage measurements, but CAR was observed by nonlocal voltage measurement. When the device was tuned near pSn region, CAR was more obvious. The result is consistent with the theoretical prediction.
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