雙局部閘極石墨烯/超導體/石墨烯元件中古柏電子對分離現象觀測

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姓名

宋國豪(Song,Guo-Hao) 查詢紙本館藏

畢業系所

物理學系

論文名稱

雙局部閘極石墨烯/超導體/石墨烯元件中古柏電子對分離現象觀測

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摘要(中)

本研究製作雙局部閘極石墨烯/超導體/石墨烯元件，主要針對超導體與石墨烯界面的古柏電子對分離(Cooper Pair Splitting,CPS)傳輸效應進行探討。古柏電子對是超導體中超導能隙內的主要電荷載子，由兩個不同自旋方向的電子所形成的量子糾纏態，當一般金屬(N)與超導體(S)形成NSN元件時，古柏電子對其離開超導體的二個電子可分別傳向兩個不同的一般金屬，此電荷傳輸過程稱為CPS，亦稱為交叉式安德烈夫反射(Crossed Andreev Reflection,CAR)。我們使用石墨烯當作古柏電子對分離的二個通道，鋁作為超導金屬，透過兩個不同位置的下閘極調控二個石墨烯通道，將其中一個石墨烯調控成主要電荷載子為電洞(p型)，另一個調控成主要電荷載子為電子(n型)，期望當元件形成pSn元件時，能提升古柏電子對分離發生的機率。我們製作了兩種不同結構的元件，分別是相連式石墨烯元件與分離式石墨烯元件，並且成功利用下閘極調控對應位置的石墨烯載子型態與濃度，使用非局域電壓量測法與電流-電流相關性量測法來觀測古柏電子對分離現象，此外也改變元件溫度以及對元件施加磁場驗證超導性質，在我們的測量中並沒有看到古柏電子對分離現象，但觀測到另一個NSN元件中會發生的效應，彈性共同穿隧(Elastic Cotunneling,EC)。

摘要(英)

We fabricated the graphene/superconductor/graphene device with two local gates, and mainly studied the Cooper pair splitting (CPS) in the system. The Cooper pair is a quantum entangled pair which consisted of two electrons with opposite spin direction. It is the major carrier in the superconducting energy gap of the superconductors. When two normal metals (N) and a superconductor (S) form an NSN device, the two electrons of a Cooper pair can leave superconductor and transport to two normal metals respectively. This transfer process called CPS, also known as crossed Andreev reflection, CAR. We used graphene as the two channels for CPS, and aluminum as the superconductor. Two local gates can tune carrier type and carrier concentration of the two graphene channels individually. We tuned one of the graphene channels as n-type whose major carrier is electron, and the other graphene channel as p-type whose major carrier is hole. And we expect this nSp configuration can promote the efficiency of CPS.
We fabricated two kinds of devices with different structure. One is a channel connected device: the two graphene channels are connected and the other is a channel separated device: the two graphene channels are separate. We successfully fabricate local gates to tune the major carrier type and the carrier concentration. Nonlocal voltage measurement and current-current correlation measurement are used for observe CPS. We verified superconductivity by changing the temperature and adding external magnetic field. But in the two different types of devices, we did not observe the CPS in our work. We only observed the elastic cotunneling (EC) which is another effect of the NSN devices.

關鍵字(中)

★ 古柏電子對
★ 超導體
★ 石墨烯

關鍵字(英)

論文目次

摘要 I
Abstract II
誌謝 III
圖目錄 VII
Chapter 1.緒論 1
1-1前言 1
1-2超導體 2
1-3超導體與一般導體介面電荷傳輸效應-安德烈夫反射(Andreev Reflection) 3
1-4NSN元件中的超導電荷傳輸行為:交叉式安德烈夫反射(Cross Andreev Reflection, CAR)與彈性共同穿隧(Elastic Cotunneling, EC) 5
1-5石墨烯(graphene) 7
1-6文獻探討 9
1-7研究目的 13
Chapter 2.元件設計與製程介紹 15
2-1元件設計 15
2-2元件製程 16
(i)電子束微影(E-beam lithography) 16
(ii)電子槍蒸鍍系統(E-gun evaporator) 18
(iii)微影製程流程 19
(iv)化學氣相沉積法石墨烯(CVD growth graphene) 20
(v)石墨烯轉置-銅箔蝕刻轉印法與氣泡剝離轉印法 21
(vi)氧電漿之石墨烯蝕刻 24
2-3壓針元件載台 26
2-4低溫量測系統 27
Chapter 3.元件基本性質、實驗量測方法與數據 29
3-1超導材料性質量測 29
3-2元件幾何結構介紹 30
(i)通道相連式石墨烯元件之幾何結構(元件A&B) 31
(ii)分離式石墨烯元件之幾何結構(元件C&D) 36
3-3量測方法介紹 40
(i)非局域電壓量測(Nonlocal Voltage measurement) 41
(ii)電流-電流相關性量測(Current-current Correlation) 43
3-4通道相連式石墨烯元件A與B的量測 47
(i)低溫石墨烯性質量測 47
(i)通道相連式石墨烯元件的非局域電壓量測 51
(ii)通道相連式石墨烯元件的電流-電流相關性量測 56
3-5通道分離式石墨烯元件C與D的量測 61
(i)低溫石墨烯性質量測 61
(ii)通道分離式石墨烯元件一次降溫的非局域電壓量測 64
(iii)分離式石墨烯元件二次降溫的非局域電壓量測 68
(iv)分離式石墨烯元件的電流-電流相關性量測 72
Chapter 4.實驗結果與討論 77
4-1石墨烯性質表現 77
4-2相連式石墨烯元件實驗結果 78
4-3分離式石墨烯元件實驗結果 79
4-4製程討論 80
Chapter 5.結論 81
參考文獻 83

參考文獻

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指導教授

陳永富

審核日期

2017-7-25

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