| Summary: | 碩士 === 國立中央大學 === 物理學系 === 105 === 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.
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