Hybrid superconductor/graphene (SC/g) junctions are excellent candidates for investigating correlations between Cooper pairs and quantum Hall (QH) edge modes. Experimental studies are challenging as Andreev reflections are extremely sensitive to junction disorder, and high magnetic fields are required to form QH edge states. We fabricated low-resistance SC/g interfaces, composed of graphene edge contacted with NbN with a barrier strength of Z ≈ 0.4, that remain superconducting under magnetic fields larger than 18 T. We establish the role of graphene’s Dirac band structure on zero-field Andreev reflections and demonstrate dynamic tunability of the Andreev reflection spectrum by moving the boundary between specular and retro Andreev reflections with parallel magnetic fields. Through the application of perpendicular magnetic fields, we observe an oscillatory suppression of the 2-probe conductance in the ν = 4 Landau level attributed to the reduced efficiency of Andreev processes at the NbN/g interface, consistent with theoretical predictions.

中文翻译：

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大磁场下 NbN/石墨烯结中的 Andreev 反射

混合超导体/石墨烯 (SC/g) 结是研究库珀对和量子霍尔 (QH) 边缘模式之间相关性的绝佳候选者。实验研究具有挑战性，因为 Andreev 反射对结无序极其敏感，并且需要高磁场来形成 QH 边缘态。我们制造了低电阻 SC/g 界面，由石墨烯边缘与 NbN 接触组成，势垒强度为Z≈ 0.4，在大于 18 T 的磁场下保持超导。我们建立了石墨烯的狄拉克带结构对零场 Andreev 反射的作用，并通过平行移动镜面反射和逆向 Andreev 反射之间的边界来证明 Andreev 反射光谱的动态可调性磁场。通过应用垂直磁场，我们观察到 ν = 4 Landau 能级中 2 探针电导的振荡抑制，这归因于 NbN/g 界面处 Andreev 过程的效率降低，这与理论预测一致。