School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK.
Nanoscale. 2018 Feb 8;10(6):3020-3025. doi: 10.1039/c7nr05904c.
Ballistic Josephson junctions are predicted to support a number of exotic physics processess, providing an ideal system to inject the supercurrent in the quantum Hall regime. Herein, we demonstrate electrical transport measurements on ballistic superconductor-graphene-superconductor junctions by contacting graphene to niobium with a junction length up to 1.5 μm. Hexagonal boron nitride encapsulation and one-dimensional edge contacts guarantee high-quality graphene Josephson junctions with a mean free path of several micrometers and record-low contact resistance. Transports in normal states including the observation of Fabry-Pérot oscillations and Sharvin resistance conclusively witness the ballistic propagation in the junctions. The critical current density J is over one order of magnitude larger than that of the previously reported junctions. Away from the charge neutrality point, the IR product (I is the critical current and R the normal state resistance of junction) is nearly a constant, independent of carrier density n, which agrees well with the theory for ballistic Josephson junctions. Multiple Andreev reflections up to the third order are observed for the first time by measuring the differential resistance in the micrometer-long ballistic graphene Josephson junctions.
弹道约瑟夫森结被预测将支持多种奇异的物理过程，为在量子霍尔区注入超导电流提供了理想的系统。在此，我们通过将石墨烯与长度可达 1.5 μm 的铌接触，在弹道超导-石墨烯-超导结上进行了电输运测量。六方氮化硼封装和一维边缘接触保证了高质量的石墨烯约瑟夫森结，其平均自由程为数微米，接触电阻创纪录的低。正常状态下的输运，包括法布里-珀罗（Fabry-Pérot）振荡和沙尔文（Sharvin）电阻的观察，都确凿地证明了结中的弹道传播。临界电流密度 J 比以前报道的结大一个数量级以上。在偏离电荷中性点的情况下，IR 乘积（I 是临界电流，R 是结的正常状态电阻）几乎是一个常数，与弹道约瑟夫森结的理论相符。通过测量微米长弹道石墨烯约瑟夫森结中的微分电阻，首次观察到多达三阶的多重安德烈夫反射。
