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Bipolar supercurrent in graphene

机译：石墨烯中的双极超电流

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Graphene—a recently discovered form of graphite only one atomic layer thick—constitutes a new model system in condensed matter physics, because it is the first material in which charge carriers behave as massless chiral relativistic particles. The anomalous quantization of the Hall conductance, which is now understood theoretically, is one of the experimental signatures of the peculiar transport properties of relativistic electrons in graphene. Other unusual phenomena, like the finite conductivity of order 4e~2/h (where e is the electron charge and h is Planck's constant) at the charge neutrality (or Dirac) point, have come as a surprise and remain to be explained. Here we experimentally study the Josephson effect in mesoscopic junctions consisting of a graphene layer contacted by two closely spaced superconducting electrodes. The charge density in the graphene layer can be controlled by means of a gate electrode. We observe a supercurrent that, depending on the gate voltage, is carried by either electrons in the conduction band or by holes in the valence band. More importantly, we find that not only the normal state conductance of graphene is finite, but also a finite supercurrent can flow at zero charge density. Our observations shed light on the special role of time reversal symmetry in graphene, and demonstrate phase coherent electronic transport at the Dirac point.
机译：石墨烯（一种最近发现的只有一个原子层厚的石墨形式）构成了凝聚态物理中的一个新的模型系统，因为它是第一种电荷载体表现为无质量手性相对论粒子的材料。霍尔电导的异常量化，这在理论上已经可以理解，这是相对论电子在石墨烯中特殊传输性质的实验特征之一。其他异常现象，例如在电荷中性点（或狄拉克）上的4e〜2 / h阶有限电导率（其中e是电子电荷，h是普朗克常数），令人惊讶，有待解释。在这里，我们通过实验研究介观结中的约瑟夫森效应，介观结由两个紧密间隔的超导电极接触的石墨烯层组成。可以通过栅电极控制石墨烯层中的电荷密度。我们观察到一个超电流，取决于栅极电压，它是由导带中的电子或价带中的空穴承载的。更重要的是，我们发现不仅石墨烯的正常状态电导是有限的，而且在零电荷密度下有限的超电流也可以流动。我们的观察揭示了时间反转对称性在石墨烯中的特殊作用，并证明了狄拉克点的相干相干电子传输。

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