Department of Physics , University of California , Berkeley , California 94720 , United States.
Department of Physics , University of California , Santa Cruz , California 95064 , United States.
Nano Lett. 2018 Aug 8;18(8):5104-5110. doi: 10.1021/acs.nanolett.8b01972. Epub 2018 Jul 30.
Graphene p-n junctions provide an ideal platform for investigating novel behavior at the boundary between electronics and optics that arise from massless Dirac Fermions, such as whispering gallery modes and Veselago lensing. Bilayer graphene also hosts Dirac Fermions, but they differ from single-layer graphene charge carriers because they are massive, can be gapped by an applied perpendicular electric field, and have very different pseudospin selection rules across a p-n junction. Novel phenomena predicted for these massive Dirac Fermions at p-n junctions include anti-Klein tunneling, oscillatory Zener tunneling, and electron cloaked states. Despite these predictions there has been little experimental focus on the microscopic spatial behavior of massive Dirac Fermions in the presence of p-n junctions. Here we report the experimental manipulation and characterization of massive Dirac Fermions within bilayer graphene quantum dots defined by circular p-n junctions through the use of scanning tunneling microscopy-based (STM) methods. Our p-n junctions are created via a flexible technique that enables realization of exposed quantum dots in bilayer graphene/hBN heterostructures. These quantum dots exhibit sharp spectroscopic resonances that disperse in energy as a function of applied gate voltage. Spatial maps of these features show prominent concentric rings with diameters that can be tuned by an electrostatic gate. This behavior is explained by single-electron charging of localized states that arise from the quantum confinement of massive Dirac Fermions within our exposed bilayer graphene quantum dots.
石墨烯 p-n 结为研究电子学和光学边界处由于无质量狄拉克费米子而产生的新型行为提供了理想的平台，例如 whispering gallery 模式和 Veselago 透镜。双层石墨烯也存在狄拉克费米子，但与单层石墨烯载流子不同，因为它们是有质量的，可以通过施加垂直电场来隙，并且在 p-n 结处具有非常不同的赝自旋选择规则。这些在 p-n 结处的大质量狄拉克费米子预测的新现象包括反 Klein 隧穿、振荡 Zener 隧穿和电子隐身态。尽管有这些预测，但实验很少关注存在 p-n 结时大质量狄拉克费米子的微观空间行为。在这里，我们通过使用基于扫描隧道显微镜的 (STM) 方法，报告了在通过圆形 p-n 结定义的双层石墨烯量子点中对大质量狄拉克费米子的实验操控和表征。我们的 p-n 结是通过一种灵活的技术创建的，该技术能够在双层石墨烯/hBN 异质结构中实现暴露的量子点。这些量子点表现出尖锐的光谱共振，其能量随着施加栅极电压的函数而分散。这些特征的空间图谱显示出明显的同心环，其直径可以通过静电栅极进行调节。这种行为可以通过在我们暴露的双层石墨烯量子点内由大质量狄拉克费米子的量子限制引起的局域态的单电子充电来解释。
