Department of Physics, Stanford University, Stanford, California 94305, USA.
Nature. 2012 Mar 14;483(7389):306-10. doi: 10.1038/nature10941.
The observation of massless Dirac fermions in monolayer graphene has generated a new area of science and technology seeking to harness charge carriers that behave relativistically within solid-state materials. Both massless and massive Dirac fermions have been studied and proposed in a growing class of Dirac materials that includes bilayer graphene, surface states of topological insulators and iron-based high-temperature superconductors. Because the accessibility of this physics is predicated on the synthesis of new materials, the quest for Dirac quasi-particles has expanded to artificial systems such as lattices comprising ultracold atoms. Here we report the emergence of Dirac fermions in a fully tunable condensed-matter system-molecular graphene-assembled by atomic manipulation of carbon monoxide molecules over a conventional two-dimensional electron system at a copper surface. Using low-temperature scanning tunnelling microscopy and spectroscopy, we embed the symmetries underlying the two-dimensional Dirac equation into electron lattices, and then visualize and shape the resulting ground states. These experiments show the existence within the system of linearly dispersing, massless quasi-particles accompanied by a density of states characteristic of graphene. We then tune the quantum tunnelling between lattice sites locally to adjust the phase accrual of propagating electrons. Spatial texturing of lattice distortions produces atomically sharp p-n and p-n-p junction devices with two-dimensional control of Dirac fermion density and the power to endow Dirac particles with mass. Moreover, we apply scalar and vector potentials locally and globally to engender topologically distinct ground states and, ultimately, embedded gauge fields, wherein Dirac electrons react to 'pseudo' electric and magnetic fields present in their reference frame but absent from the laboratory frame. We demonstrate that Landau levels created by these gauge fields can be taken to the relativistic magnetic quantum limit, which has so far been inaccessible in natural graphene. Molecular graphene provides a versatile means of synthesizing exotic topological electronic phases in condensed matter using tailored nanostructures.
单层石墨烯中无质量狄拉克费米子的观测催生了一个新的科学和技术领域，旨在利用在固态材料中表现出相对论行为的电荷载流子。无质量和有质量的狄拉克费米子都已在包括双层石墨烯、拓扑绝缘体的表面态和铁基高温超导体在内的越来越多的狄拉克材料中得到了研究和提出。由于这种物理现象的可及性取决于新材料的合成，因此对狄拉克准粒子的探索已经扩展到了人工系统，例如由超冷原子组成的晶格。在这里，我们报告了在一个完全可调谐的凝聚态系统——通过原子操纵一氧化碳分子在铜表面上组装而成的分子石墨烯——中出现的狄拉克费米子。利用低温扫描隧道显微镜和光谱学，我们将二维狄拉克方程的对称性嵌入到电子晶格中，然后可视化和塑造由此产生的基态。这些实验表明，在该系统中存在线性色散的无质量准粒子，同时伴随着石墨烯特有的态密度。然后，我们局部调整晶格位点之间的量子隧穿，以调整传播电子的相位积累。晶格扭曲的空间纹理产生具有原子尖锐的 p-n 和 p-n-p 结器件，可对狄拉克费米子密度进行二维控制，并赋予狄拉克粒子质量的能力。此外，我们局部和全局施加标量和矢量势，以产生拓扑上不同的基态，最终产生嵌入的规范场，其中狄拉克电子对其参考系中存在但实验室系中不存在的“赝”电场和磁场做出反应。我们证明，通过这些规范场产生的朗道能级可以被带到相对论磁量子极限，这在天然石墨烯中迄今是无法达到的。分子石墨烯为使用定制的纳米结构在凝聚态物质中合成奇异拓扑电子相提供了一种通用手段。
