Department of Physics and Astronomy, Rutgers University, Piscataway, NJ, USA.
National Institute for Materials Science, Tsukuba, Japan.
Nature. 2019 Sep;573(7772):91-95. doi: 10.1038/s41586-019-1460-4. Epub 2019 Jul 31.
Bilayer graphene can be modified by rotating (twisting) one layer with respect to the other. The interlayer twist gives rise to a moiré superlattice that affects the electronic motion and alters the band structure. Near a 'magic angle' of twist, where the emergence of a flat band causes the charge carriers to slow down, correlated electronic phases including Mott-like insulators and superconductors were recently discovered by using electronic transport. These measurements revealed an intriguing similarity between magic-angle twisted bilayer graphene and high-temperature superconductors, which spurred intensive research into the underlying physical mechanism. Essential clues to this puzzle, such as the symmetry and spatial distribution of the spectral function, can be accessed through scanning tunnelling spectroscopy. Here we use scanning tunnelling microscopy and spectroscopy to visualize the local density of states and charge distribution in magic-angle twisted bilayer graphene. Doping the sample to partially fill the flat band, we observe a pseudogap phase accompanied by a global stripe charge order that breaks the rotational symmetry of the moiré superlattice. Both the pseudogap and the stripe charge order disappear when the band is either empty or full. The close resemblance to similar observations in high-temperature superconductors provides new evidence of a deeper link underlying the phenomenology of these systems.
双层石墨烯可以通过旋转（扭曲）一个相对于另一个的层来进行修饰。层间扭转产生了莫尔超晶格，这影响了电子运动并改变了能带结构。在扭转的“魔法角度”附近，由于出现平坦能带导致载流子减速，最近通过电子输运发现了相关的电子相，包括类莫特绝缘体和超导体。这些测量结果揭示了魔法角扭曲双层石墨烯和高温超导体之间令人着迷的相似性，这促使人们对其潜在的物理机制进行了深入研究。解决这个难题的关键线索，如光谱函数的对称性和空间分布，可以通过扫描隧道谱来获得。在这里，我们使用扫描隧道显微镜和光谱学来可视化魔法角扭曲双层石墨烯中的局域态密度和电荷分布。通过对样品进行部分掺杂以填充平坦能带，我们观察到赝能隙相伴随着打破莫尔超晶格旋转对称性的全局条纹电荷有序。当能带为空或满时，赝能隙和条纹电荷有序都会消失。与高温超导体中类似观察结果的密切相似性为这些系统的现象学提供了更深层次联系的新证据。
