Superconductors, orbital magnets and correlated states in magic-angle bilayer graphene
Superconductors, orbital magnets and correlated states in magic-angle bilayer graphene
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DOI：
10.1038/s41586-019-1695-0
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发表时间：
2019-10-31
期刊：
影响因子：
64.8
通讯作者：
Efetov, Dmitri K.
中科院分区：
文献类型：
--
作者：
Lu, Xiaobo;Stepanov, Petr;Efetov, Dmitri K.
超导性可以在接近破缺对称母态的条件下发生(1)。在双层石墨烯中，一层相对于另一层的扭转角度约为1度，导致了超平坦莫尔超晶格微带的出现。这些带是强关联物理(2-5)的丰富和高度可调谐的来源，特别是超导电性，它出现在接近相互作用诱导的绝缘态(6，7)。本文报道了具有高度均匀扭转角的魔角扭转双层石墨烯器件的制备。扭角无序的减小揭示了在四重自旋谷简并平坦导带和价带的所有整数占有处都存在绝缘态，即在莫尔带填充因子nu=0，+/-1，+/-2，+/-3处。在nu接近-2时，在高达3Kelvin的临界温度以下观察到超导电性。我们还在更低的温度下观察到了三个新的超导穹顶，接近nu=0和。=+/-1个绝缘态。值得注意的是，在nu=+/-1时，我们发现了陈数不为零的态。当垂直磁场大于3.6特斯拉时，当nu=-1时，绝缘态表现出急剧的滞后电阻增强，这与场驱动相变是一致的。我们的研究表明，破坏对称态、相互作用驱动的绝缘体、轨道磁体、具有非零陈氏数的态和超导穹顶频繁地出现在广泛的莫尔平带填充中，包括接近电荷中性。这项研究提供了魔角扭曲双层石墨烯现象学的更详细的观点，增加了我们对其出现性质的不断演变的理解。
Superconductivity can occur under conditions approaching broken-symmetry parent states(1). In bilayer graphene, the twisting of one layer with respect to the other at 'magic' twist angles of around 1 degree leads to the emergence of ultra-flat moire superlattice minibands. Such bands are a rich and highly tunable source of strong-correlation physics(2-5), notably superconductivity, which emerges close to interaction-induced insulating states(6,7). Here we report the fabrication of magic-angle twisted bilayer graphene devices with highly uniform twist angles. The reduction in twist-angle disorder reveals the presence of insulating states at all integer occupancies of the fourfold spin-valley degenerate flat conduction and valence bands-that is, at moire band filling factors nu = 0, +/- 1, +/- 2, +/- 3. At nu approximate to -2, superconductivity is observed below critical temperatures of up to 3 kelvin. We also observe three new superconducting domes at much lower temperatures, close to the nu = 0 and. = +/- 1 insulating states. Notably, at nu = +/- 1 we find states with non-zero Chern numbers. For nu = -1 the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field greater than 3.6 tesla is applied, which is consistent with a field-driven phase transition. Our study shows that broken-symmetry states, interaction-driven insulators, orbital magnets, states with non-zero Chern numbers and superconducting domes occur frequently across a wide range of moire flat band fillings, including close to charge neutrality. This study provides a more detailed view of the phenomenology of magic-angle twisted bilayer graphene, adding to our evolving understanding of its emergent properties.