The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity—which cannot be explained by weak electron–phonon interactions—in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°—the first ‘magic’ angle—the electronic band structure of this ‘twisted bilayer graphene’ exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature–carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 1011 per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids.

中文翻译：

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魔角石墨烯超晶格中的非常规超导性

强相关材料的行为，特别是非常规超导体的行为，已经被广泛研究了几十年，但仍然没有得到很好的理解。缺乏理论理解推动了研究此类行为的实验技术的发展，例如使用超冷原子晶格来模拟量子材料。在这里，我们报告了在二维超晶格中实现了固有的非常规超导性——这无法用弱电子 - 声子相互作用来解释——通过堆叠两片相对于彼此扭曲小角度的石墨烯形成。对于大约 1.1° 的扭曲角——第一个“魔角”——这种“扭曲双层石墨烯”的电子能带结构表现出接近零费米能量的平坦带，导致半填充时的相关绝缘状态。在远离这些相关绝缘状态的材料静电掺杂后，我们观察到可调谐零电阻状态，临界温度高达 1.7 开尔文。扭曲双层石墨烯的温度-载流子密度相图类似于氧化铜（或铜酸盐），并包括对应于超导性的圆顶形区域。此外，材料纵向电阻的量子振荡表明在相关绝缘态附近存在小的费米面，类似于未掺杂的铜酸盐。考虑到如此小的费米面（相当于每平方厘米约 1011 个载流子密度），扭曲双层石墨烯的超导临界温度相对较高，使其成为电子间配对强度最强的超导体之一。扭曲双层石墨烯是一种精确可调的纯碳基二维超导体。因此，它是研究强相关现象的理想材料，可以深入了解高临界温度超导体和量子自旋液体的物理学。