Evidence for unconventional superconductivity in twisted trilayer graphene
Evidence for unconventional superconductivity in twisted trilayer graphene
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DOI：
10.1038/s41586-022-04715-z
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发表时间：
2022-06-16
期刊：
影响因子：
64.8
通讯作者：
Nadj-Perge, Stevan
中科院分区：
文献类型：
--
作者：
Kim, Hyunjin;Choi, Youngjoon;Nadj-Perge, Stevan
魔角扭曲三层石墨烯（MATTG）已经成为一种莫尔材料，表现出强烈的电子相关性和非常规的超导性（1，2）。然而，该系统的局部光谱研究仍然缺乏。在这里，我们进行高分辨率的扫描隧道显微镜和光谱的MATTG，揭示了广泛的区域有利于镜像对称堆叠的原子重建。在这些区域中，我们观察到类似于魔角双层中的破坏电子跃迁和掺杂依赖的能带结构变形，正如理论上预期的那样，给出了平坦能带的共性（3，4）。最值得注意的是，在每个莫尔晶胞跨越两到三个孔的密度窗口中，超导性的光谱特征表现为费米能级处的隧穿电导的明显下降，伴随着在升高的温度和磁场下逐渐被抑制的相干峰。所观察到的掺杂的电导的演变是一致的门可调的过渡从带隙超导体的节点超导体，这是理论上兼容的急剧过渡从Bardeen-Cooper-Schrieffer超导体的玻色-爱因斯坦凝聚超导体与节点序参数。在这个掺杂窗口，我们还检测到峰-谷-峰结构，表明超导性是由强耦合到玻色子模式的MATTG。我们的研究结果将使人们能够进一步理解扭曲双层之外的石墨烯莫尔结构中的超导性和相关态（5）。
Magic-angle twisted trilayer graphene (MATTG) has emerged as a moire material that exhibits strong electronic correlations and unconventional superconductivity(1,2). However, local spectroscopic studies of this system are still lacking. Here we perform high-resolution scanning tunnelling microscopy and spectroscopy of MATTG that reveal extensive regions of atomic reconstruction favouring mirror-symmetric stacking. In these regions, we observe symmetry-breaking electronic transitions and doping-dependent band-structure deformations similar to those in magic-angle bilayers, as expected theoretically given the commonality of flat bands(3,4). Most notably in a density window spanning two to three holes per moire unit cell, the spectroscopic signatures of superconductivity are manifest as pronounced dips in the tunnelling conductance at the Fermi level accompanied by coherence peaks that become gradually suppressed at elevated temperatures and magnetic fields. The observed evolution of the conductance with doping is consistent with a gate-tunable transition from a gapped superconductor to a nodal superconductor, which is theoretically compatible with a sharp transition from a Bardeen-Cooper-Schrieffer superconductor to a Bose-Einstein-condensation superconductor with a nodal order parameter. Within this doping window, we also detect peak-dip-hump structures that suggest that superconductivity is driven by strong coupling to bosonic modes of MATTG. Our results will enable further understanding of superconductivity and correlated states in graphene-based moire structures beyond twisted bilayers(5).