Zhang Canxun, Zhu Tiancong, Kahn Salman, Li Shaowei, Yang Birui, Herbig Charlotte, Wu Xuehao, Li Hongyuan, Watanabe Kenji, Taniguchi Takashi, Cabrini Stefano, Zettl Alex, Zaletel Michael P, Wang Feng, Crommie Michael F
Department of Physics, University of California, Berkeley, CA, USA.
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
Nat Commun. 2021 May 4;12(1):2516. doi: 10.1038/s41467-021-22711-1.
The discovery of interaction-driven insulating and superconducting phases in moiré van der Waals heterostructures has sparked considerable interest in understanding the novel correlated physics of these systems. While a significant number of studies have focused on twisted bilayer graphene, correlated insulating states and a superconductivity-like transition up to 12 K have been reported in recent transport measurements of twisted double bilayer graphene. Here we present a scanning tunneling microscopy and spectroscopy study of gate-tunable twisted double bilayer graphene devices. We observe splitting of the van Hove singularity peak by ~20 meV at half-filling of the conduction flat band, with a corresponding reduction of the local density of states at the Fermi level. By mapping the tunneling differential conductance we show that this correlated system exhibits energetically split states that are spatially delocalized throughout the different regions in the moiré unit cell, inconsistent with order originating solely from onsite Coulomb repulsion within strongly-localized orbitals. We have performed self-consistent Hartree-Fock calculations that suggest exchange-driven spontaneous symmetry breaking in the degenerate conduction flat band is the origin of the observed correlated state. Our results provide new insight into the nature of electron-electron interactions in twisted double bilayer graphene and related moiré systems.
在莫尔范德华异质结构中发现相互作用驱动的绝缘和超导相，引发了人们对理解这些系统中新型关联物理的浓厚兴趣。虽然大量研究集中在扭曲双层石墨烯上，但在最近对扭曲双双层石墨烯的输运测量中，已经报道了相关绝缘态和高达12 K的类似超导转变。在此，我们展示了对栅极可调扭曲双双层石墨烯器件的扫描隧道显微镜和光谱研究。我们观察到在传导平带半填充时，范霍夫奇点峰分裂约20 meV，同时费米能级处的局域态密度相应降低。通过绘制隧穿微分电导，我们表明这个关联系统呈现出能量分裂态，这些态在莫尔晶胞的不同区域中空间离域，这与仅源于强局域化轨道内的在位库仑排斥的有序性不一致。我们进行了自洽哈特里 - 福克计算，结果表明简并传导平带中交换驱动的自发对称性破缺是观察到的关联态的起源。我们的结果为扭曲双双层石墨烯及相关莫尔系统中电子 - 电子相互作用的本质提供了新的见解。
