Tunable spin-polarized correlated states in twisted double bilayer graphene
Tunable spin-polarized correlated states in twisted double bilayer graphene
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DOI：
10.1038/s41586-020-2458-7
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发表时间：
2020-07-09
期刊：
影响因子：
64.8
通讯作者：
Kim, Philip
中科院分区：
文献类型：
--
作者：
Liu, Xiaomeng;Hao, Zeyu;Kim, Philip
将晶格中电子的能量带宽降低到长程库仑相互作用能以下会促进相关效应。莫尔超晶格是通过堆叠具有受控扭转角（1-3）的货车德瓦尔斯异质结构而产生的，其使得能够工程化电子能带结构。奇异的量子相位可以出现在工程莫尔平坦带中。最近在魔角扭曲双层石墨烯的平带中发现了相关绝缘体态、超导性和量子反常霍尔效应（4-8），这引发了对其他莫尔系统中相关电子态的探索（9-11）。货车范德瓦耳斯莫尔超晶格的电子性质可以通过调节层间耦合或组成层的能带结构来进一步调节（9）。在这里，我们使用扭曲双层石墨烯（TDBG）的货车德瓦尔斯异质结构，证明了一个平坦的电子带，在一个扭曲角的范围内不受垂直电场的影响。与魔角扭曲的双层石墨烯类似，TDBG在半填充和四分之一填充的平带处显示出能隙，表明相关绝缘体态的出现。我们发现，这些绝缘体状态的间隙增加面内磁场，这表明铁磁秩序。在掺杂的半填充绝缘体，电阻率的突然下降，观察到随着温度的降低。这种临界行为被限制在一个小的区域中的密度电场平面，并归因于从正常金属的自旋极化的相关态的相变。电场可调谐TDBG中自旋极化关联态的发现为设计相互作用驱动的量子相位提供了一条新的途径。
Reducing the energy bandwidth of electrons in a lattice belowthe long-range Coulomb interaction energy promotes correlation effects. Moire superlattices-which are created by stacking van der Waals heterostructures with a controlled twist angle(1-3)-enable the engineering of electron band structure. Exotic quantum phases can emerge in an engineered moire flat band. The recent discovery of correlated insulator states, superconductivity and the quantum anomalous Hall effect in the flat band of magic-angle twisted bilayergraphene(4-8) has sparked the exploration of correlated electron states in other moire systems(9-11). The electronic properties of van der Waals moire superlattices can further be tuned by adjusting the interlayer coupling' orthe band structure of constituent layers(9). Here, using van der Waals heterostructures oftwisted double bilayergraphene (TDBG), we demonstrate a flat electron band that istunable by perpendicular electric fields in a range oftwist angles. Similarly to magic-angle twisted bilayer graphene, TDBG shows energygaps at the half- and quarter-filled flat bands, indicating the emergence of correlated insulator states. We find that the gaps of these insulator states increase with in-plane magnetic field, suggesting a ferromagnetic order. On doping the half-filled insulator, a sudden drop in resistivity is observed with decreasing temperature. This critical behaviour is confined to a small area in the density-electric-field plane, and is attributed to a phase transition from a normal metal to a spin-polarized correlated state. The discovery of spin-polarized correlated states in electric-field-tunable TDBG provides a new route to engineering interaction-driven quantum phases.