Half- and quarter-metals in rhombohedral trilayer graphene
Half- and quarter-metals in rhombohedral trilayer graphene
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DOI：
10.1038/s41586-021-03938-w
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发表时间：
2021-09-01
期刊：
影响因子：
64.8
通讯作者：
Young, Andrea F.
中科院分区：
文献类型：
--
作者：
Zhou, Haoxin;Xie, Tian;Young, Andrea F.
铁磁性在过渡金属化合物中最常见，其中电子占据高度局部化的d轨道。然而，铁磁有序也可能出现在低密度二维电子系统中（1-5）。在这里，我们表明，门调谐货车霍韦奇异性在菱形三层石墨烯（6）驱动电子系统的自发铁磁极化到一个或多个自旋和谷的味道。使用电容和运输测量，我们观察到一个级联的过渡调谐到密度和电子位移场之间的相位，其中量子振荡有四倍，两倍或一倍的简并性，与自旋和谷简并正常金属，自旋极化的“半金属”，自旋和谷极化的“四分之一金属”，分别。对于电子掺杂，数据的显著特征被包括谷-各向异性相互作用的现象学Stoner模型（7）很好地捕获。对于孔填充，我们观察到一个更丰富的相图具有微妙的相互作用的对称性和费米表面拓扑结构的转变打破。最后，我们介绍了一种使用旋转排列的六方氮化硼衬底的莫尔超晶格（5，8）。值得注意的是，我们发现，只有弱扰动的同位旋顺序，与莫尔电位催化形成的拓扑非平凡的间隙状态时，巡回半或四分之一金属状态发生在半或四分之一超晶格带填充。我们的研究结果表明，菱形石墨烯是多体理论的良好控制测试的理想平台，并揭示了莫尔材料（4，5，9，10）中的磁性本质上是巡回的。
Ferromagnetism is most common in transition metal compounds where electrons occupy highly localized d orbitals. However, ferromagnetic order may also arise in low-density two-dimensional electron systems(1-5). Here we show that gate-tuned van Hove singularities in rhombohedral trilayer graphene(6) drive spontaneous ferromagnetic polarization of the electron system into one or more spin and valley flavours. Using capacitance and transport measurements, we observe a cascade of transitions tuned to the density and electronic displacement field between phases in which quantum oscillations have fourfold, twofold or onefold degeneracy, associated with a spin- and valley-degenerate normal metal, spin-polarized 'half-metal', and spin- and valley-polarized 'quarter-metal', respectively. For electron doping, the salient features of the data are well captured by a phenomenological Stoner model(7) that includes valley-anisotropic interactions. For hole filling, we observe a richer phase diagram featuring a delicate interplay of broken symmetries and transitions in the Fermi surface topology. Finally, we introduce a moire superlattice using a rotationally aligned hexagonal boron nitride substrate(5,8). Remarkably, we find that the isospin order is only weakly perturbed, with the moire potential catalysing the formation of topologically nontrivial gapped states whenever itinerant half- or quarter-metal states occur at half- or quarter-superlattice band filling. Our results show that rhombohedral graphene is an ideal platform for well-controlled tests of many-body theory, and reveal magnetism in moire materials(4,5,9,10) to be fundamentally itinerant in nature.