Flavour Hund's coupling, Chern gaps and charge diffusivity in moire graphene
Flavour Hund's coupling, Chern gaps and charge diffusivity in moire graphene
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DOI：
10.1038/s41586-021-03366-w
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发表时间：
2021-04-01
期刊：
影响因子：
64.8
通讯作者：
Jarillo-Herrero, Pablo
中科院分区：
文献类型：
--
作者：
Park, Jeong Min;Cao, Yuan;Jarillo-Herrero, Pablo
相互作用驱动的自发对称性破缺是许多物质量子相的核心。在莫尔系统中，平带中的自旋/谷"味道"对称性的破坏是最终出现相关和拓扑基态的母态的基础（1 - 10）。然而，这种味道对称性破缺的微观机制及其与低温相的联系还不清楚。在这里，我们研究了破缺对称多体基态的魔角扭曲双层石墨烯（MATBG）和它的非平凡拓扑结构，同时使用热力学和输运测量。我们直接观察味对称性破缺的化学势钉扎在所有整数填充的莫尔超晶格，证明味洪德的耦合在多体基态的重要性。基本的平带的拓扑性质表现在打破时间反演对称性，在那里我们测量的能隙对应于陈绝缘体状态与陈数3，2，1，分别在填充因子1，2，3，符合味对称性破缺MATBG的霍夫施塔特蝴蝶谱。此外，电阻率和化学势的同时测量提供了MATBG在奇异金属区域中的温度依赖性电荷扩散率（11）-先前仅在超冷原子中探索的量（12）。我们的研究结果使我们更接近一个统一的框架来理解在有和没有磁场的情况下，MATBG拓扑带中的相互作用。
Interaction-driven spontaneous symmetry breaking lies at the heart of many quantum phases of matter. In moire systems, broken spin/valley 'flavour' symmetry in flat bands underlies the parent state from which correlated and topological ground states ultimately emerge(1-10). However, the microscopic mechanism of such flavour symmetry breaking and its connection to the low-temperature phases are not yet understood. Here we investigate the broken-symmetry many-body ground state of magic-angle twisted bilayer graphene (MATBG) and its nontrivial topology using simultaneous thermodynamic and transport measurements. We directly observe flavour symmetry breaking as pinning of the chemical potential at all integer fillings of the moire superlattice, demonstrating the importance of flavour Hund's coupling in the many-body ground state. The topological nature of the underlying flat bands is manifested upon breaking time-reversal symmetry, where we measure energy gaps corresponding to Chern insulator states with Chern numbers 3, 2, 1 at filling factors 1, 2, 3, respectively, consistent with flavour symmetry breaking in the Hofstadter butterfly spectrum of MATBG. Moreover, concurrent measurements of resistivity and chemical potential provide the temperature-dependent charge diffusivity of MATBG in the strange-metal regime(11)-a quantity previously explored only in ultracold atoms(12). Our results bring us one step closer to a unified framework for understanding interactions in the topological bands of MATBG, with and without a magnetic field.