Cascade of phase transitions and Dirac revivals in magic-angle graphene
Cascade of phase transitions and Dirac revivals in magic-angle graphene
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DOI：
10.1038/s41586-020-2373-y
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发表时间：
2020-06-01
期刊：
影响因子：
64.8
通讯作者：
Ilani, S.
中科院分区：
文献类型：
--
作者：
Zondiner, U.;Rozen, A.;Ilani, S.
在魔角（1-4）附近的扭曲双层石墨烯表现出丰富的电子相关物理，显示出绝缘相（3-6）、磁性相（7，8）和超导相（4-6）。该系统的电子能带被预测（1，2）在魔角附近明显变窄（9，10），导致各种可能的破缺基态（11-17）。在这里，使用测量的本地电子的可压缩性，我们表明，这些相关的相位起源于一个不寻常的序列带人口的高能量状态。当载体被添加到系统中时，对应于自旋和谷自由度的四种电子“味道”并没有被相等地填充。相反，它们是通过一系列尖锐的相变，这似乎是强烈的非对称跳跃的电子压缩性附近的整数填充的莫尔晶格填充填充。在每一次跃迁中，一个单一的自旋/谷味从其部分填充的对等体中带走所有载流子，将它们“重置”到电荷中性点附近。其结果是，在电荷中性附近观察到的Dirac-like字符在每个整数填充后重新出现。测量填充因子1附近的化学势的面内磁场依赖性揭示了大的自发磁化，进一步证实了对称性破缺的级联的这幅图片。相变和狄拉克复苏的顺序观察温度远高于超导和相关绝缘状态的发病。这表明，我们在这里报告的状态，具有强烈的电子味对称性破缺和恢复的Dirac类电子特征，在魔角石墨烯的物理学中很重要，形成母态，从母态中出现更脆弱的超导和相关的绝缘基态。角度扭曲的双层石墨烯表明，该系统的绝缘相和超导相都来自于高能量的破缺态。
Twisted bilayer graphene near the magic angle(1-4)exhibits rich electron-correlation physics, displaying insulating(3-6), magnetic(7,8)and superconducting phases(4-6). The electronic bands of this system were predicted(1,2)to narrow markedly(9,10)near the magic angle, leading to a variety of possible symmetry-breaking ground states(11-17). Here, using measurements of the local electronic compressibility, we show that these correlated phases originate from a high-energy state with an unusual sequence of band population. As carriers are added to the system, the four electronic 'flavours', which correspond to the spin and valley degrees of freedom, are not filled equally. Rather, they are populated through a sequence of sharp phase transitions, which appear as strong asymmetric jumps of the electronic compressibility near integer fillings of the moire lattice. At each transition, a single spin/valley flavour takes all the carriers from its partially filled peers, 'resetting' them to the vicinity of the charge neutrality point. As a result, the Dirac-like character observed near charge neutrality reappears after each integer filling. Measurement of the in-plane magnetic field dependence of the chemical potential near filling factor one reveals a large spontaneous magnetization, further substantiating this picture of a cascade of symmetry breaking. The sequence of phase transitions and Dirac revivals is observed at temperatures well above the onset of the superconducting and correlated insulating states. This indicates that the state that we report here, with its strongly broken electronic flavour symmetry and revived Dirac-like electronic character, is important in the physics of magic-angle graphene, forming the parent state out of which the more fragile superconducting and correlated insulating ground states emerge.Local electronic compressibility measurements of magic-angle twisted bilayer graphene show that the insulating and superconducting phases of this system are both derived from a high-energy symmetry-broken state.