Dynamical Correlations and Order in Magic-Angle Twisted Bilayer Graphene,Physical Review X

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Dynamical Correlations and Order in Magic-Angle Twisted Bilayer Graphene
Physical Review X ( IF 11.6 ) Pub Date : 2024-09-11 , DOI: 10.1103/physrevx.14.031045
Gautam Rai ₁ , Lorenzo Crippa ₂ , Dumitru Călugăru ₃ , Haoyu Hu ₄ , Francesca Paoletti ₂ , Luca de’ Medici ₅ , Antoine Georges _{6,

7,

8,

9} , B. Andrei Bernevig _{3,

4,

10} , Roser Valentí ₁₁ , Giorgio Sangiovanni ₂ , Tim Wehling _{1,

12}

Affiliation

The interplay of dynamical correlations and electronic ordering is pivotal in shaping phase diagrams of correlated quantum materials. In magic-angle twisted bilayer graphene, transport, thermodynamic, and spectroscopic experiments pinpoint a competition between distinct low-energy states with and without electronic order, as well as between localized and delocalized charge carriers. In this study, we utilize dynamical mean-field theory on the topological heavy fermion model of twisted bilayer graphene to investigate the emergence of electronic correlations and long-range order in the absence of strain. We contrast moment formation, Kondo screening, and ordering on a temperature basis and explain the nature of emergent correlated states based on three central phenomena: (i) the formation of local spin and valley isospin moments around 100 K, (ii) the ordering of the local isospin moments around 10 K preempting Kondo screening, and (iii) a cascadic redistribution of charge between localized and delocalized electronic states upon doping. At integer fillings, we find that low-energy spectral weight is depleted in the symmetric phase, while we find insulating states with gaps enhanced by exchange coupling in the zero-strain ordered phases. Doping away from integer filling results in distinct metallic states: a “bad metal” above the ordering temperature, where scattering off the disordered local moments suppresses electronic coherence, and a “good metal” in the ordered states with coherence of quasiparticles facilitated by isospin order. This finding reveals coherence from order as the microscopic mechanism behind the Pomeranchuk effect observed experimentally by Rozen et al. [Nature (London) 592, 214 (2021)] and by Saito et al. [Nature (London) 592, 220 (2021)]. Upon doping, there is a periodic charge reshuffling between localized and delocalized electronic orbitals leading to cascades of doping-induced Lifshitz transitions, local spectral weight redistributions, and periodic variations of the electronic compressibility ranging from nearly incompressible to negative. Our findings highlight the essential role of charge transfer, hybridization, and ordering in shaping the electronic excitations and thermodynamic properties in twisted bilayer graphene and provide a unified understanding of the most puzzling aspects of scanning tunneling spectroscopy, transport, and compressibility experiments.

中文翻译：

魔术角扭曲双层石墨烯中的动力学相关性和有序

动力学相关性和电子排序的相互作用在塑造相关量子材料的相图方面至关重要。在魔术角扭曲双层石墨烯中，输运、热力学和光谱实验确定了具有和没有电子有序的不同低能态之间以及局域和离域电荷载流子之间的竞争。在本研究中，我们利用扭曲双层石墨烯拓扑重费米子模型的动力学平均场理论来研究在没有应变的情况下电子相关性和长程有序的出现。我们对比了矩形成、Kondo 筛选和基于温度的排序，并根据三个中心现象解释了新兴相关态的性质：（i）在 100 K 附近形成局部自旋和谷类等旋矩，（ii）在 10 K 附近局部等旋矩的排序抢占了 Kondo 筛选，以及（iii）掺杂后局部和离域电子态之间的电荷级联再分布。在整数填充中，我们发现低能量谱权重在对称相中耗尽，而我们发现零应变有序相中的交换耦合增强了带有间隙的绝缘态。从整数填充中掺杂会产生不同的金属状态：高于有序温度的“坏金属”，其中无序局部矩的散射抑制了电子相干性，而“好金属”处于有序状态，由同位自旋有序促进了准粒子的相干性。这一发现揭示了顺序的一致性，这是 Rozen 等人在实验中观察到的 Pomeranchuk 效应背后的微观机制。 [Nature （London）592， 214 （2021）] 和 Saito 等人 [Nature （London）592， 220 （2021）]。掺杂后，局域和非局域电子轨道之间存在周期性电荷重新洗牌，导致掺杂诱导的 Lifshitz 跃迁级联、局部光谱权重重新分布以及电子可压缩性的周期性变化，范围从几乎不可压缩到负。我们的研究结果强调了电荷转移、杂化和有序在塑造扭曲双层石墨烯中的电子激发和热力学特性中的重要作用，并为扫描隧穿光谱、传输和可压缩性实验中最令人费解的方面提供了统一的理解。

更新日期：2024-09-11

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