Flat electronic bands, characteristic of ‘magic-angle’ twisted bilayer graphene, host many correlated phenomena^{1,2,3,4,5,6,7,8,9}. Nevertheless, many properties of these bands and emerging symmetry-broken phases are still poorly understood. Here we use scanning tunnelling spectroscopy to examine the evolution of the twisted bilayer graphene bands and related gapped phases as the twist angle between the two graphene layers changes. We detect filling-dependent flattening of the bands that is appreciable even when the angle is well above the magic angle value and so the material is nominally in a weakly correlated regime. Upon approaching the magic angle, we further show that the most prominent correlated gaps begin to emerge when band flattening is maximized around certain integer fillings of electrons per moiré unit cell. Our observations are consistent with a model that suggests that a significant enhancement of the density of states caused by the band flattening triggers a cascade of symmetry-breaking transitions. Finally, we explore the temperature dependence of the cascade and identify gapped features that develop in a broad range of band fillings where superconductivity is expected. Our results highlight the role of interaction-driven band flattening in defining the electronic properties of twisted bilayer graphene.

扁平电子带，“魔角”扭曲双层石墨烯的特征，承载许多相关现象^{1,2,3,4,5,6,7,8,9}. 然而，这些带的许多特性和新兴的对称破缺相仍然知之甚少。在这里，我们使用扫描隧道光谱来检查扭曲双层石墨烯带和相关间隙相的演变，因为两个石墨烯层之间的扭曲角发生变化。我们检测到带的填充相关变平，即使角度远高于魔角值，这种变平也很明显，因此材料名义上处于弱相关状态。在接近魔角时，我们进一步表明，当带展平在每个莫尔晶胞的某些整数电子填充附近最大化时，最显着的相关间隙开始出现。我们的观察结果与一个模型一致，该模型表明由带展平引起的状态密度的显着增强触发了一系列对称破坏跃迁。最后，我们探索了级联的温度依赖性，并确定了在预期超导性的广泛带填充中发展的间隙特征。我们的结果强调了相互作用驱动的带展平在定义扭曲双层石墨烯的电子特性中的作用。