Of the two stable forms of graphite, hexagonal and rhombohedral, the former is more common and has been studied extensively. The latter is less stable, which has so far precluded its detailed investigation, despite many theoretical predictions about the abundance of exotic interaction-induced physics1–6. Advances in van der Waals heterostructure technology7 have now allowed us to make high-quality rhombohedral graphite films up to 50 graphene layers thick and study their transport properties. Here we show that the bulk electronic states in such rhombohedral graphite are gapped8 and, at low temperatures, electron transport is dominated by surface states. Because of their proposed topological nature, the surface states are of sufficiently high quality to observe the quantum Hall effect, whereby rhombohedral graphite exhibits phase transitions between a gapless semimetallic phase and a gapped quantum spin Hall phase with giant Berry curvature. We find that an energy gap can also be opened in the surface states by breaking their inversion symmetry by applying a perpendicular electric field. Moreover, in rhombohedral graphite thinner than four nanometres, a gap is present even without an external electric field. This spontaneous gap opening shows pronounced hysteresis and other signatures characteristic of electronic phase separation, which we attribute to emergence of strongly correlated electronic surface states. High-quality rhombohedral graphite films are found to offer an alternative to twisted bilayer graphene as a platform for studying correlated physics in carbon materials.

中文翻译：

---

多层菱形石墨中的电子相分离

在石墨的两种稳定形式，六角形和菱形，前者更为常见，并已被广泛研究。后者不太稳定，尽管有许多关于奇异相互作用诱导物理学的丰富性的理论预测 1-6，但到目前为止，这已经排除了对其的详细研究。范德华异质结构技术的进步 7 现在使我们能够制造高达 50 层石墨烯层的高质量菱形石墨薄膜并研究它们的传输特性。在这里，我们表明这种菱面体石墨中的体电子态是有间隙的，并且在低温下，电子传输由表面态主导。由于它们提出的拓扑性质，表面态具有足够高的质量来观察量子霍尔效应，由此菱形石墨表现出无间隙半金属相和具有巨大贝里曲率的有间隙量子自旋霍尔相之间的相变。我们发现，通过施加垂直电场打破它们的反转对称性，也可以在表面态中打开能隙。此外，在薄于四纳米的菱形石墨中，即使没有外部电场，也存在间隙。这种自发的间隙打开显示出明显的滞后和电子相分离的其他特征，我们将其归因于强相关电子表面态的出现。发现高质量的菱形石墨薄膜提供了一种替代扭曲双层石墨烯的替代品，作为研究碳材料相关物理的平台。