Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
Department of Physics, University of California at Berkeley, Berkeley, CA, USA.
Nature. 2019 Aug;572(7768):215-219. doi: 10.1038/s41586-019-1393-y. Epub 2019 Jul 17.
Understanding the mechanism of high-transition-temperature (high-T) superconductivity is a central problem in condensed matter physics. It is often speculated that high-T superconductivity arises in a doped Mott insulator as described by the Hubbard model. An exact solution of the Hubbard model, however, is extremely challenging owing to the strong electron-electron correlation in Mott insulators. Therefore, it is highly desirable to study a tunable Hubbard system, in which systematic investigations of the unconventional superconductivity and its evolution with the Hubbard parameters can deepen our understanding of the Hubbard model. Here we report signatures of tunable superconductivity in an ABC-trilayer graphene (TLG) and hexagonal boron nitride (hBN) moiré superlattice. Unlike in 'magic angle' twisted bilayer graphene, theoretical calculations show that under a vertical displacement field, the ABC-TLG/hBN heterostructure features an isolated flat valence miniband associated with a Hubbard model on a triangular superlattice where the bandwidth can be tuned continuously with the vertical displacement field. Upon applying such a displacement field we find experimentally that the ABC-TLG/hBN superlattice displays Mott insulating states below 20 kelvin at one-quarter and one-half fillings of the states, corresponding to one and two holes per unit cell, respectively. Upon further cooling, signatures of superconductivity ('domes') emerge below 1 kelvin for the electron- and hole-doped sides of the one-quarter-filling Mott state. The electronic behaviour in the ABC-TLG/hBN superlattice is expected to depend sensitively on the interplay between the electron-electron interaction and the miniband bandwidth. By varying the vertical displacement field, we demonstrate transitions from the candidate superconductor to Mott insulator and metallic phases. Our study shows that ABC-TLG/hBN heterostructures offer attractive model systems in which to explore rich correlated behaviour emerging in the tunable triangular Hubbard model.
理解高温超导（high-T）的机制是凝聚态物理的核心问题。人们通常推测，掺杂莫特绝缘体中的 Hubbard 模型可以产生高温超导。然而，由于莫特绝缘体中的电子-电子相关性很强， Hubbard 模型的精确解极具挑战性。因此，研究可调谐 Hubbard 系统非常重要，在这种系统中，可以对非常规超导及其与 Hubbard 参数的演化进行系统研究，从而加深我们对 Hubbard 模型的理解。在这里，我们报告了在 ABC 三层石墨烯（TLG）和六方氮化硼（hBN）莫尔超晶格中可调谐超导的特征。与“魔角”扭曲双层石墨烯不同，理论计算表明，在垂直位移场下，ABC-TLG/hBN 异质结构具有与三角超晶格上的 Hubbard 模型相关的孤立扁平价带微带，其中带宽可以通过垂直位移场连续调谐。通过施加这种位移场，我们在实验中发现，ABC-TLG/hBN 超晶格在四分之一和一半填充状态下分别在 20 开尔文以下和 1 开尔文以下显示出莫特绝缘状态，这对应于每个单元分别有一个和两个空穴。进一步冷却后，在四分之一填充的莫特态的电子和空穴掺杂侧，超导的特征（“穹顶”）在 1 开尔文以下出现。ABC-TLG/hBN 超晶格的电子行为预计将敏感地依赖于电子-电子相互作用和微带带宽之间的相互作用。通过改变垂直位移场，我们演示了从候选超导体到莫特绝缘体和金属相的转变。我们的研究表明，ABC-TLG/hBN 异质结构提供了有吸引力的模型系统，可以在其中探索可调谐三角 Hubbard 模型中出现的丰富相关行为。
