Six-layer graphene displays superconductivity and orbital magnetism

Cornell University

Graphene, a material made of carbon atoms arranged in an atomically thin sheet, continues to surprise scientists with its remarkable electronic properties, according to Xiaomeng Liu, assistant professor of physics in the College of Arts and Sciences. 

Liu and members of his research group have discovered that stacking six graphene layers in a rhombohedral arrangement – in which each graphene sheet is shifted slightly sideways from the one beneath it, like steps in a staircase – creates a versatile platform for exploring two unusual quantum phenomena: superconductivity and electrically controllable orbital magnetism.

These findings reveal how a seemingly simple carbon-based material can host a remarkably rich collection of quantum states, Liu said. They also establish rhombohedral multilayer graphene as a promising platform for investigating the fundamental mechanisms of superconductivity and magnetism. Potentially, it could be used to develop new ways of controlling quantum materials.

“Superconductivity and Ferroelectric Orbital Magnetism in Semimetallic Rhombohedral Hexalayer Graphene” was published in Nature Physics on Oct. 22. Co-first authors are Kavli postdoctoral associates Jinghao Deng and Hongyuan Li (a former Klarman Postdoctoral Fellow in A&S) and doctoral student Jiabin Xie. Liu is the corresponding author. 

Jie Shan, visiting professor in Cornell Duffield Engineering and Kin Fai Mak, adjunct professor in physics in A&S, also contributed, as did researchers from the National Institute for Materials Science in Tsukuba, Japan.

The Liu research group explores emergent quantum phenomena arising from the collective motion of electrons in quantum materials, specializing in van der Waals heterostructures, which are created by stacking layers of two-dimensional materials – six-layer graphene is such a material.

In this study, the researchers found that superconducting states emerge in six-layer graphene when two types of charge carriers – electrons and holes – coexist in the material, with a surprising resilience to the in-plane magnetic field.

“This unusual coexistence offers new clues about how superconductivity can arise through interactions among electrons,” Liu said.

The team also discovered a form of magnetism, arising from the electron’s orbital motion, that is closely coupled to the material's electric polarization. Remarkably, this magnetic state can be switched using an electric field, suggesting opportunities to control magnetic properties through electrical signals.

“What excites us is that a material made entirely of carbon can support such rich collective behaviors of electrons,” Liu said. “Six-layer graphene gives us a platform to investigate how superconductivity and magnetism emerge and how we can control them electrically. This advances our broader goal of understanding and controlling quantum states in materials built from atomically thin layers.”