A collaborative team of scientists from Cornell University in the U.S., Japan's National Institute for Materials Science, and Germany's Max Planck Institute for the Structure and Dynamics of Matter has uncovered unusual quantum states within six-layer graphene. While graphene is known for its honeycomb carbon structure and technological potential, stacking multiple layers can yield distinct properties. The researchers focused on rhombohedral hexalayer graphene, a configuration where six layers are shifted sideways in a repeating pattern.
The discovery was somewhat accidental. Senior author Xiaomeng Liu explained that the team initially aimed to study twisted rhombohedral graphene to create isolated topological bands. However, in one device constructed by stacking two three-layer sheets, the layers relaxed into alignment, inadvertently forming the six-layer rhombohedral structure. Initial measurements on this material hinted at behaviors not previously reported in studies of five-layer graphene, prompting a deeper exploration of its phase diagram.
To investigate these properties, the team prepared flakes with the specific six-layer stacking pattern and sandwiched them between insulating layers of hexagonal boron nitride with graphite-based gates. They conducted extensive measurements by varying temperature, magnetic fields, charge carrier numbers, and electric field intensities. These tests revealed vanishing electrical resistance indicative of superconductivity, as well as an anomalous Hall response suggesting orbital magnetism driven by electron motion. Additionally, quantum oscillations allowed the team to reconstruct the Fermi surface and identify the types of charge carriers present.
The study identified several electronic states, including interaction-driven insulating states, regions exhibiting superconducting behavior, and two distinct types of orbital magnetism. Notably, two superconducting-like states emerged specifically under conditions where electron and hole carriers coexisted. Liu stated that interactions between these two carrier types appear to play a central role in forming these unusual states. The primary findings include the discovery of a new superconducting state and a new multiferroic orbital magnet within this strongly correlated semimetallic regime.
These results highlight the potential of six-layer graphene for studying superconductivity and magnetic behaviors driven by electron interactions. The researchers are now further investigating the microscopic origins of the observed multiferroic orbital magnetic state. Liu noted that ongoing work raises the possibility that some carriers form an electron crystal while others remain in a conducting, liquid-like state. Determining whether this crystal produces orbital magnetism or realizes a topological electron solid remains an open question for future research.
Sources: Phys.org.