Researchers at Cornell University, the National Institute for Materials Science (Japan) and the Max Planck Institute for the Structure and Dynamics of Matter have found new superconducting and magnetic states in rhombohedral hexalayer graphene, a stack of six graphene layers arranged in a staggered, repeating pattern.
It includes correlated insulators, several pockets of superconductivity and two distinct multiferroic orbital magnetic orders.
One of the magnetic states is a ferroelectric orbital magnet.
It switches sharply under unipolar electric fields, which signals spontaneous electric polarization, and it shows magnetic hysteresis that the electric field can reverse.
Rhombohedral graphene was also the subject of an earlier report on superconductivity and magnetism emerging in the same material.
Researchers at Cornell University, the National Institute for Materials Science (Japan) and the Max Planck Institute for the Structure and Dynamics of Matter have found new superconducting and magnetic states in rhombohedral hexalayer graphene, a stack of six graphene layers arranged in a staggered, repeating pattern.
Rhombohedral multilayer graphene has become a platform for exploring correlated and topological quantum phases, thanks to flat electronic bands that carry Berry curvature. Earlier studies mostly looked at the regime where the conduction and valence bands are separated. The new work explores the semimetallic regime of the six-layer material, in which the two bands overlap and electrons and holes coexist.
The team mapped a rich phase diagram in this regime. It includes correlated insulators, several pockets of superconductivity and two distinct multiferroic orbital magnetic orders. Two of the superconducting-like states appear only in regions where electron and hole Fermi surfaces coexist, which suggests that both types of carriers play a part in forming them.
One of the magnetic states is a ferroelectric orbital magnet. It switches sharply under unipolar electric fields, which signals spontaneous electric polarization, and it shows magnetic hysteresis that the electric field can reverse. The researchers say this differs from multiferroicity reported earlier near zero electric field, and that their results are consistent with a model of coupled electric and magnetic polarization.
The team says it is now investigating how electrons and their interactions give rise to the multiferroic orbital magnetic state on a microscopic scale, including whether some carriers form an electron crystal while others remain in a liquid-like conducting state.
Rhombohedral graphene was also the subject of an earlier report on superconductivity and magnetism emerging in the same material.
