Featured Research

Ferromagnetism with a New Twist

Posted: June 22, 2019

Localized orbitals in the superlattice of twisted graphene bilayers near the magic angle. Strong interactions in those orbitals lead to a novel Mott state with ferromagnetic correlations.

In a new study published in Physical Review Letters, OU postdoc Kangjun Seo and physics professor Bruno Uchoa have found numerical evidence of a novel Mott state that exhibits ferromagnetism at low temperatures. Recent experiments found that when two sheets of graphene are twisted by a very small angle, dubbed a 'magic angle', the combined system behaves as a very strongly correlated system in ways that remind several of the outstanding properties observed in high temperature superconductors. The new study shows that the Mott state in twisted graphene bilayers departs from other known examples in fundamental ways. 

Mott physics has been extensively investigated in the last decades in high-temperature cuprate superconductors, systems that can carry charge currents at relatively high temperature without any dissipation. In the Mott phase, the motion of charge carriers is confined by strong repulsive interactions. That leads instead to insulating behavior and anti-ferromagnetism — antiparallel spin alignment —  as a result of the Pauli exclusion principle, which states that two electrons cannot occupy the same quantum state. The new work shows that symmetry restrictions imposed by the lattice of twisted graphene bilayers can strongly favor ferromagnetism, even in the presence of strong repulsive interactions.

This new phenomenon is unheard of in conventional Mott insulators and sheds a conceptual light on the nature of the novel insulating state observed experimentally in twisted graphene bilayers. The new work may also provide profound insights about the spin symmetry in the superconducting phase recently discovered in the metallic regime of this system.

See also: https://eurekalert.org/pub_releases/2019-06/uoo-ops061919.php