MIT Discovers Multiple Superconducting States in Rhombohedral Graphene

Key Findings

MIT researchers have identified multiple superconducting states in rhombohedral graphene, a natural microscopic structure found in graphite. The findings, published in Nature, reveal four distinct superconducting states—three of which persist in magnetic fields up to approximately 9 tesla, or about 180,000 times Earth's magnetic field.

One superconducting state actually strengthens in a perpendicular magnetic field, with its critical temperature rising from 55 millikelvin to about 90 millikelvin. The material can also sustain 50–60% more electrical current before losing superconductivity.

Understanding the Material

Graphene is a single-atom-thick sheet of carbon atoms arranged in a lattice. Rhombohedral graphene consists of four to five graphene layers stacked in a staircase-like pattern. This structure occurs naturally in graphite and can be isolated via exfoliation—a process of peeling away layers.

"People might assume this is a simple, boring carbon material. But we can control this material by tuning experimental 'knobs,' such as electrical voltages." — Long Ju, MIT associate professor

Unusual Superconducting Behavior

A surprising aspect of this discovery is that superconductivity was observed when electrons were removed from the material (hole doping) at specific electron densities. This contrasts with earlier experiments that added electrons.

The persistence of superconductivity in magnetic fields is particularly unusual. In conventional superconductivity, magnetic fields destroy the effect by breaking apart Cooper pairs—electrons with opposite spins. A proposed explanation is that electrons in this material may form Cooper pairs with aligned spins, which remain stable under magnetic fields.

Broader Implications

Lead author Junseok Seo, an MIT graduate student, emphasized the unique control researchers have over this material:

"We can control the simplest of chemicals—carbon—and structurally alter the material... We're not only dealing with what nature gives us, but we're applying additional controls to change it to something that nature does not give us, but that can exist in the same material."

Previous work by the same group had already discovered chiral superconductivity and fractional electron charge in similar structures.

Research Collaboration & Support

The study involved collaboration with Dominik Zumbuhl's group at the University of Basel, Switzerland, and other institutions. Device fabrication was conducted at MIT.nano, and the work was supported by the U.S. Office of Naval Research.