Physicists in Associate Professor Long Ju's laboratory at MIT have discovered that rhombohedral graphene, a natural stacking arrangement of four or five carbon layers found in ordinary graphite, can host several distinct superconducting states. The study, published in Nature with collaborators at the University of Basel, Florida State University, the University of Florida and Japan's National Institute for Materials Science, departs from earlier work by progressively removing electrons from the material rather than adding them.
Measuring electrical resistance under magnetic fields applied both parallel and perpendicular to the graphene plane, the team identified four superconducting states at particular electron densities. Three survived a parallel field of about 9 tesla, roughly 180,000 times the strength of Earth's magnetic field. Most strikingly, one state actually strengthened in a perpendicular field: its transition temperature rose from about 55 to 90 millikelvin, and it tolerated 50 to 60 percent more current before superconductivity collapsed.
Magnetic fields ordinarily destroy superconductivity by breaking apart the paired electrons that carry lossless current, so field-hardened states challenge conventional theory. The researchers propose that electrons in this system may pair with their spins aligned, though the mechanism remains unconfirmed. The findings mark out a simple carbon material as a playground for unconventional superconductivity, with implications for how robust quantum materials might eventually be engineered.