In a recent study published in Nature, researchers from MIT have discovered that ordinary graphite exhibits remarkable multifaceted properties at the microscale. The study reveals that a certain microscopic structure found in natural graphite can host multiple superconducting states. Superconductivity is an electronic state where electrons pair up and glide through a material with zero resistance. Although thousands of materials are known as superconductors, it is rare for one material to exhibit multiple forms of superconductivity. The team found these multiple superconducting states in atomically thin exfoliations of graphite, known as graphene.
Specifically, graphene is a single-atom-thin sheet of carbon atoms arranged in a precise microscopic lattice. The discoveries were made in samples of rhombohedral graphene, which consists of a stack of four or five graphene layers. Interestingly, several of the new superconducting states in rhombohedral graphene persist in the presence of a magnetic field, which typically destroys superconductivity, and surprisingly, these states even strengthen under magnetic exposure. This indicates a new family of unconventional superconducting states within a seemingly simple material. Long Ju, the Associate Professor of Physics at MIT, states, "People might assume that this is a simple, boring carbon material, but we can control this material by tuning certain experimental 'knobs,' such as electrical voltages."
The exact origins of these superconducting states and their persistence in a magnetic field remain unclear. Ju notes, "From a fundamental physics perspective, it’s very exotic that a magnetic field doesn’t kill superconductivity, but instead boosts it." The research team conducted experiments by progressively removing electrons from rhombohedral graphene while measuring electrical resistance. They found that at certain electron densities, four distinct superconducting states emerged. Remarkably, three of these states persisted in magnetic fields up to about 9 tesla, which is around 180,000 times stronger than Earth’s magnetic field. When the magnetic field was applied parallel to the material’s plane, superconductivity not only persisted but increased when a perpendicular field was applied, surviving beyond the material’s critical temperature. Ju explained, "The superconductivity actually is enhanced, with the transition temperature going from 55 millikelvin to probably 90 millikelvin."
The researchers propose that in rhombohedral graphene, electrons may pair with aligned spins at certain densities, allowing them to maintain superconductivity even under magnetic influence. This intriguing idea warrants further experimental and theoretical investigation. For now, these findings showcase the potential for new and exotic phenomena to emerge in seemingly simple materials, with appropriate measurements and controls. Junseok Seo, the lead author, emphasizes, "We can control the simplest chemical and structural material—crystalline carbon—as part of the fun."