Researchers at MIT have found that ordinary graphite, commonly used in pencil lead, can exhibit complex properties at the microscale. A study published in Nature reveals that a specific microscopic structure within natural graphite can support several superconducting states. Superconductivity allows electrons to pair and move through a material without resistance. While many superconductors exist, it’s unusual for a single material to exhibit multiple superconducting states.<\/p>\n
The team identified these states in graphene, specifically in rhombohedral graphene, which consists of four or five layers stacked in a precise arrangement. Notably, some of the discovered states remain stable in a magnetic field, which typically disrupts superconductivity, and even become stronger under such conditions. This discovery introduces a new class of unconventional superconducting states in a seemingly simple material.<\/p>\n
“People might think of this as a simple, boring carbon material,” said Long Ju, the Lawrence C. and Sarah W. Biedenharn Associate Professor of Physics at MIT. “But we can manipulate this material by adjusting experimental ‘knobs,’ such as electrical voltages, revealing diverse superconducting properties.” The exact mechanisms behind these states and their persistence in magnetic fields remain unclear. Ju added, “It’s very exotic that a magnetic field doesn’t kill superconductivity, and instead it boosts it.”<\/p>\n
The study involved MIT co-authors including Junseok Seo, Shenyong Ye, and others, alongside external collaborators such as Armel Cotten and Dominik Zumbuhl\u2019s group from the University of Basel, as well as partners from Florida State University, the University of Florida, and Japan’s National Institute for Materials Science.<\/p>\n
Graphene and other atomically thin materials can display unexpected electronic and physical properties. Ju\u2019s group has explored graphene’s capabilities, focusing on naturally occurring structures like rhombohedral graphene. This configuration, comprising stacked layers offset like a staircase, has shown surprising electronic characteristics.<\/p>\n
To study rhombohedral graphene, researchers exfoliated graphite to find this specific pattern. They isolated samples to examine its properties, discovering rare forms of superconductivity and fractional electron charge. In their latest study, the team investigated the effects of removing electrons from rhombohedral graphene.<\/p>\n
By adjusting electron density and applying external currents and magnetic fields, the team identified four distinct superconducting states. Three of these states persisted under a high magnetic field, which typically disrupts superconductivity by breaking electron pairs. However, in Ju’s experiments, three states survived in a magnetic field up to about 9 tesla.<\/p>\n
When the magnetic field was perpendicular to the graphene plane, they observed an increase in superconductivity, allowing the material to function at higher temperatures than expected. At specific electron densities, rhombohedral graphene’s superconductivity exceeded its critical temperature in a zero magnetic field. Ju noted, “The transition temperature goes from 55 millikelvin to probably 90 millikelvin,” with the material handling more current before losing superconductivity, which is unusual.<\/p>\n
The researchers suggest that instead of the usual pairing of electrons with opposite spins, in rhombohedral graphene, electrons may pair with aligned spins, allowing the magnetic field to maintain their alignment and preserve superconductivity.<\/p>\n