Researchers at MIT, University of Basel, Florida State University, the University of Florida and the National Institute for Materials Science in Japan have reported a new family of unconventional superconducting states in rhombohedral multilayer graphene, revealing magnetic-field-enhanced superconductivity that persists far beyond conventional limits.
The work focuses on rhombohedral graphene - naturally occurring stacks of four or five graphene layers arranged in a staircase-like configuration within graphite. Unlike engineered twisted structures, this system combines intrinsic structural order with exceptionally flat electronic bands, enabling strong electronic correlations that can be tuned via electrostatic gating. By exfoliating graphite and isolating these rare stacking domains, the team accessed ultraclean samples suitable for probing delicate superconducting phases.
Transport measurements on tetralayer and pentalayer rhombohedral graphene revealed multiple distinct superconducting states emerging as a function of carrier density. In pentalayer devices, the researchers identified three types of field-enhanced and field-induced superconductivity, alongside additional superconducting phases stabilized by proximitized spin-orbit coupling (SOC). Notably, four superconducting states were observed overall at specific electron densities when carriers were removed from the system.
A defining feature of these states is their robustness - and even enhancement - under applied magnetic fields. Three superconducting phases persist under in-plane magnetic fields up to approximately 8.5-9 tesla, exceeding the conventional Pauli limit by tens of times. This is in stark contrast to standard Bardeen-Cooper-Schrieffer superconductors, where magnetic fields typically suppress superconductivity by breaking Cooper pairs.
Even more strikingly, the team observed superconductivity that strengthens under a perpendicular magnetic field. At certain carrier densities, the superconducting transition temperature increases from about 55 millikelvin to roughly 90 millikelvin under field, while the critical current rises by 50-60%. This indicates a genuine field-enhanced superconducting state rather than simple resilience.
Compared to Bernal-stacked bilayer graphene - where only in-plane field enhancement has been reported - rhombohedral pentalayer graphene supports superconductivity enhanced by both in-plane and out-of-plane magnetic fields. These phases also occur at relatively low gate electric fields, owing to the flatter band dispersion inherent to rhombohedral stacking, which simplifies experimental access and device integration.
While the microscopic origin of these states remains under investigation, one proposed mechanism involves unconventional pairing. In contrast to standard spin-singlet Cooper pairs, which are vulnerable to magnetic fields, the data are consistent with spin-aligned pairing configurations that can survive - or even benefit from - applied fields. Additionally, the introduction of spin-orbit coupling generates new superconducting phases without degrading material quality, suggesting a pathway toward engineered topological superconductivity.
“From a fundamental physics point of view, it’s very exotic that a magnetic field doesn’t kill superconductivity, and instead it boosts it,” said MIT's Long Ju. “We have provided a lot of experimental results and provided the nutrition that people can absorb to try to think about what’s going on here.”
Overall, the results establish rhombohedral graphene as a highly tunable platform for studying clean-limit, field-enhanced superconductivity. With accessible gate control and compatibility with interfacial engineering, the system offers a promising route toward realizing topological superconducting states and non-Abelian quasiparticles in a crystalline carbon platform.
