Researchers find rhombohedral graphene can host multiple magnetic-field-boosted states

Researchers from the Massachusetts Institute of Technology, University of Basel, Florida State University, the University of Florida, and the National Institute for Materials Science in Japan have identified a new class of unconventional superconducting states in rhombohedral multilayer graphene. These states exhibit magnetic-field-enhanced superconductivity that surpasses conventional limits.

The study centers on rhombohedral graphene, which consists of naturally occurring stacks of four or five graphene layers arranged in a staircase-like pattern within graphite. This configuration, unlike engineered twisted structures, combines intrinsic structural order with flat electronic bands, facilitating strong electronic correlations adjustable through electrostatic gating. By exfoliating graphite and isolating these rare stacking domains, the researchers obtained ultraclean samples ideal for examining delicate superconducting phases.

Transport measurements on tetralayer and pentalayer rhombohedral graphene revealed multiple distinct superconducting states as a function of carrier density. In pentalayer devices, the team identified three types of field-enhanced and field-induced superconductivity, along with additional phases stabilized by proximitized spin-orbit coupling. Notably, four superconducting states were observed at specific electron densities when carriers were removed from the system.

A key characteristic of these states is their robustness and enhancement under applied magnetic fields. Three superconducting phases persist under in-plane magnetic fields up to approximately 8.5-9 tesla, far exceeding the conventional Pauli limit. This contrasts with standard Bardeen-Cooper-Schrieffer superconductors, where magnetic fields typically disrupt superconductivity by breaking Cooper pairs.

Remarkably, the team observed superconductivity that strengthens under a perpendicular magnetic field. At certain carrier densities, the superconducting transition temperature increased from about 55 millikelvin to roughly 90 millikelvin under field, while the critical current rose by 50-60%. This indicates a genuine field-enhanced superconducting state rather than mere 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 occur at relatively low gate electric fields, due to the flatter band dispersion inherent to rhombohedral stacking, simplifying experimental access and device integration.

While the microscopic origin of these states is still under investigation, one proposed mechanism involves unconventional pairing. Unlike standard spin-singlet Cooper pairs, which are susceptible to magnetic fields, the data suggest spin-aligned pairing configurations that can withstand or even benefit from applied fields. Additionally, the introduction of spin-orbit coupling generates new superconducting phases without degrading material quality, indicating a potential 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 Long Ju of MIT. "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 findings position 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.