In traditional Bardeen-Cooper-Schrieffer (BCS) superconductors, magnetic fields destroy Cooper pairs, which are the entities that carry dissipationless current.
Rhombohedral graphene, with its crystallographically pure lattice and gate-tunable electron interactions, has become a verified model system for studying these exotic phases.
A rare electronic state emerges in five-layer grapheneResearchers discovered that some of the new superconducting states in rhombohedral Graphene can survive even when a magnetic field is applied, a situation that usually destroys superconductivity.
In their latest study, the team tried something new: instead of adding electrons to rhombohedral Graphene, they carefully removed them.
To their surprise, this revealed four different superconducting states.
Recently, condensed matter physicists have been pushing the boundaries of the Golden Rule restrictions on electron pairing. MIT Researchers have now reported a striking discovery in rhombohedral multilayer graphene: a new family of superconductors that thrive not in spite of magnetic fields, but because of them.
Superconductivity, as a phase, is inherently delicate, easily disrupted by impurities or external magnetic fields. In traditional Bardeen-Cooper-Schrieffer (BCS) superconductors, magnetic fields destroy Cooper pairs, which are the entities that carry dissipationless current. However, time-reversal symmetry can be broken with gauge one in some non-conventional superconductors where magnetic-field-enhanced superconductivity emerges.
The results are surprising since they seem to defy current understanding, where it is expected that magnetic fields suppress the superconducting state; whereas these materials actually show strengthening of the superconducting state in high magnetic fields, a behavior theorized long ago but rarely seen in low-disorder materials. Rhombohedral graphene, with its crystallographically pure lattice and gate-tunable electron interactions, has become a verified model system for studying these exotic phases.
Using transport measurements on rhombohedral tetralayer and pentalayer graphene, researchers uncovered a spectrum of superconducting behaviors in the clean limit. The pentalayer, in particular, revealed three distinct types of field-enhanced and field-induced superconductivity in atomically thin exfoliated graphite, known as Graphene. The team made its discoveries in samples of rhombohedral Graphene, a natural structure in graphite consisting of a stack of four or five graphene layers.
A rare electronic state emerges in five-layer graphene
Researchers discovered that some of the new superconducting states in rhombohedral Graphene can survive even when a magnetic field is applied, a situation that usually destroys superconductivity. Even more surprising, these states don’t just endure the magnetic field; they actually become stronger because of it. Taken together, the results reveal an entirely new class of unconventional superconductors hidden within what appears to be a simple material.
Long Ju, the Lawrence C. and Sarah W. Biedenharn Associate Professor of Physics at MIT, said, “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. This is how a simple physical material can exhibit so many different superconducting properties.”
Contrary to common understanding, rhombohedral graphene is not an artificial laboratory construct but rather arises naturally from bulk graphite. Identification is your main struggle. The researchers start with mechanical exfoliation of graphite, often from adhesive tape, and then typically systematically search the resulting flakes for their distinctive stacked ‘staircase’ appearance, moving gradually away from the rhombohedral phase. As soon as such rare domains are identified, they can be isolated for in-depth characterization.
Using this method, the researchers isolated and studied four- and five-layer rhombohedral graphene. They found that this particular layered structure must support exotic superconductivity, specifically an extremely rare chiral superconducting state, and behaviors such as fractional electron charge. They show that conventional graphite hides an extremely rich arena for studying highly correlated electronic states in contemporary physics.
In their latest study, the team tried something new: instead of adding electrons to rhombohedral Graphene, they carefully removed them. To their surprise, this revealed four different superconducting states. Even more remarkable, three of these states survived in strong magnetic fields up to 9 tesla, far beyond the field strength that usually destroys superconductivity.
When the magnetic field was applied perpendicular to the graphene layers, the material did something extraordinary: superconductivity not only persisted but became stronger, raising its critical temperature and carrying more current than before.
Although conventional superconductors, in coplanar structures, have electrons that pair with opposite spins and thus resist attempts to be disrupted by a magnetic field, it appears graphene’s electrons may sometimes stack up in planes with opposite-aligned spins. The exact mechanism remains somewhat hazy, but the experiments show that a carbon lattice can form an unconventional superconducting state.
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