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Superconductivity is a widely sought after material property, which entails an electrical resistance of zero below a specific critical temperature. So far, it has been observed in various materials, including recently in so-called multilayer graphene allotropes (i.e., materials that consist of several layers of a hexagonal carbon lattice).

Recent studies found that when bilayer graphene is placed on a WSe₂ (tungsten-diselenide) substrate, its superconducting phase is enhanced. This results in a greater charge carrier density and higher critical temperature (i.e., the temperature at which a material becomes a superconductor).

Researchers at University of California at Santa Barbara and California Institute of Technology have carried out a study aimed at further investigating this enhancement in the graphite allotrope Bernal bilayer graphene. Their paper, in Nature Â鶹ÒùÔºics, reports the observation of two distinct superconducting states in this material, challenging current models of electron pairing in graphite allotropes.

"Prior to this work, we had observed superconductivity in bilayer graphene without WSe₂ and our collaborators from Caltech, Prof. Stevan Nadj-Perge and Yiran Zhang, a graduate student at the time, had told us about their recent results of higher critical temperature when bilayer graphene is proximitized with WSe₂," Ludwig Holleis, first author of the paper, told Â鶹ÒùÔº. "We started looking into this newly found superconductivity and the enhancement of critical temperatures and magnetic fields."

The main goal of the recent study carried out by Holleis and his colleagues was to better understand the enhancement of the critical temperatures and magnetic fields previously reported in bilayer graphene in proximity to WSe₂, as well as the ground state that it emerges from. To do this, they examined the same superconductor that was found to exhibit the highest critical temperature during a previous study carried out at Caltech.

"We found the same superconductor again in the sample we measured and also observed the second superconductor which has a much smaller critical temperature," explained Holleis. "In principle, observing the superconductor is the easy part as we just performed resistance measurements. Understanding its properties then is harder.

"To do this, we performed high resolution quantum oscillation measurements, which measure the Fermi surface of the electrons—in simple terms, the states in momentum space the electron can live on."

Interestingly, the researchers found that the measurements they collected were not compatible with the rotational symmetry of the crystal they examined. Instead, they observed a preferential direction, which is known as nematicity.

"Nematicity has been found in other superconducting materials such as iron superconductors, and it could play an important role for superconductivity here as well," said Holleis. "With the second main result, the limit of in-plane critical fields by orbital depairing, we tried to understand some more mysterious data.

"Basically, the in-plane critical magnetic field is generically set by either the Pauli limit, or by the Ising spin-orbit coupling, as should be the case here. Neither of these seemed to fit any of the experimental data."

After discussing their measurements with theoretical physicist Prof. Erez Berg at Weizman Institute and his student Yaar Vituri, Holleis and his colleagues proposed a new depairing mechanism for superconductivity of in-plane orbital moments. Their work could soon inspire new studies exploring the distinct superconducting phases they observed, while also helping to constrain theories predicting the pairing mechanisms in graphite allotropes.

"We have already submitted on superconductivity on trilayer graphene with WSe₂, led by two other graduate students in our lab, Cailtin Patterson and Owen Sheekey," added Holleis. "More generally, understanding these (now many) superconductors in multilayer graphene is difficult, and currently we are working on new experimental techniques to extract their secrets."

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