Spin state of Cooper pairs in magic-angle twisted trilayer graphene
In plain words
In magic-angle trilayer graphene, superconductivity survives in-plane magnetic fields two to three times larger than the field that breaks pairs of electrons with opposite spins, suggesting pairs with parallel spins (spin triplet).
Precise statement
Cao et al. (arXiv:2103.12083, 2021) found in magic-angle twisted trilayer graphene an in-plane critical field exceeding the Pauli limit $B_{P} \sim 18.6\ \mathrm{kG}$ per kelvin of $T_{c}$ by a factor 2 to 3, and re-entrant superconductivity at higher in-plane fields. Determine whether the pairs are spin-triplet, a singlet-triplet mixture allowed by mirror-symmetry breaking under displacement field, or singlets protected by spin-orbit or orbital effects. An answer gives the spin structure of the order parameter with direct evidence.
What would settle it
Spin-resolved tunnelling or a measurement of the spin susceptibility below $T_c$, or Josephson coupling to a known spin-singlet superconductor.