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Title: Spin state of Cooper pairs in magic-angle twisted trilayer graphene
Plain: 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: 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.
Settled by: Spin-resolved tunnelling or a measurement of the spin susceptibility below $T_c$, or Josephson coupling to a known spin-singlet superconductor.
Kind: well-posed
Answer type: classification
References: (empty)
Literature status: open
Status note: (empty)
Posed since: 2021