Gap symmetry and pairing mechanism of bilayer $\mathrm{La_3Ni_2O_7}$
In plain words
In $\mathrm{La_3Ni_2O_7}$ electrons in two nickel orbitals take part, and it is unknown whether pairs form mainly between the two layers of each bilayer or within each layer as in cuprates.
Precise statement
For bilayer $\mathrm{La_3Ni_2O_7}$ (hydrostatic pressure above $\sim 140\,\mathrm{kbar}$, or compressively strained films), determine the gap symmetry (interlayer $s\pm$ between bonding and antibonding d_z2-derived bands, or d-wave dominated by d_x2-y2) and the dominant pairing interaction (interlayer superexchange of d_z2 orbitals, Hund-coupled spin fluctuations). An answer is a momentum-resolved or phase-sensitive gap measurement plus a model reproducing $T_c \sim 80\,\mathrm{K}$.
What would settle it
ARPES or quasiparticle interference on superconducting films resolving gap magnitudes and signs on each Fermi sheet, matched by a model calculation of $T_{\mathrm{c}}$.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Gap-symmetry measurements reported in 2025, including a d-wave claim under pressure, have not converged.
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