A nickelate finally gets its gap measured, and it does not look like a cuprate

The useful part of this work is which question it closes, and how. Donglai Feng, one of the co-senior authors, describes the problem as a change in the number of orbitals you have to keep track of: cuprates are largely a single Cu 3dx2-y2 band at a d9 configuration, while bilayer nickelates sit at d7.5 with both Ni 3dx2-y2 and 3dz2 close to the Fermi level, which is what has kept the pairing mechanism under theoretical and experimental dispute ^[6]. Co-senior author Yuefeng Nie names the two things nobody had pinned down: whether the 3dz2-dominated gamma band actually crosses the Fermi level and what it does there, and what the pairing symmetry of the gap is ^[7].

Neither question could be answered by transport measurements, and the tool that answers them was locked out by the sample environment. ARPES reads the energy and momentum of electrons kicked out of a surface, so it needs a pristine one, and it does not go inside the pressure cells that were originally required to make bulk nickelate crystals superconduct at all ^[8]. That is the reason the field had bulk superconductivity near 80 K under very high pressure ^[5] and no momentum-resolved look at the superconducting state to go with it ^[7].

The workaround was not a better pressure cell. Co-senior author Dawei Shen says compressively strained thin films had recently been shown to superconduct at ambient pressure, which opened the door, but that growing films with high phase purity, uniform Tc, and atomically flat clean surfaces at the same time was exceptionally difficult ^[9]. What the Nanjing group reports doing about it is unglamorous and specific: many optimization cycles on oxide molecular-beam epitaxy plus in-situ ozone annealing to stabilize strontium-doped films flat enough for a surface-sensitive probe ^[3], measured on the micro-ARPES BL-03U beamline at the Shanghai Synchrotron Radiation Facility that Shen's group built ^[2].

Read the result conservatively. The gap that opens when Cooper pairs form is a forbidden energy range ^[10], and the account of the paper says the films' gap is consistent with a pairing mechanism distinct from the d-wave case in cuprates ^[1]. The single temperature quoted anywhere in it is the 80 K bulk high-pressure figure; no Tc for the films and no gap magnitude appear, and the pairing statement is framed as a difference from cuprates rather than as a named symmetry ^[11]. So the load-bearing claim is a ruling-out, obtained on samples that only a handful of growth chambers can currently make.

That is still the part that redirects work. If ambient-pressure strained films can be grown clean enough to survive ARPES ^[9][3], then the scarce resource in nickelate research stops being high-pressure hardware and becomes epitaxy plus beamtime, and the theories that borrowed their gap structure from cuprates now have a measurement to argue with.