Nickelate superconductors: infinite-layer and bilayer families

5 problems

• Nature of the density-wave order in ambient-pressure $\mathrm{La_3Ni_2O_7}$

  Contested

  At normal pressure $\mathrm{La_3Ni_2O_7}$ does not superconduct but forms a density wave, a periodic pattern of spins and possibly charges, near $150\ \mathrm{K}$; its form and its link to superconductivity are uncertain.

• Gap symmetry and pairing mechanism of bilayer $\mathrm{La_3Ni_2O_7}$

  Contested

  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.

• Which $\mathrm{La_3Ni_2O_7}$ polymorph carries bulk superconductivity under pressure?

  Partially resolved

  Early pressure experiments found only a small superconducting volume, and $\mathrm{La_3Ni_2O_7}$ crystals contain two stacking patterns of nickel-oxide layers, so which structure superconducts in bulk was unclear.

• Why infinite-layer nickelates superconduct only as thin films

  Contested

  Infinite-layer nickelates superconduct when made as thin films but not, so far, as bulk crystals made by the same chemical reduction step.

• How high can $T_{c}$ reach in ambient-pressure nickelate films?

  Open

  Squeezing bilayer nickelate films by growing them on a substrate with smaller atomic spacing mimics high pressure; the question is whether this can reach the 80 K seen under pressure.