Cuprates and Nickelates and More Nickelates, Oh My

The discovery of cuprate superconductivity in 1986 led soon afterwards to speculations of whether high temperature superconductivity existed in other transition metal oxides, in particular low valence (d⁹) nickelates. In 2019, this was discovered in the infinite-layer nickelate Sr-doped LaNiO₂. But four years later, a surprising discovery was made of high temperature superconductivity under pressure in two layered (d⁷) nickelates, La₃Ni₂O₇ and La₄Ni₃O₁₀, followed by a subsequent discovery at ambient pressure in strained films. In this talk, I will review what we know about these two classes of nickelates and their similarities and differences with cuprates. A focus will be on the relation of superconductivity to the charge and spin density waves found in these materials, as well as the contrasting roles of dx²-y² and dz² nickel orbitals.