  Following the discovery of superconductivity in the cuprates and the seminal
work by Anderson, the theoretical efforts to understand high-temperature
superconductivity have been focusing to a large extent on a simple model: the
one-band Hubbard model. However, superconducting cuprates need to be doped, and
the doped holes go into the oxygen orbitals. This requires a more elaborate
multi-band model such as the three-orbital Emery model. The recently discovered
nickelate superconductors appear, at first glance, to be even more complicated
multi-orbital systems. Here, we analyse this multi-orbital system and find that
it is instead the nickelates which can be described by a one-band Hubbard
model, albeit with an additional electron reservoir and only around the
superconducting regime. Our calculations of the critical temperature Tc are in
good agreement with experiment, and show that optimal doping is slightly below
the 20% Sr-doping of Ref. 11. Even more promising than 3d nickelates are 4d
palladates.
