The discovery of superconductivity in nickelates by Harold Hwang and coworkers in 2019 [1] marked the beginning of a new age of superconductivity, the nickel age. These novel superconductors are on the one hand extremely similar to the cuprates but on the other hand also distinctly different. Hence, there is a great hope that we are now better able to distinguish the essentials from the incidentals for high-temperature superconductivity and thus eventually solve the arguably biggest quest of solid state physics: what is the microscopic mechanism for high-temperature superconductivity?

The novel (Sr-doped) NdNiO$_2$ superconductors are not only isostructural to the well known cuprate superconductor CaCuO$_2$ but also both, Ni and Cu, are formally $3d^9$ in the respective parent compound. At second glance, the nickelates appear to be more complicated than their cuprate peers. Besides the Ni $d_{x^2 -y^2}$ band that crosses the Fermi level, there are additional pockets around the $A$ momentum and at low doping possibly the $\Gamma$ momentum. However, we and others [2,3] have argued that the $A$ pocket is merely a passive bystander, an electron reservoir while the actual physics is governed by the Ni $d_{x^2 -y^2}$ band. With the thus derived one-band Hubbard model we were able to predict the superconducting phase diagram in nickelates [3] prior to experiment [4] to good accuracy. This gives us some hope that we are on a good way toward a more thorough understanding. Other groups [5,6] emphasize the multi-orbital character and experimentally even fundamental studies such as angular resolved photoemission spectroscopy are still pending. There is a lot to be done both on the experimental and theoretical side.



[1] D. Li et al., Nature 572, 624 (2019).
[2] J. Karp et al., Phys. Rev. X 10, 021061 (2020).
[3] M. Kitatani et al., npj Quantum Materials 5, 59 (2020)
[4] D. Li et al., Phys. Rev. Lett. 125, 027001 (2020).
[5] F. Lechermann, Phys. Rev. B101, 081110 (2020).
[6] V. Petocchi et al., Phys. Rev. X 10, 041047 (2020).