The last decade has witnessed a dramatic increase in scientific research aimed at finding even high Tc superconductors. Most are based on conventional electron phonon driven mechanisms using machine learning and Eliasberg theory in a materials genome like search for potentially interesting materials. This search has extended to the class of unconventional yet not understood mechanisms of which the most spectacular finding is Tc of up to 140K in cuprate based materials in which the common CuO2 planes form the platform and the effective valence electron or hole concentration seems to be the most important but not exclusive variable. The inclusion of Ni based oxides with NiO2 planes in this search has led to claims of superconductivity “onsets” at elevated temperatures up to 80 K mostly at high pressures or in thin strained epitaxial films simulating an effective high pressure. The discovery of superconductivity in Sr substituted NdNiO2 epitaxial thin films by the Liu et al (Stanford group) in 2020 started a dramatic search for other potential Ni based superconductors with some success in bulk materials at high pressure and in epitaxial (strained) films. The class of materials discovered to be of interest are the Ruddlesden-Popper-phases with chemical composition of An+1BnX3n+1 B in our case is Ni and A is mostly a rare earth with some alkaline earth substitution. This material also contains NiO2 planes like the cuprates but these can occur as single double or triple NiO2 planes separated by one or more LaO planes forming various polymorphs. NdNiO2 corresponds to n approaching infinity LaNiO3 material but with the O missing in the La layers also called the infinite layer compound in which the O between the single NiO2 layers is missing. These materials in some cases also exhibit what one suggests are high temperature superconducting onsets at high pressure but also in strained thin films.
In this talk I will stress the extra theoretical complications that are present in the Nickelates relative to the cuprates aside from any additional problems regarding the stoichiometry of the samples that exhibit apparent superconductivity. We have used both density function and model Hamiltonian exact diagonalization methods to study the electronic, magnetic structures and their interaction with the local lattice distortions resulting in charge and spin density waves for some of the magnetic structures. We demonstrate that the basic starting point can best be making all the Ni 2+(d8 S=1) and then addition the additional holes which could be on mainly Ni or as we find mostly on O. We demonstrate that the exact many body cluster calculations and the DFT calculations done using hybrid functional methods HSE06 have a lot of qualitative agreement as to where the additional holes are located i.e. mainly on the in plane O 2p orbitals with a strong hybridization or exchange interaction with the dx2-y2 orbitals. This is like the basic ideas that have developed over time regarding the cuprates. The most surprising to us in this detailed study of 7 different magnetic structures is huge dependence of the electronic structure to the type of magnetic order. The main conclusion is that these materials although like the cuprates in some regards but also very different in others. For example, here we would be in the very over doped region of the cuprate phase diagram and in addition the importance of the partly occupied 3dz2-r2 orbital not at all active in the optimal doping range of the cuprates and here very prominent throughout.
If time permits, I will very briefly discuss an interesting new(?) idea concerning the mechanism for pairing interactions between electrons or holes propagating in the different planes in the La3Ni2O7 and La4Ni3O10.