Role of correlations in Ruddlesden-Popper bilayer nickelates under compressive strain

The recent discovery of superconductivity in thin films of the bilayer Ruddlesden-Popper (RP) nickelate La$_3$Ni$_2$O$_7$ (La327) under compressive strain has generated enormous interest, opening up further opportunities to stabilize superconductivity in this class of materials at ambient pressure. To better understand the many-body normal state from which superconductivity arises, it is important to ascertain the nature and role of correlations in its electronic structure. To provide insights into this question, we use a fully charge self-consistent DFT+e-DMFT (eDMFT) approach to study La327 at several compressive strain levels. At the strain level where superconductivity has been observed experimentally (-2\%), in contrast with DFT and DFT+$U$ results, the so-called $\gamma$ pocket emerges and the associated band, of mostly $d_{z^2}$ character, crosses the Fermi level exhibiting `flat band''-like features when dynamical correlations are included. Larger strain levels suppress the $\gamma$ pocket, which may have implications for superconductivity or its pairing symmetry.