Electronic correlations and spin-charge-density stripes in double-layer La$_3$Ni$_2$O$_7

Using \emph{ab initio} band structure and DFT+dynamical mean-field theory methods we examine the effects of electron-electron interactions on the electronic structure, magnetic state, and structural phase stability of the recently discovered double-layer perovskite superconductor La$_3$Ni$_2$O$_7$ (LNO) under pressure. Our results show the emergence of a double spin-charge-density stripe state characterized by a wave vector ${\bf q}=(\frac{1}{4},\frac{1}{4})$ arrangement of the nominally high-spin Ni$^{2+}_\mathrm{A}$ and low-spin Ni$^{3+}_\mathrm{B}$ ions (diagonal hole stripes oriented at $45^\circ$ to the Ni-O bond) which form zigzag ferromagnetic chains alternating in the $ab$ plane. The phase transition is accompanied by cooperative breathing-mode distortions of the lattice structure and leads to a reconstruction of the low-energy electronic structure and magnetic properties of LNO. We obtain a narrow-gap correlated insulator with a band gap value of $\sim$0.2 eV characterized by strong localization of the Ni $3d$ states and significant spin-orbital polarizations of the charge deficient Ni$^{3+}_\mathrm{B}$ ions. Our results suggest the importance of double exchange to determine the magnetic properties of LNO, similarly to that in charge-ordered manganites. We propose that spin and charge stripe fluctuations play an important role to tune superconductivity in LNO under pressure.