Using ab initio band structure and 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 ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ (LNO) under pressure. Our results show the emergence of a double spin-charge-density stripe state characterized by a wave vector $\mathbf{q}=(\frac{1}{4},\frac{1}{4})$ arrangement (diagonal hole stripes oriented at ${45}^{\circ }$ to the Ni-O bond) of the nominally high-spin ${\mathrm{Ni}}_A^{2+}$ and low-spin ${\mathrm{Ni}}_B^{3+}$ (with a predominant $3d^8\underline{L}$ configuration with a hole localized in the O $2p$ band) ions 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. Our results indicate a strong orbital dependence of correlation effects in LNO. We obtain a narrow-gap correlated insulator with a band gap value of $\sim 0.2\mathrm{eV}$ characterized by strong localization of the Ni $3d$ states and significant spin-orbital polarizations of the charge deficient ${\mathrm{Ni}}_B^{3+}$ ions. We note the importance of double exchange to determine the magnetic properties of LNO, similarly to that in charge-ordered manganites and nickelates. We propose that spin and charge stripe fluctuations play an important role in tuning superconductivity in LNO under pressure.

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• Charge density waves
• First-principles calculations
• Quantum phase transitions
• Spin density waves
• Strongly correlated systems
• Unconventional superconductors
• Band structure methods
• Dynamical mean field theory
• Electron-correlation calculations