The s$^\pm$-Wave Pairing and the Destructive Role of Apical-Oxygen Deficiencies in La$_3$Ni$_2$O$_7$ Under Pressure

Abstract

Recently, the bilayer perovskite nickelate La$_3$Ni$_2$O$_7$ has been reported to show evidence of high-temperature superconductivity (SC) under a moderate pressure of about 14 GPa. To investigate the superconducting mechanism, pairing symmetry, and the role of apical-oxygen deficiencies in this material, we perform a random-phase-approximation based study on a bilayer model consisting of the $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals of Ni atoms in both the pristine crystal and the crystal with apical-oxygen deficiencies. Our analysis reveals an $s^{\pm}$-wave pairing symmetry driven by spin fluctuations. The crucial role of pressure lies in that it induces the emergence of the $\gamma$-pocket, which is involved in the strongest Fermi-surface nesting. We further found the emergence of local moments in the vicinity of apical-oxygen deficiencies, which significantly suppresses the $T_c$. Therefore, it is possible to significantly enhance the $T_c$ by eliminating oxygen deficiencies during the synthesis of the samples.