Possible Enhancement of Superconductivity in Ambient-Pressure La$_3$Ni$_2$O$_7$ Thin Film

Abstract

As an unconventional superconducting system capable of reaching 40K under ambient pressure, the La$_3$Ni$_2$O$_7$ film superconductor has become a recent focal point in the field of superconductivity, demanding further theoretical exploration of possible pairing mechanisms. In this work, we employ the fluctuation exchange (FLEX) approximation to systematically analyze the superconducting properties of the two-orbital, two-site model for the La$_3$Ni$_2$O$_7$ film in the weakly correlated regime, focusing on its dependence on hole-doped concentration. Through a more detailed examination of the Fermi surface topology, we find that when a $δ$ pocket composed of the $d_{z^{2}}$ orbital emerges near the $Γ$-point, its nesting with the $γ$-pocket, along with the nesting between the $α$- and $β$-pockets, leads to a mutual enhancement of $s_{\pm}$-wave pairing at the corresponding wave vector. Furthermore, we propose that this nesting-driven enhancement of spin-fluctuation-induced pairing may be a viable mechanism to enhance superconductivity.

Figures (4)

• Figure 1: (a), The schematic crystal structure of half-UC thin film of La$_3$Ni$_2$O$_7$ using the lattice parameters of La$_{2.85}$Pr$_{0.15}$Ni$_2$O$_7$ grown on SrLaALO$_4$ substrate Zhou_2024_AmbientSC, where nickel atoms form octahedral coordination with surrounding oxygen atoms. For a better illustration, we put a exaggeratedly small vacuum than the one used in calculation above the thin film. (b), The band structure of the half-UC system. The orange lines show the DFT bands, green lines the TB bands. We also show the projected bands of the $e_g$ orbitals, with the red (blue) dots the $d_{x^2-y^2}$ ($d_{z^2}$) orbital.
• Figure 2: The Fermi surface of $n=1.3$ (a) and $n=1.42$ (b). The spin susceptibility of $n=1.3$ (c) and $n=1.42$ (d). The four Fermi pocket named as $\delta,\alpha,\beta,\gamma$ are labeled in (b). The nesting wave vectors are marked in the corresponding figures.
• Figure 3: (a) The $\lambda$ as a function of electron filling $n$ at $U=1.5$, with the red line corresponding to $d_{xy}$-wave symmetry and the black line the $s$-wave symmetry. (b)-(c) The gap function of the linearized Eliashberg equation within $s$- and $d_{xy}$-wave symmetry, respectively, at $n=1.3$. (d) The $\lambda$ in $s$-wave symmetry as a function of $U$, with the red line corresponding to $n=1.42$ and the black line $n=1.3$. (e)-(f) The gap function of the linearized Eliashberg equation within $s$- and $d_{xy}$-wave symmetry , respectively, at $n=1.42$.
• Figure 4: (a)(b) The FS which the $\delta$ pocket is removed at $n=1.3$ and $n=1.42$. (d)(e) The gap function of $s$-wave symmetry calculated by FS like (a) and (b). (c)(f) The $\lambda$ as a function of U($n$), n = 1.42 in (c) and U = 1.5 in (f).