Compressive Strain Turns $s^{\pm}$ into $d$-Wave Pairing in One-unit-cell La$_3$Ni$_2$O$_7$ Thin Film Via Substrate-Induced Hole Doping

TL;DR

This study shows that applying compressive strain to a one-unit-cell La$_3$Ni$_2$O$_7$ thin film induces a transition from $s^{ iny extrm{ extpm}}$ to $d$-wave pairing, driven by substrate-induced hole doping, which enhances superconductivity.

Contribution

It reveals how strain and substrate effects induce hole doping and change pairing symmetry from $s^{ iny extrm{ extpm}}$ to $d$-wave in La$_3$Ni$_2$O$_7$ thin films, providing insights into tuning superconductivity.

Findings

Strain suppresses inter-layer hopping and increases crystal-field splitting.

Hole doping enhances spin-fluctuation-driven pairing correlations.

Pairing symmetry transitions from $s^{ iny extrm{ extpm}}$ to $d_{x^2-y^2}$ and $d_{xy}$ with doping.

Abstract

Motivated by recent reports of ambient-pressure superconductivity in La$_3$Ni$_2$O$_7$ films grown on LaSrAlO$_4$, we investigate the superconducting instability in a one-unit cell thin film using {\it ab initio} and random-phase approximation techniques. Compared to the high-pressure bulk system, the ratio of inter-layer $d_{3z^2-r^2}$ hopping to intra-layer $d_{x^2-y^2}$ hopping is suppressed in the 1UC thin film, and the crystal-field splitting of the $e_g$ orbitals is increased. Our calculation indicates that spin-fluctuation-driven pairing correlations are weak for the stoichiometric case at ambient pressure, but increase significantly under hole doping. The leading pairing symmetry is also found to change by hole doping. Specifically, we obtain a leading $d_{x^2-y^2}$ pairing state at moderate hole doping, followed by a $d_{xy}$ state at higher doping. These states are driven by intra-band spin-fluctuation scattering {\it within} the $\gamma$ hole pocket centered around the M point, and arise primarily from states in the Ni layer {\it farther} from the substrate. These results strongly suggest that the thin-film superconducting samples are hole-doped and that pairing in this system predominantly arises in the layer, as opposed to the inter-layer pairing in the pressurized bulk system.