Superconductivity governed by Janus-faced fermiology in strained bilayer nickelates

Abstract

High-temperature superconductivity in pressurized and strained bilayer nickelates (La,Pr)$_3$Ni$_2$O$_7$ has emerged as a new frontier. One of the key unresolved issues concerns the fermiology that underlies superconductivity. On both theoretical and experimental sides, no general consensus has been reached, and conflicting results exist regarding whether the relevant Fermi surface involves a $γ$ pocket -- a hole pocket with $d_{z^2}$-orbital character centered at the Brillouin zone corner. Here, we address this issue by unveiling a Janus-faced role of the $γ$ pocket in spin-fluctuation-mediated superconductivity. We show that this pocket simultaneously induces dominant pair-breaking and pair-forming channels for the leading $s_\pm$-wave pairing. Consequently, an optimal superconducting transition temperature $T_\mathrm{c}$ is achieved when the $γ$ pocket surfaces at the Fermi level, placing the system near a Lifshitz transition. This suggests that superconductivity can emerge, provided the maximum energy level of the $γ$ pocket lies sufficiently close to the Fermi level, either from below or above. Our finding not only reconciles two opposing viewpoints on the fermiology, but also naturally explains recent experiments on (La,Pr)$_3$Ni$_2$O$_7$ thin films, including the superconductivity under compressive strain, two conflicting measurements on the Fermi surface, and the dome shape of $T_\mathrm{c}$ as a function of hole doping.

Figures (3)

• Figure 1: (a) Crystal structure of La$_3$Ni$_2$O$_7$ drawn using VESTA VESTA. (b) Fermi surface obtained from the MLWF fit of the DFT band structure at zero strain. The color bar indicates the relative orbital weight. (c) The evolution of the band structure with strain. $+$ and $-$ denote tensile and compressive strain, respectively.
• Figure 2: (a) The leading $\lambda_\mathrm{sc}$ as a function of strain. Circles: $s_\pm$-wave gap; squares: $d_{x^2-y^2}$-wave gap. The inset shows the FSs for several values of strain. The blue arrow highlights the marginal presence of the $\gamma$ pocket at $- 2.25$ % strain. (b) Comparison between our calculated $\lambda_\mathrm{sc}$ (red line; left ticks) with experimental $T_\mathrm{c}$sun_signatures_2023li2025_100gpawang2024la2prni2o7ko2025thinfilm (right ticks) as a function of the in-plane lattice constant $a_\mathrm{LNO}$. Sun et al.: Ref. sun_signatures_2023; Li et al.: Ref. li2025_100gpa; Wang et al.: Ref. wang2024la2prni2o7; Ko et al.: Ref. ko2025thinfilm. The filled triangles denote the data for the strained thin films ko2025thinfilm, while the open symbols denote those for the pressurized samples sun_signatures_2023li2025_100gpawang2024la2prni2o7. (c) The nonzero elements of the $s_\pm$-wave $\Delta_{lm}(\bm{k},i\omega_0)$ in the BA basis at $- 2.25$ % strain. (d) Orbital weight on the FS at $- 1$ % strain. The top panel shows the $x_+$ and $x_-$ contributions and the bottom panel shows the $z_+$ and $z_-$ contributions. $\bm{q}^\mathrm{pb} = (\epsilon_1, \epsilon_2)$ with $\epsilon_1, \epsilon_2 < \pi$ being small numbers and $\bm{q}^\mathrm{pf} = (\pi, 0)$ denote wave vectors associated with the dominant pair-breaking and pair-forming channels, respectively, for the $s_\pm$-wave pairing. (e) The pair-breaking interaction $\Gamma^\mathrm{s}_{z_+ z_+ z_+ z_+ }(\bm{q},i\nu_0) ~\{=\Gamma^\mathrm{s}_{z_- z_- z_- z_- }(\bm{q},i\nu_0) \}$ (top) and the associated irreducible susceptibility $\chi^0_{z_+ z_+ z_+ z_+ }(\bm{q},i\nu_0)$ (bottom). (f) The pair-forming interaction $\Gamma^\mathrm{s}_{z_+ z_- z_- z_+ }(\bm{q},i\nu_0)~\{ =\Gamma^\mathrm{s}_{z_- z_+ z_+ z_- }(\bm{q},i\nu_0) \}$ (top) and the associated irreducible susceptibility $\chi^0_{z_+ z_- z_+ z_- }(\bm{q},i\nu_0)$ (bottom).
• Figure 3: (a) $\lambda_\mathrm{sc}$ as a function of hole doping $x$ for $-2.5$ % and $-1.5$ % strain values (right), and the corresponding FSs for selected cases (left). (b) Experimental phase diagram of $-2$ % strained La$_{3-x}$Sr$_{x}$Ni$_2$O$_7$ thin films grown on SrLaAlO$_4$ substrate with $x$ denoting hole doping. The yellow circles with error bars are the experimental data at which the resistivity reaches 98 % of the extrapolated normal-state resistivity hao2025superconductivityphasediagramsrdoped.