Spin correlations in La$_3$Ni$_2$O$_7$ superconducting thin films

Abstract

The discovery of ambient-pressure superconductivity with $T_{c,\text{onset}} > 40$ K in {\LNO} (LNO) thin films grown on the SrLaAlO$_4$ (SLAO) substrate with compressive ($\varepsilon\approx-2\%$) epitaxial strain provides a unique platform for investigating the superconducting mechanisms in nickelate superconductors. Here, we use resonant inelastic X-ray scattering (RIXS) to unveil the dispersive spin excitations in the LNO/SLAO superconducting thin film and establish the strain dependence of the electronic and spin excitations in LNO thin films with strain ranging from $\varepsilon\approx-2\%$ to $+1.9\%$. Compared with the bulk crystal, the LNO/SLAO thin film (with $\varepsilon\approx-2\%$) exhibits similar $dd$ excitations and spin dynamics with larger bandwidth. By contrast, tensile-strained LNO/SrTiO$_3$ ($\varepsilon \approx +1.9\%$) exhibits a marked suppression of both the spin excitations and the Ni 3{\dz}-derived $dd$ excitations. The strain dependence of the spin excitations reflects significant changes in the interlayer exchange coupling $J_z$, and the diminishing $dd$ excitations in tensile-strained samples indicate weaker Ni 3{\dz}-O 2$p_{z}$ hybridization. This strain evolution of the spin excitations and $J_z$ is attributed to the strain-tuned $c$-axis Ni-O-Ni bond angle $\varphi$, which controls the Ni 3{\dz}-O 2$p_{z}$ hybridization. Since superconductivity is observed only in films grown on SLAO, and spin correlations are enhanced along with the emergence of superconductivity, our results identify $\varphi$ as a key structural lever controlling $J_z$ and provide direct spectroscopic support for interlayer spin-fluctuation-mediated pairing scenarios in bilayer nickelates.

Figures (4)

• Figure 1: (a) Ni-O bilayer structure in LNO. The NiO$_6$ octahedra tilting causes the interlayer and intralayer Ni-O-Ni bond angles ($\varphi$ and $\varphi'$) to deviate from $180^\circ$. $a_{\rm o}$ and $b_{\rm o}$ ($a_{\rm T}$ and $b_{\rm T}$) represent the orthorhombic (pseudo-tetragonal) primitive vectors. (b) Scattering geometry for RIXS measurements. The scattering plane is defined by the incident vector $k_{\rm i}$ and the scattered vector $k_{\rm f}$. The electric field vector of $\pi$-polarized ($\sigma$-polarized) incident X-rays is parallel (perpendicular) to the scattering plane. $q_\parallel$ is the projection of $q$ onto the $H$-$K$ plane. The sample rotation angle ($\theta$) is defined as the angle between the incident X-ray and $ab$ plane of samples. (c) $q_\parallel$-dependent RIXS spectra of LNO/SLAO, measured at $T=20$ K with scattering angle $2\theta_s=90^{\circ}$ and $110^{\circ}$, and $\pi$ polarization. Elastic scattering has been subtracted. The red circles mark the undamped energy $E_0(q_\parallel)$ for the spin excitations in LNO/SLAO, whereas the solid and dashed blue curve marks the $E_0(q_\parallel)$ fitted from $\pi$- and $\sigma$-polarized data of La$_3$Ni$_2$O$_7$ single crystal, adapted from Ref. chen2024electronic. (d), (e) Comparison of spin excitations at $q_{\parallel}=(0.06, 0)$ (d) and $(0.18, 0)$ (e) between LNO/SLAO thin film (red curves) and LNO single crystal (blue dashed curves) chen2024electronic. (f) Bond‑angle control of interlayer exchange. The Ni-O$_{\rm AP}$-Ni angle $\varphi$ sets $J_z$, a key ingredient in interlayer‑mediated pairing lu2024interlayer. Selected Ni sites show the partially filled $3d{z^2}$/$3d_{x^2-y^2}$ orbitals and the associated hoppings ($t_\perp$, $t_\parallel$) and exchanges ($J_z$, $J_\parallel$). (g) Absolute values of interlayer $J_z \equiv J_3$ and intralayer $J_1$ and $J_2$ as a function of epitaxial strain.
• Figure 2: (a) XAS of LNO films near the Ni-$L_3$ edge with $\pi$ and $\sigma$ polarization after subtracting the La-$M_4$ peaks at the grazing incident angle $\theta=20^\circ$. The arrows indicate the satellite peaks, which are mainly from the $3d^8\underline{L}$ configuration chen2024electronic. (b)-(d) Incident-energy ($E_i$) dependence of the excitations of LNO films on LAO (#2) (b), LSAT (c) and STO (d) substrates with $\pi$ polarization. The white curves are corresponding XAS spectra and the magenta dashed line mark the resonance with the $\sim1.6$ eV excitation. All RIXS data in (b)-(d) were normalized to the incident photon flux and collected at $\theta = 20^\circ$. (e) Comparison of representative RIXS spectra of LNO/SLAO and LNO/LAO (#1) measured at $E_{i}=851.7$ eV and $q_{\parallel}=(0.21, 0)$ with $2\theta_{s}=90^{\circ}$ and $\theta=10^\circ$.
• Figure 3: Comparison of $q_\parallel$-dependent RIXS spectra between LNO/SLAO and LNO/LAO (#1) (a), and between LNO/LAO (#2) and LNO/STO (b), measured along the $[H, 0]$ and $[H, H]$ directions with $\pi$ polarization in grazing incidence geometry. The energy resolution is $\Delta E\approx32$ meV for (a) and $\Delta E\approx45$ meV for (b). Scattering angle: in (b), $2\theta_{s}=150^\circ$; in (a), $2\theta_{s}=110^\circ$ for $|q_\parallel|\geq0.26$ and $2\theta_{s}=90^\circ$ otherwise. Vertical bars mark the peak positions of the spin excitations. (c) Spin-excitation spectra of LNO thin films extracted from the DHO fitting of the data shown in (a) and (b). The RIXS spectral intensities were normalized to the incident photon flux and unit-cell number across different samples, and to the 1 eV $dd$ excitations across different $q_{\parallel}$s.
• Figure 4: (a) The undamped energy $E_{0}$ (circles) and damping factor $\gamma$ (diamonds) of the spin excitations in LNO films extracted from the fitting of the RIXS spectra using DHO function. The red/blue/black solid and green dashed curves are fittings of the dispersions of thin films and bulk LNO chen2024electronic following a Heisenberg model ni2025spinSI. (b) Comparison of the magnon spectral weight between LNO/LAO (#1) and LNO/SLAO, and between LNO/LAO (#2) and LNO/STO. The error bars in (a) and (b) were estimated by the uncertainty of the elastic peak position and the standard deviation of the fits.