Doping evolution of spin excitations in La$_{3-x}$Sr$_{x}$Ni$_2$O$_7$/SrLaAlO$_4$ superconducting thin films

Abstract

Ambient-pressure superconductivity in compressively strained bilayer nickelate films enables direct spectroscopic tests of pairing scenarios, yet how magnetism evolves with carrier doping remains largely unexplored. Here we use Ni $L_3$-edge resonant inelastic x-ray scattering (RIXS) to track electronic and spin excitations in coherently strained La$_{3-x}$Sr$_x$Ni$_2$O$_7$/SrLaAlO$_4$ thin films ($x=0$, $0.09$, $0.21$ and $0.38$), spanning superconducting and overdoped non-superconducting regimes at essentially fixed epitaxial strain. Transport confirms superconductivity for $x\le0.21$ and a weakly insulating normal state at $x=0.38$. The $dd$-excitation manifold evolves weakly up to $x=0.21$, whereas the $\sim0.4$ eV and $\sim1.6$ eV features broaden and lose intensity at $x=0.38$. In the superconducting films, dispersive spin excitations persist along both $[H, H]$ and $[H, 0]$ with nearly doping-independent undamped dispersions and only a modest reduction of spectral weight, consistent with robust double-stripe correlations. By contrast, at $x=0.38$ the magnetic response becomes strongly broadened and weakened, with enhanced damping and $\sim50\%$ lower spectral weight, indicating a collapse of coherent double-stripe spin excitations. The concomitant suppression of magnetic coherence and superconductivity establishes a direct doping-controlled link between magnetism and superconductivity in bilayer nickelate films.

Figures (4)

• Figure 1: Epitaxial growth and superconducting phase diagram of La$_{3-x}$Sr$_x$Ni$_2$O$_7$ thin films on SrLaAlO$_4$.a, Structural schematic of the La$_{3-x}$Sr$_x$Ni$_2$O$_7$ (LSNO327) film coherently grown on a (001)-oriented SrLaAlO$_4$ (SLAO) substrate. b– e, Reciprocal space maps in $[H,0,L]$ plane (in reciprocal lattice units) collected around the SLAO $(10 \underline{11})$ reflection showing the LSNO327 $(10 \underline{17})$ film peak for $x = 0$ (b), $0.09$ (c), $0.21$ (d) and $0.38$ (e); the vertical alignment of the film and substrate peaks indicates coherent in-plane strain. f, X-ray diffraction $\theta-2\theta$ scans showing $(00L)$ reflections of the LSNO327 phase (indexed) for films with $x = 0, 0.09, 0.21$ and $0.38$; asterisks denote substrate-related peaks. g, Temperature dependence of the in-plane resistivity $\rho$($T$) for representative films, highlighting superconducting transitions with $T_{c,98\%} = 43.2$ K ($x = 0$), $45.3$ K ($x = 0.09$) and $35.3$ K ($x = 0.21$), while the $x = 0.38$ film shows weakly insulating behaviour; $T_{c,98\%}$ is defined at $98\%$ of the normal-state resistivity. h, Sr-doping evolution of the phase diagram summarizing $T_{c,98\%}$ (open circles) and $T_{c,50\%}$ (solid squares; midpoint criterion). The shaded region marks the superconducting regime.
• Figure 2: Sr-doping evolution of dd excitations in LSNO/SLAO thin films.a– d, Incident-energy-dependent Ni-$L_3$ RIXS intensity maps measured at a grazing-incidence angle $\theta$ = 20$^\circ$ for films with $x=0$ ( a), 0.09 ( b), 0.21 ( c) and 0.38 ( d), plotted as a function of incident photon energy ($E_i$) and energy loss. The solid white curves show the corresponding Ni-$L_3$ X-ray absorption spectra (scaled for clarity). The black dashed squares mark the $\sim$1.6 eV $dd$ excitations. The vertical dashed line marks $E_i=856.4$ eV, the incident energy used to extract the spectra in e. e, Comparison of Ni-$L_3$ RIXS spectra measured at $E_i=856.4$ eV and a grazing-incidence angle $\theta=15.7^\circ$ for the four dopings, highlighting the doping-dependent evolution of the $dd$ excitations; vertical arrows indicate the changes of the $\sim0.4$ eV and $\sim1.6$ eV features. Data for $x=0$ are from Ref. zhong2025spin. The inset in e shows a comparison of the $0.4$ eV excitation between $x=0.21$ and $0.38$.
• Figure 3: Sr-doping evolution of spin excitations in LSNO/SLAO thin films.a– d, False-colour maps of the low-energy Ni–$L_3$ RIXS intensity plotted as a function of energy loss and in-plane momentum transfer $\mathbf{q}_\parallel$ along the $[H,H]$ direction (left part of each panel) and the $[H,0]$ direction (right part), for $x=0$ ( a), 0.09 ( b), 0.21 ( c) and 0.38 ( d). Elastic scattering has been fitted and subtracted from the color maps. The scattering angle $2\theta_s= 110^\circ$ for $|\mathbf{q}_\parallel|\ge0.26$ and $2\theta_s= 90^\circ$ otherwise. The symbols overlaid on the maps denote the undamped spin-excitation dispersion $E_0(\mathbf{q}_\parallel)$ extracted from fits to the RIXS spectra (error bars indicate fitting uncertainties). e, Waterfall plots of representative low-energy Ni–$L_3$ RIXS spectra measured with $\pi$-polarized incident X-rays at $T\approx 20$ K for $x=0$, 0.09, 0.21 and 0.38, along ($\mathbf{q}_\parallel,0$) (left) and $(\mathbf{q}_\parallel,\mathbf{q}_\parallel)/\sqrt{2}$ (right); spectra are vertically offset for clarity and $\mathbf{q}_\parallel$ values are indicated. Dispersive, well-defined spin excitations persist up to $x=0.21$, whereas for $x=0.38$ the magnetic response is strongly broadened and its intensity is markedly reduced over the entire measured $\mathbf{q}_\parallel$ range. Data for $x=0$ are from Ref. zhong2025spin.
• Figure 4: Dispersions, damping and spectral weight of spin excitations across Sr doping in LSNO/SLAO thin films.a, Undamped spin-excitation energy $E_0(\mathbf{q}_\parallel)$ (symbols; left axis) and damping factor $\gamma(\mathbf{q}_\parallel)$ (data points connected by dashed lines; right axis) extracted from damped-harmonic-oscillator (DHO) fits to the low-energy Ni–$L_3$ RIXS spectra for $x=0$, $0.09$, $0.21$ and $0.38$, plotted along the $[H, H]$ (left) and $[H, 0]$ (right) directions. The vertical grey line marks $\Gamma$ ($\mathbf{q}_\parallel=0$), and the solid black curve shows the fitted dispersion for comparison. For $x\le 0.21$, the dispersions nearly coincide and the modes remain weakly damped, whereas for $x=0.38$ the dispersion softens and the damping increases markedly over the full $\mathbf{q}_\parallel$ range. b, Momentum dependence of the magnon spectral weight $W(\mathbf{q}_\parallel)$, obtained by energy integration of the fitted DHO lineshapes. Error bars in a, b represent uncertainties from the fits; dashed lines are guides to the eye.