Superconductivity and phase diagram in Sr-doped La${}_{3-x}$Sr${}_x$Ni${}_2$O${}_7$ thin films

Abstract

Recent studies have demonstrated ambient pressure superconductivity in compressively strained La${}_3$Ni${}_2$O${}_7$ thin films, yet the phase diagram of heterovalent doping$-$critical for advancing the field$-$remains unexplored. Here, we report superconductivity in Sr${}^{2+}$-doped La${}_{3-x}$Sr${}_x$Ni${}_2$O${}_7$ films synthesized via molecular beam epitaxy with ozone-assisted post-annealing. The superconducting transition temperature ($T_{\mathrm{c}}$) follows an asymmetric dome-like profile, persisting across a wide doping range ($0\le x\le 0.21$) before diminishing at $x\approx 0.38$. Optimally doped films ($x=0.09$) achieve $T_{\mathrm{c}}$ of $\sim$ 42 K, with high critical current ($J_{\mathrm{c}}>1.4$ $kA/c{\mathrm{m}}^2$ at 2 K) and upper critical fields ($\hat{I}{¼}_0{H}_{\mathrm{c},\parallel }(0)=83.7$ $\mathrm{T}$, $\hat{I}{¼}_0{H}_{\mathrm{c},\perp }(0)=110.3$ $\mathrm{T}$), comparable to reported La${}_{3-x}$Pr${}_x$Ni${}_2$O${}_7$ films. Scanning transmission electron microscopy reveals oxygen vacancies predominantly occupy at planar NiO${}_2$ sites$-$unlike apical-site vacancies in bulk samples$-$due to Coulomb repulsion destabilizing planar oxygen under compressive strain. Additionally, the elongated out-of-plane Ni-O bonds, exceeding those in pressurized bulk samples by $4\%$, likely weaken the interlayer $d_{z^2}$ coupling, thus contributing to the reduced $T_{\mathrm{c}}$ in strained films. This work establishes heterovalent Sr${}^{2+}$ doping as a robust tuning parameter for nickelate superconductivity, unveiling a unique phase diagram topology.

31 pages, 12 figures