Abstract
The discovery of Ruddlesden–Popper (RP) bilayer nickelate superconductors under high pressure has opened a new chapter in high-transition-temperature superconductivity1,2,3,4,5,6,7,8. However, the high-pressure conditions and presence of impurity phases have hindered comprehensive investigations into their superconducting properties and potential applications. Here we report ambient-pressure superconductivity onset above the McMillan limit (40 K) in RP bilayer nickelate epitaxial thin films. Three-unit-cell-thick La2.85Pr0.15Ni2O7 pure-phase single-crystal films are grown using the gigantic-oxidative atomic layer-by-layer epitaxy on SrLaAlO4 substrates9. Resistivity measurements and magnetic field responses indicate onset transition temperature of 45 K. The transition to zero resistance shows characteristics consistent with a Berezinskii–Kosterlitz–Thouless (BKT) behaviour, with TBKT = 9 K. The Meissner diamagnetic effect is observed at 8 K by using a mutual inductance setup, in agreement with the BKT-like transition. In- and out-of-plane critical magnetic fields show anisotropy. Scanning transmission electron microscopy images and X-ray reciprocal space mappings reveal that the RP bilayer nickelate films adopt a tetragonal phase under roughly 2% coherent epitaxial compressive strain in the NiO2 planes relative to the bulk. Our findings pave the way for comprehensive investigations of nickelate superconductors under ambient pressure conditions and for exploring superconductivity at higher transition temperatures through strain engineering in heterostructures.
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Acknowledgements
We acknowledge the discussions with G.-M. Zhang, D.-X. Yao and H. Yuan. This work was supported by the National Key R&D Program of China (grant no. 2022YFA1403101), the Natural Science Foundation of China (grants 92265112, 12374455 and 52388201), the Guangdong Provincial Quantum Science Strategic Initiative (grants GDZX2401004 and GDZX2201001), the Shenzhen Municipal Funding Co-Construction Program Project (grants SZZX2401001 and SZZX2301004) and the Shenzhen Science and Technology Program (grant KQTD20240729102026004). H.W. acknowledges support by the China Postdoctoral Science Foundation (grants GZB20240294 and 2024M751287). W.-Q.C. acknowledges the support by Center for Computational Science and Engineering at Southern University of Science and Technology.
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Q.-K.X. and Z.C. supervised the project. Z.C. initiated the study and coordinated the research efforts. G.Z., W.L. and Z.N. performed thin-film growth with assistance from Y.C. H.W. performed low-temperature measurements and analysis. Y.L. and H.H. contributed on STEM analysis. H.H. contributed on XRD analysis. W.-Q.C. provided theoretical support. Y.-J.S. provided support in data interpretation. Z.C. wrote the manuscript with key input from Q.-K.X. and all other authors. G.Z., W.L., H.W. and Z.N. contributed equally to this work.
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Extended data figures and tables
Extended Data Fig. 1 Growth.
a, Schematic of synthesizing (La,Pr)3Ni2O7 on SrLaAlO4 with gigantic-oxidative atomically layer-by-layer epitaxy (GOALL-Epitaxy). b and c, RHEED oscillations and patterns of La3Ni2O7 and La2.85Pr0.15Ni2O7 growth on SrLaAlO4. Blue and orange blocks represent the growth of LaOx/(La0.95Pr0.05)Ox and NiOx layer, respectively. d-g, Time evolution of RHEED pattern during the growth of La2.85Pr0.15Ni2O7 film on SrLaAlO4 substrate. RHEED pattern of the substrate SrLaAlO4 (d), 1 unit cell of La2.85Pr0.15Ni2O7 (e), 2 unit cells (f) and 3 unit cells (g), respectively along [100] direction. Red solid line in (d) represents the intensity profile, which is obtained by integrating the intensity vertically in the rectangular area marked with red dashed line. h, Evolution of the distance between 00 and 01 diffraction spots, which is determined by the peak position of the intensity profile and the error bar is calculated by the FWHM of the peak.
Extended Data Fig. 2 Post-annealing.
a, Resistivity-versus-temperature curves of a 3UC La2.85Pr0.15Ni2O7/SrLaAlO4 sample before and after pure ozone flow annealing. This is a different sample from the one shown in Figs. 1 and 2 in the main text. b, X-ray diffraction (XRD) of the same sample shown in (a) before and after pure ozone flow annealing. c, Resistivity-temperature curves for La2.85Pr0.15Ni2O7 films on SrLaAlO4 substrates with different annealing conditions as follows. Sample 1 (3UC): 400 °C for 30 mins, with an O3 pressure of 1.57 × 10−1 mbar. Sample 2 (3UC): 500 °C for 30 mins, with an O3 pressure of 1.52 × 10−1 mbar. Sample 3 (3UC): 700 °C for 30 mins, with an O3 pressure of 1.10 × 10−1 mbar. Sample 4 (3UC): 600 °C for 30 mins, with an O3 pressure of 1.00 × 10−1 mbar. Sample 5 (3UC): 600 °C for 30 mins, with an O3 pressure of 1.38 × 10−1 mbar. Sample 6 (3UC): 600 °C for 30 mins, with an O3 pressure of 1.38 × 10−1 mbar. Sample 7 (3UC): 600 °C for 30 mins, with an O3 pressure of 1.30 × 10−1 mbar. Sample 8 (6UC): 700 °C for 30 mins, with an O3 pressure of 1.67 × 10−1 mbar. Sample 9 (6UC): 700 °C for 30 mins, with an O3 pressure of 2.03 × 10−1 mbar. The growth conditions for all films were identical as described in the methods section.
Extended Data Fig. 3 Oxygen loss.
a, Resistance changes as functions of time, with sample maintained at vacuum and different temperature inside the quantum design physical property measurement system (PPMS). b, An Arrhenius fit yields an activation energy of 0.34 eV for the oxygen loss process. c, XRD of the same sample shown in Figs. 1 and 2 in the main text after post-annealing and after 6 h in ultrahigh vacuum and 6 days in 1 atm O2 and at ambient temperature. Resistance increase rate in 1 atm O2 at ambient temperature is approximately 2 Ω per hour.
Extended Data Fig. 4 Current-voltage (I-V) characteristics.
I-V curves from 4 K to 15 K, with 1 K interval. Dashed red lines are linear fits to the log-log I-V curves. Inset: the power law exponent α obtained from the fit as a function of temperature. It is important to note that, due to the significant heating effect caused by the applied current (a result of the high critical current associated with the elevated TC), the actual sample temperatures are higher than the recorded values.
Extended Data Fig. 5 Sample without Pr.
Resistivity-temperature curve and magnetic field responses (inset) for a 3UC La3Ni2O7 film on SrLaAlO4.
Extended Data Fig. 6 Hall effect raw data.
a, 3UC La2.85Pr0.15Ni2O7 film on SrLaAlO4. b, 3UC La3Ni2O7 film on SrLaAlO4.
Extended Data Fig. 7 Penetration depth.
Extracted penetration depth λ as a function of temperature from mutual inductance measurements assuming superconducting thickness of 4 ± 3 nm. The flux-leak is lower than 2% in our measurement setup as calibrated by a 5 mm × 5 mm × 0.5 mm Nb plate under 2 K.
Extended Data Fig. 8 Sr interfacial diffusion.
STEM HAADF and atomically-resolved EDS of an 3UC La2.85Pr0.15Ni2O7/SrLaAlO4 superconducting sample (a) and a 3UC La3Ni2O7/SrLaAlO4 sample (b). Data in a are the same set of data as shown in Fig. 3d in the main text. c, Sr EDS intensity for both samples as a function of distance across the interface.
Extended Data Fig. 9 Absorption spectroscopy.
Electron energy loss spectroscopy (EELS) of Ni L (a) and O K (b) edges, comparing film and substrate, in an annealed 3UC La2.85Pr0.15Ni2O7/SrLaAlO4 superconducting sample. c, Synchrotron X-ray absorption spectroscopy (XAS) spectra of Ni-L2 edge comparing a superconducting La2.85Pr0.15Ni2O7/SrLaAlO4 sample (same sample shown in Fig. 1) and a superconducting Nd0.8Sr0.2NiO2/(LaAlO3)0.3(Sr2TaAlO6)0.7 sample.
Extended Data Fig. 10 Surface and thickness.
a,b, Atomic force microscope (AFM) image of the SrLaAlO4 substrate annealed at 1030 °C and 1080 °C, respectively. The root-mean-square roughness is 536.6 pm and 439.6 pm, respectively. c, AFM image of a 3UC La3Ni2O7 film on SrLaAlO4 substrate. The root-mean-square roughness is 308.6 pm. d, AFM image of a 3UC La2.85Pr0.15Ni2O7 film on SrLaAlO4 substrate. The root-mean-square roughness is 407.3 pm. e, X-ray reflectivity (XRR) of 5UC La3Ni2O7/LaAlO3 (top), 3UC La3Ni2O7/SrLaAlO4 (middle) and 3UC La2.85Pr0.15Ni2O7/SrLaAlO4 (bottom) films. f, The relationship between electron density and thickness obtained by fitting the XRR results. The film thickness and surface roughness obtained by fitting are also listed.
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Zhou, G., Lv, W., Wang, H. et al. Ambient-pressure superconductivity onset above 40 K in (La,Pr)3Ni2O7 films. Nature 640, 641–646 (2025). https://doi.org/10.1038/s41586-025-08755-z
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DOI: https://doi.org/10.1038/s41586-025-08755-z
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