Abstract
The newly discovered nickelate superconductors so far only exist in epitaxial thin films synthesized by a topotactic reaction with metal hydrides1. This method changes the nickelates from the perovskite to an infinite-layer structure by deintercalation of apical oxygens1,2,3. Such a chemical reaction may introduce hydrogen (H), influencing the physical properties of the end materials4,5,6,7,8,9. Unfortunately, H is insensitive to most characterization techniques and is difficult to detect because of its light weight. Here, in optimally Sr doped Nd0.8Sr0.2NiO2H epitaxial films, secondary-ion mass spectroscopy shows abundant H existing in the form of Nd0.8Sr0.2NiO2Hx (x ≅ 0.2–0.5). Zero resistivity is found within a very narrow H-doping window of 0.22 ≤ x ≤ 0.28, showing unequivocally the critical role of H in superconductivity. Resonant inelastic X-ray scattering demonstrates the existence of itinerant interstitial s (IIS) orbitals originating from apical oxygen deintercalation. Density functional theory calculations show that electronegative H– occupies the apical oxygen sites annihilating IIS orbitals, reducing the IIS–Ni 3d orbital hybridization. This leads the electronic structure of H-doped Nd0.8Sr0.2NiO2Hx to be more two-dimensional-like, which might be relevant for the observed superconductivity. We highlight that H is an important ingredient for superconductivity in epitaxial infinite-layer nickelates.
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Data availability
The data that support the findings of this study are available from the Figshare data repository, https://figshare.com/articles/dataset/Critical_Role_of_Hydrogen_for_Superconductivity_in_Infinite-layer_Nickelates/21591546. Source data are provided with this paper.
Code availability
The code that supports the findings of this study is available from the corresponding author upon request.
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Acknowledgements
L.Q. acknowledges the support by National Natural Science Foundation of China (grant nos. 12274061, 52072059 and 11774044), Science and Technology Department of Sichuan Province (grant nos. 2021JDJQ0015 and 2022ZYD0014) and Fundamental Research Funds for the Central Universities (grant no. ZYGX2020J023). X.S. and B.H. acknowledge the support from the NSFC (grant no. 12088101) and NSAF (grant no. U2230402). We thank Diamond Light Source for providing beam time under proposal ID NT30296. S.L. thanks Australian Research Council Discovery Projects (grant nos. DP220103229 and DP19013661) for the financial support. Computations were done at Tianhe-JK cluster at CSRC. TOF–SIMS was conducted in the University of New South Wales Mark Wainwright Analytical Centre.
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L.Q. designed the project. L.Q., K.-J.Z. and B.H. interpreted the experimental and theoretical data and supervised the project. X.D. performed thin film growth, XRD measurement, electrical properties and Hall effect characterizations. M.X. and Y.Z. helped with film growth and post-reduction. H.L. helped with electrical properties measurement. X.W. and Q.W. helped with film growth and physical property system measurements. C.C.T., J.C., M.G.-F., S.A. and K.-J.Z. performed XAS and RIXS measurements. J.Z. and S.L. performed SIMS measurements. M.W. and P.G. performed the transmission electron microscopy measurements. X.S. and B.H. performed DFT calculations. H.X. and X.Z. helped with DFT theory study. All the authors participated in data analysis and discussion. K.-J.Z., X.D., L.Q. and B.H. drafted the manuscript with input from all authors.
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Extended data figures and tables
Extended Data Fig. 1 Characterization of perovskite structure and description of topotactical reduction process.
a, b, Typical RHEED intensity oscillation and RHEED pattern along SrTiO3 [101] azimuth direction for Nd0.8Sr0.2NiO3 films. The oscillations indicate that the film grows layer by layer during the initial stage of growth. The layer-by-layer growth gradually disappeared as the film thickened. c, Typical XRD of as-grown Nd0.8Sr0.2NiO3 films. Nd0.8Sr0.2NiO3 (001) and (002) diffraction peaks are located at 23.7° and 48.3°, respectively d, Illustration of the reduction heating process. During the reduction time the sample is maintained at the temperature of 300 °C.
Extended Data Fig. 2 X-ray diffraction for all Nd0.8Sr0.2NiO2Hx films.
XRD scans of films with reduction time varied from 1 min to 420 min. The insert picture shows the corresponding c lattice parameters.
Extended Data Fig. 3 SIMS spectra of other elements in thin film and substrate.
SIMS signals of O, Ti, Ni, Sr, Nd secondary ions in the films under different H doping levels.
Extended Data Fig. 4 Crystal structure and SIMS spectrum of mica.
a, Crystal structure of mica with the formula of KAl2[AlSi3O10](OH)2. b, SIMS spectrum of H, O, Al, K, Si secondary ions in mica as a reference sample for calibration.
Extended Data Fig. 5 Zoom-in view of ρ(T) with different H concentrations.
The temperature-dependent resistivities of Nd0.8Sr0.2NiO2Hx films with x varying from 0.19 to 0.33.
Extended Data Fig. 6 Crystal structure of Nd0.8Sr0.2NiO2Hx.
a-c, Lowest energy configurations of Nd0.8Sr0.2NiO2Hx (x = 0, 0.25 and 0.5) in 2×2×2 supercell, where H forms 1D chain structure along c direction [see details in Supplementary Note 12]. The green shaded areas indicate the unit cells. These structures are used for electronic structure calculations in Fig. 4.
Extended Data Fig. 7 RIXS spectra of Nd0.8Sr0.2NiO2Hx.
Low-energy RIXS intensity maps (a-c) and the integrated quasielastic region (d–f) of three Nd0.8Sr0.2NiO2Hx samples with x = 0.19 (a, d), 0.26 (b, e) and 0.33 (c, f), respectively. No sign of CDWs is present in any probed samples.
Supplementary information
Supplementary Information
Supplementary Notes 1–18, Figs. 1–19, Tables 1 and 2 and references.
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Ding, X., Tam, C.C., Sui, X. et al. Critical role of hydrogen for superconductivity in nickelates. Nature 615, 50–55 (2023). https://doi.org/10.1038/s41586-022-05657-2
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DOI: https://doi.org/10.1038/s41586-022-05657-2
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