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Electronic Structure of Compressively Strained Bilayer Nickelate Thin Film

Bai Yang Wang1,2,*,†, Sebastien N. Abadi1,3,4,*, Yidi Liu1,3, Yu Zhang5, Yong Zhong1,2, Yijun Yu1,2,‡, Berit H. Goodge6, Xiaoliang Zhang5, Yi-Ming Wu7 et al.

Ruohan Wang1,3, Jiarui Li1,3, Yaoju Tarn1,2, Eun Kyo Ko1,2, Vivek Thampy8, Chun Lin8, Makoto Hashimoto8, Donghui Lu8, Young S. Lee1,3, Thomas P. Devereaux1,4,9, Chunjing Jia5, Harold Y. Hwang1,2, and Zhi-Xun Shen1,2,3,4,§

  • *These authors contributed equally to this work.
  • †Contact author: bwang87@stanford.edu
  • ‡Present address: State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai, China.
  • §Contact author: zxshen@stanford.edu
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Phys. Rev. X 16, 031008 – Published 15 July, 2026

DOI: https://doi.org/10.1103/4h5x-btvx

Abstract

The discovery of superconductivity in bulk bilayer nickelates under high pressure, and its subsequent stabilization in compressively strained thin films at ambient pressure, has established a new platform for exploring high-Tc superconductivity beyond the cuprates. Central to this development is the prominent role of the 3dz2 orbital in shaping the low-energy electronic structure, imposing constraints on microscopic theories and fueling debate over the superconducting mechanism. Here we report a systematic in situ angle-resolved photoemission spectroscopy study of compressively strained bilayer nickelate thin films spanning Ca doping, oxygen stoichiometry and film thickness. Despite variations in oxygen-vacancy disorder and surface termination, the electronic structure remains robust and exhibits a systematic strain-driven evolution consistent with an intermediate-correlation regime. In particular, we demonstrate a Ca-doping-induced electronic structure evolution that is consistent with Fermi-level crossing of the γ band and is decoupled from the presence of superconductivity, suggesting that, while important for the γ band to be near the Fermi level, the debated γ crossing and the resulting Fermi pocket may not be a prerequisite for superconductivity. Together, our results establish key spectroscopic constraints on the minimal fermiology and correlation strength relevant to superconductivity in bilayer nickelates, providing an experimental foundation for microscopic theories of their pairing mechanism.

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Popular Summary

Determining the precise electronic states that enable high-temperature superconductivity in bilayer nickelate thin films at ambient pressure remains a major challenge due to competing theories regarding their orbital configurations and correlation strengths. We addressed this uncertainty by using angle-resolved photoemission spectroscopy on films with varied thicknesses, growth methods, and chemical compositions to directly map their complete electronic structure. Our measurements reveal electronic behaviors consistent with intermediate electronic correlations and show that the highly debated band derived from out-of-plane nickel orbitals shifts sensitively with epitaxial strain and substitutional doping. Crucially, we demonstrated that while this orbital band can cross the Fermi level, its presence there is not a strict requirement for the emergence of superconductivity. These results provide direct spectroscopic constraints that rule out several microscopic theories of nickelate superconductivity. Our work clarifies how strain, doping, and orbital character can be systematically deployed to tune superconductivity in next-generation quantum materials.

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Phys. Rev. X 16, 031008– Published 15 July, 2026
  • Received 27 May 2026
  • Accepted 17 June 2026
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