Recently, the research team led by Professor Yuefeng Nie and Associate Professor Dianxiang Ji from the College of Engineering and Applied Sciences at Nanjing University has made significant progress in nickelate superconductivity. Utilizing oxide molecular beam epitaxy (OMBE) technology, the team successfully synthesized Sr-doped La₃Ni₂O₇ thin films exhibiting ambient-pressure superconductivity and, for the first time, systematically mapped the carrier doping-dependent superconducting phase diagram of the bilayer nickelate. This research achievement has been published in Nature Materials under the title "Superconductivity in Sr-doped La₃Ni₂O₇ thin films" (DOI: 10.1038/s41563-025-02327-2).
Research Background
In recent years, the emergence of novel nickel-based superconducting materials has provided new opportunities for unraveling the mechanisms of unconventional high-temperature superconductivity, becoming a forefront hotspot in condensed matter physics. Among them, the bilayer nickelate La₃Ni₂O₇, with a superconducting transition temperature exceeding that of liquid nitrogen and exhibiting numerous properties distinct from cuprate superconductors, has attracted widespread attention.
In cuprate and iron-based superconductors, carrier doping suppresses the antiferromagnetic parent state to induce a high-temperature superconducting state, and the phase diagram can also encompass complex phenomena such as pseudogap, nematic phase, strange metal behavior, and Fermi liquid regime. Studying these quantum phenomena is crucial for revealing the mechanisms of superconductivity. However, the preparation of superconducting La₃Ni₂O₇ thin films faces numerous challenges, as the superconducting state is extremely sensitive to epitaxial growth parameters and ozone annealing conditions. Heterovalent doping with Sr²⁺ ions and the approximately -2% in-plane compressive strain induced by the substrate further increase the preparation difficulty. Consequently, the carrier-tuned phase diagram of La₃Ni₂O₇ had not been reported until now.
Breakthrough in Material Preparation
Professor Yuefeng Nie's research group has long been dedicated to developing thin film preparation techniques based on oxide molecular beam epitaxy (OMBE) with single atomic layer precision control, applying them to the study of quantum materials such as ferroelectrics, multiferroics, and high-temperature superconductors. This study builds upon the team's extensive accumulation in OMBE techniques. By combining in-situ real-time monitoring via reflection high-energy electron diffraction (RHEED) with ex-situ X-ray diffraction characterization, the team established methods for high-precision stoichiometry calibration and doping control. Using an improved layer-by-layer deposition process, they prepared high-quality Sr-doped La₃Ni₂O₇ thin films and successfully observed the high-temperature superconducting state.
Unique Superconducting Phase Diagram
Experimental studies reveal that La₃₋ₓSrₓNi₂O₇ thin films epitaxially grown on SrLaAlO₄ substrates exhibit a unique superconducting phase diagram. Within the doping range of 0 ≤ x ≤ 0.21, the superconducting transition temperature (Tc) remains relatively stable with varying doping concentration, with the optimal Tc around 42 K. As the doping concentration increases further, Tc gradually decreases, forming an incomplete dome-like phase diagram. This characteristic is distinctly different from the two separate, complete dome-shaped phase diagrams typically formed through hole and electron doping in cuprate superconductors.
Figure 1. Structural characterization, superconducting properties, and phase diagram of Sr-doped La₃Ni₂O₇ thin films.
Regulation of Structure and Properties by Epitaxial Strain
Atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM) characterization revealed that oxygen vacancies introduced during the sample preparation process are predominantly located within the NiO₂ planes. This distribution characteristic significantly differs from the phenomenon observed in bulk single crystals, where oxygen vacancies primarily occupy the interlayer apical oxygen sites. Theoretical calculations indicate that in-plane biaxial compressive strain enhances the overlap of in-plane electron wavefunctions, increasing the Coulomb repulsion energy at the in-plane oxygen sites and thereby lowering the formation energy of oxygen vacancies at these sites.
The study also found that epitaxial compressive strain causes significant changes in Ni-O bond lengths: the in-plane Ni-O bonds shorten, while the out-of-plane Ni-O bonds elongate by approximately 4%. This structural change weakens the interlayer coupling strength of the dz² orbitals, providing a plausible explanation for understanding the difference in superconducting transition temperatures between thin film and bulk samples.
Research Significance and Outlook
This study is the first to reveal the characteristic incomplete dome-like superconducting phase diagram in carrier-doped La₃Ni₂O₇ superconducting thin films and elucidates the regulation role of epitaxial strain on oxygen vacancy distribution and interlayer coupling. These results provide an important experimental foundation for further understanding the mechanisms of nickelate superconductivity. Follow-up work could focus on exploring the feasibility of constructing a complete phase diagram through substitution of La³⁺ with tetravalent ions and investigating possible competing ordered phases in different doping regimes. These studies will contribute to deepening the understanding of superconducting mechanisms in nickelate superconductors.
Bo Hao, Maosen Wang, and Wenjie Sun (Ph.D. students from the College of Engineering and Applied Sciences, Nanjing University) and Yang Yang (Ph.D. student from the Department of Applied Physics, The Hong Kong Polytechnic University) are the co-first authors of the paper, Professor Yuefeng Nie and Associate Professor Dianxiang Ji the corresponding authors. Professor Jian Zhou provided crucial theoretical support for this work. This work was supported by the National Key R&D Program of China from the Ministry of Science and Technology, the National Natural Science Foundation of China, the Ministry of Education's "Chang Jiang Scholars Program", the National Natural Science Foundation for Young Scientists, the General Research Fund (GRF) from the Hong Kong Research Grants Council (RGC), the Natural Science Foundation of Jiangsu Province, the Postdoctoral Innovative Talent Support Program, and other funding projects. The National Laboratory of Solid State Microstructures at Nanjing University, the Collaborative Innovation Center of Advanced Microstructures, the Jiangsu Key Laboratory of Artificial Functional Materials, the Department of Applied Physics at The Hong Kong Polytechnic University, and the Jiangsu Physical Science Research Center provided significant support for this research.
