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常压镍基高温超导电性的发现，为深入探索镍基超导机理带来了新平台。然而，Ruddlesden-Popper镍氧化物在热力学上处于亚稳态，对其结构和氧含量的精准控制极具挑战。本文介绍了利用强氧化原子逐层外延生长技术在LaAlO3和SrLaAlO4衬底上制备单相、高质量的Ln3Ni2O7（Ln为镧系元素）薄膜的系统方法。其中，(La,Pr,Sm)3Ni2O7/SrLaAlO4超导薄膜的超导起始转变温度（Tc,onset）达到50 K。阳离子化学计量偏差、逐层原子覆盖度、薄膜与衬底界面重构和氧化条件是影响薄膜Ln3Ni2O7晶体质量和超导性能的四个重要因素： 1）精准的阳离子化学计量控制会抑制晶体杂相的产生； 2）原子逐层的完整覆盖和3）优化的界面重构可以减少薄膜的堆垛层错； 4）准确的氧含量调控则是实现超导单转变和高Tc,onset的关键。这些发现对各类氧化物高温超导薄膜的逐层外延生长具有借鉴意义。
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关键词:
- 镍氧化物超导薄膜 /
- Ruddlesden-Popper相 /
- 强氧化原子逐层外延 /
- 界面重构
The discovery of ambient-pressure nickelate high-temperature superconductivity provides a new platform for probing the underlying superconducting mechanisms. However, the thermodynamic metastability of Ruddlesden-Popper nickelates Lnn+1NinO3n+1 (Ln = lanthanide) presents significant challenges in achieving precise control over their structure and oxygen stoichiometry. This study establishes a systematic approach for growing phase-pure, high-quality Ln3Ni2O7 thin films on LaAlO3 and SrLaAlO4 substrates using gigantic-oxidative atomic-layerby-layer epitaxy. The films grown under an ultrastrong oxidizing ozone atmosphere are superconducting without further post annealing. Specifically, the optimal Ln3Ni2O7/SrLaAlO4 superconducting film exhibitsan onset transition temperature (Tc,onset) of 50 K. Four critical factors governing the crystalline quality and superconducting properties of Ln3Ni2O7 films are identified: 1) precise cation stoichiometric control suppresses secondary phase formation. In a Ni-rich sample (+7%), the thin film forms a Ln4Ni3O10 secondary phase, and the R-T curve correspondingly exhibits metallic behavior. In contrast, a Ni-deficient sample forms a Ln2NiO4 secondary phase, with its R-T curve indicating insulating behavior over the entire temperature range. 2) Complete atomic layer-by-layer coverage minimizes stacking faults. Deviation from ideal monolayer coverage induces in-plane atomic number mismatch, whichdirectly triggers out-of-plane lattice collapse or uplift near bulkequilibrium positions. 3) Optimized interface reconstruction can improve the atomic arrangement at the interface. This can be achieved through methods such as annealing the SrLaAlO4 substrate or pre-depositing a 0.5-unit-cell-thick Ln2NiO4-phase buffer layer, which enhances the energy difference between the Ln-site and Ni-site layers to promote proper stacking. 4) Accurate oxygen content regulation is essential for achieving a single superconducting transition and high Tc,onset. Although the under-oxidized sample demonstrates a relatively high Tc,onset (50 K), it displays a two-step superconducting transition. Conversely, the over-oxidized sample exhibits a reduced Tc,onset of 37 K and similarly manifests a two-step transition. These findings provide valuable insights for the layer-by-layer epitaxy growth of diverse oxide high-temperature superconducting films-
Keywords:
- nickelate superconducting thin film /
- Ruddlesden-Popper phase /
- gigantic-oxidative atomic-layer-by-layer epitaxy /
- surface structure
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