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常压镍基高温超导电性的发现, 为深入地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 further exploring the underlying superconducting mechanisms. However, the thermodynamic metastability of Ruddlesden-Popper nickelates Lnn+1NinO3n+1 (Ln = lanthanide) poses significant challenges for precise control over their structures and oxygen stoichiometry. This study establishes a systematic approach to growing phase-pure, high-quality Ln3Ni2O7 thin films on LaAlO3 and SrLaAlO4 substrates by using gigantic-oxidative atomic-layer-by-layer epitaxy. The films grown under an ultrastrong oxidizing ozone atmosphere are superconducting without further post-annealing. Specifically, the optimal Ln3Ni2O7/SrLaAlO4 superconducting film exhibits an onset transition temperature (Tc,onset) of 50 K. Four critical factors governing the crystalline quality and superconducting properties of Ln3Ni2O7 films are identified as follows. 1) Precise cation stoichiometric control suppresses secondary phase formation. In an Ni-rich sample (+7%), the thin film forms an Ln4Ni3O10 secondary phase, and the R-T curve correspondingly exhibits metallic behavior. In contrast, an Ni-deficient sample forms an 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, which directly triggers out-of-plane lattice collapse or uplift near bulk-equilibrium 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 into 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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图 1 (a) GAE生长示意图; n = 1, 2, 3, 4镍氧化物RP相 (b) XRD θ-2θ扫描结果和(c)结构示意图; (d) (00 4n+2)峰位对于RP相A/B位名义摩尔比的依赖关系[22]
Fig. 1. (a) Schematic diagram of GAE; (b) XRD θ-2θ scan results and (c) structural schematic diagram for RP phase nickelates with n = 1–4; (d) dependence of the (00 4n+2) peak position on the nominal A/B-site molar ratio of the RP phase[22].
图 2 (a) SLAO衬底上富镍7%(S1)、阳离子化学计量平衡(S2)和缺镍11%(S3)(La, Pr, Sm)3Ni2O7样品XRD θ-2θ扫描结果; (b) R-T曲线和(c) 起始转变区域放大图. 阳离子化学计量通过改变LnOx与NiOx靶材的轰击脉冲数比例实现调控
Fig. 2. (a) XRD θ-2θ scans and (b) R-T curves for 7% Ni-rich (S1), cation-stoichiometric (S2), and 11% Ni-deficient (S3) (La, Pr, Sm)3Ni2O7 samples on SLAO substrates; (c) an enlarged plot showing the onset of the transition. The cation stoichiometry is controlled by the pulse ratio of the LnOx and NiOx targets.
图 3 阳离子化学计量平衡样品 (a) RHEED振荡曲线及其(b) 生长前后RHEED衍射图样; 缺镍11%样品(c), (d)和富镍7%样品(e), (f) RHEED振荡曲线及其区域放大图. 各样品中LnOx层的沉积对应了RHEED强度的下降, 而NiOx层的沉积则会使得强度上升
Fig. 3. (a) RHEED intensity oscillations and (b) diffraction patterns before and after growth for the cation-stoichiometric sample; RHEED oscillation curves and their zoom-in views for the (c), (d) Ni-deficient (–11%) and (e), (f) Ni-rich (+7%) sample. For all samples, the deposition of the LnOx layer corresponds to a decrease in RHEED intensity, while that of the NiOx layer leads to an increase.
图 4 逐层原子覆盖度分别为116.0%, 101.5%和100.0%样品 (a) XRD θ-2θ扫描结果; (b) X射线反射率及其相应(c) R-T曲线. 样品的逐层原子覆盖度通过同比缩放LnOx与NiOx轰击脉冲数实现调控
Fig. 4. (a) XRD θ-2θ scan results, (b) X-ray reflectivity profiles, and their corresponding (c) R-T curves for samples with layer-by-layer atomic coverages of 116.0%, 101.5%, and 100.0%, respectively. The layer-by-layer atomic coverage is controlled by proportionally scaling the number of laser pulses for the LnOx and NiOx targets.
图 5 在未处理(S7)、退火处理(S8)和预沉积缓冲层(S2)衬底上生长(La, Pr, Sm)3Ni2O7薄膜 (a) 结构示意图; (b) RHEED振荡曲线; (c) XRD θ-2θ扫描结果; (d) R-T曲线. 其中, S2样品的生长臭氧分压为1.2×10–2 mbar, S7和S8样品则为2.0×10–2 mbar, 其他生长参数保持一致. S8样品较低的Tc,onset源于样品的过氧化
Fig. 5. (La, Pr, Sm)3Ni2O7 thin films grown on as-received (S7), annealed (S8), and pre-deposited buffer layer (S2) substrates: (a) Structural schematic diagrams; (b) RHEED intensity oscillations; (c) XRD θ-2θ scan results; (d) R-T curves. The ozone partial pressure during growth was 1.2×10–2 mbar for sample S2, and 2.0×10–2 mbar for samples S7 and S8, while all other growth parameters were kept consistent. The lower Tc of S8 originates from over-oxidation.
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