Preparation and optimization of high-temperature superconducting Ruddlesden-Popper nickelate thin films

摘要

本研究采用巨大氧化原子层外延（GAE）方法在LaAlO₃和SrLaAlO₄衬底上成功制备了相纯、高质量的Ruddlesden-Popper镍酸盐Ln₃Ni₂O₇薄膜。在超强氧化性臭氧气氛下生长的薄膜无需后续退火即可超导，其中优化的Ln₃Ni₂O₇/SrLaAlO₄薄膜起始转变温度（T_c,onset）可达50 K。系统研究发现四个关键因素决定薄膜结晶质量和超导性能：精确控制阳离子化学计量可抑制二次相生成；完整的原子层逐层覆盖结合优化的界面重构能减少堆垛层错；准确调控氧含量是实现单一超导转变和高T_c,onset的必要条件。研究还发现，阳离子化学计量偏离会导致Ni-rich或Ni-deficient二次相形成，分别引发金属-绝缘体转变或高度绝缘行为，而原子层覆盖率偏差（如101.5%）虽仍可维持超导但产生残余电阻。界面重构（如预沉积半晶胞Ln₂NiO₄或对SrLaAlO₄衬底进行退火处理）可显著改善薄膜结晶性。这些结果为高质量氧化物高温超导薄膜的逐层外延生长提供了重要指导。

材料

• (La、Pr、Sm)₃Ni₂O₇
• La₃Ni₂O₇

方法

• gigantic-oxidative atomic-layer-by-layer epitaxy (GAE)
• X-ray diffraction (XRD)
• X-ray reflectivity (XRR)
• reflection high-energy electron diffraction (RHEED)
• transport measurements (R-T)
• magnetron sputtering for electrode deposition

关键词

• cation stoichiometry
• atomic layer coverage
• interface reconstruction
• oxygen content
• stacking faults
• secondary phase
• superconducting transition temperature

亮点

• Films grown under an ultrastrong oxidizing ozone atmosphere are superconducting without further post annealing.
• The optimal Ln₃Ni₂O₇/SrLaAlO₄ superconducting film exhibits an onset transition temperature (T_c,onset) of 50 K.
• Deviations in cation stoichiometry lead to the formation of Ni-rich or Ni-deficient secondary phases, inducing metal-insulator transitions or highly insulating behavior, respectively.
• Interface reconstruction, such as predisposing half-unit-cell Ln₂NiO₄ or annealing the SrLaAlO₄ substrate, significantly improves film crystallinity.

结论

• This study systematically investigates four critical factors governing the crystalline quality and superconducting properties of Ln₃Ni₂O₇ thin films: cation stoichiometric precision, atomic layer coverage, interface reconstruction, and oxidation conditions.
• Accurate cation stoichiometry suppresses the formation of secondary RP phases, enhancing crystalline phase purity.
• Precise atomic layer coverage significantly mitigates stacking fault generation, thereby reducing XRD peak splitting and anomalous position shifts.
• For SLAO substrates, thermal annealing or pre-deposition of a half-unit-cell 214-phase buffer layer enhances layer-by-layer growth at the interface, which can substantially improve overall crystalline structure and superconducting performance.
• Under optimal ozone partial pressure, samples exhibit maximized T_c with single sharp superconducting transition.

主要论断

• Precise cation stoichiometric control suppresses secondary phases (n=1 or n=3 RP phases) and is essential for superconductivity.
  • 证据: full_text Fig. 2: Ni-rich (S₁) shows 4310 phase; Ni-deficient (S₃) shows 214 phase; only stoichiometric (S₂) superconducts,abstract: 'precise cation stoichiometric control suppresses secondary phase formation'
• Complete atomic layer coverage (100%) minimizes stacking faults; deviations (e.g., 101.5%) still allow superconductivity but with residual resistivity.
  • 证据: full_text Fig. 4: S₄ (116%) shows peak splitting; S₅ (101.5%) shows asymmetric peaks; S₆ (100%) symmetric,S₅ still superconducting but with residual resistivity
• Interface reconstruction (via annealed substrate or pre-deposited half-UC buffer) significantly improves crystallinity and superconducting performance.
  • 证据: full_text Fig. 5: as-received substrate gives insulating film; annealed or buffered substrate gives superconducting film with complete XRD peaks,abstract: 'optimized interface reconstruction minimizes stacking faults'
• Accurate oxygen content (optimal ozone partial pressure ≈1.2×10^-2 mbar) is critical for single sharp superconducting transition and high T_c,onset.
  • 证据: full_text Fig. 6: under-oxidized and over-oxidized show two-step transitions; optimally oxidized gives single transition at 50 K,abstract: 'accurate oxygen content regulation is essential'

研究流程

• film_growth — GAE enables single-phase growth without post-annealing.
  • 材料: Ln₃Ni₂O₇ (Ln = La, Pr, Sm); substrates: LaAlO₃ (LAO), SrLaAlO₄ (SLAO)
  • 方法: gigantic-oxidative atomic-layer-by-layer epitaxy (GAE); pulsed laser deposition of Ln₂O₃ and NiO_x targets
  • 观察: RHEED oscillations indicate precise layer coverage
• characterization — Four critical factors govern quality: stoichiometry, coverage, interface, oxygen content.
  • 材料: as-grown Ln₃Ni₂O₇ films with varying stoichiometry, coverage, interface, oxidation
  • 方法: X-ray diffraction (XRD) θ-2θ scans; X-ray reflectivity (XRR); transport measurements
  • 观察: Optimal films show sharp XRD peaks and T_c,onset up to 50K