Superconductor-insulator transitions in infinite-layer nickelates controlled via operando monitored reduction

摘要

该研究通过开发原位监测还原（OMR）方法，实现了对无限层镍酸盐超导体中Ni 3d轨道电子占据从约3d⁷到3d⁹的超宽范围连续调控，从而可控地驱动超导体-绝缘体转变（SIT）。结合同步辐射X射线吸收谱和扫描透射电镜对氧原子的分析，精确标定了电子占据态，并进一步利用离子液体门控和磁场调制SIT。Nernst效应测量显示，与铜氧化物不同，配对在电阻开始下降时即已启动，而迈斯纳效应仅在零电阻态才出现，标志着全局相位相干性的建立。角度依赖的磁输运研究表明，转变温度区间内超导呈现二维与三维特征的混合，表明所观察的SIT偏离了经典二维模型。这些结果在氧含量-磁场-温度参数空间内为理解无限层镍酸盐中结构与电子相变的相互作用提供了独特视角。

材料

• Nd₀.8Sr₀.2NiO₂

方法

• Operando monitored reduction (OMR)
• X-ray absorption spectroscopy (XAS)
• STEM
• Electrical transport
• Mutual inductance
• Nernst effect
• Ionic liquid gating

关键词

• superconductor insulator transition
• ni 3d orbital occupancy
• nernst effect
• phase coherence
• mixed dimensional superconductivity

亮点

• Our results provide a unique perspective for understanding the interplay between structural and electronic phase transitions in the infinite-layer nickelates across the oxygen content–magnetic field–temperature parameter space.
• The OMR technique enables ultrawide-range continuous modulation of Ni 3d orbital electron occupancy from ≈3d7 to ≈3d9.

结论

• Nernst effect measurements reveal that, unlike in cuprates, pairing initiates as soon as the resistance starts to drop, while the Meissner effect only appears in the zero-resistance state, marking the establishment of global phase coherence.
• Angle-dependent magnetotransport shows that within the transition temperature range, superconductivity exhibits a mixture of two-dimensional and three-dimensional characteristics.
• The observed SIT deviates from the canonical 2D model.

主要论断

• The operando monitored reduction (OMR) method enables continuous modulation of Ni 3d orbital electron occupancy over an ultrawide range from ≈3d7 to ≈3d9.
  • 证据: Abstract: 'B_y developing an in situ monitoring reduction (OMR) method, this study achieved continuous modulation of the Ni 3d orbital electron occupancy… from approximately 3d7 to 3d9'
• Nernst effect measurements reveal that pairing initiates at the onset of the resistive drop, distinct from cuprates where fluctuations extend above T_c.
  • 证据: Abstract: 'Nernst effect measurements reveal that, unlike in cuprates, pairing initiates as soon as the resistance starts to drop'

研究流程

• Sample preparation and operando reduction — OMR enables precise control of oxygen content and electron occupancy
  • 材料: Nd₀.8Sr₀.2NiO₃ thin films on LSAT (001)
  • 方法: Pulse laser deposition; Operando monitored reduction (OMR) with CaH₂
  • 观察: Continuous modulation of Ni 3d occupancy from ≈3d7 to ≈3d9
• Multi-probe characterization — Pairing initiates at the onset of resistive drop, distinct from cuprates
  • 材料: Reduced films at different stages
  • 方法: X-ray absorption spectroscopy (XAS); Scanning transmission electron microscopy (STEM); Transport, Nernst, angle-dependent magnetoresistance
  • 观察: SIT tuned by oxygen content, ionic liquid gating, and magnetic field; Nernst effect shows pairing begins at resistive drop, Meissner effect at zero resistance
• Dimensionality analysis — SIT deviates from canonical 2D model
  • 材料: Angle-dependent magnetoresistance data
  • 方法: Combined 2D Tinkham and 3D GL model
  • 观察: Mixed 2D and 3D superconducting characters