Superconducting dome and field-enhanced superconductivity of PLD synthesized Nd₁₋ₓEuₓNiO₂ thin films

摘要

研究人员采用脉冲激光沉积结合氢化钙拓扑还原法，成功制备了一系列无限层结构 Nd_1-xEu_xNiO₂ 薄膜，掺杂范围扩展至 x=0–0.7。电输运测量揭示在 0.2≤x≤0.5 区间存在一个超导穹顶，其掺杂宽度大于分子束外延制备的样品，与化学溶液法相当。其中 x=0.3 的薄膜表现出最优超导转变温度约 31 K，显著高于其他真空外延技术所得数值，表明脉冲激光沉积是制备高质量、高转变温度该类镍氧化物超导薄膜的有效途径。磁输运实验在欠掺杂和过掺杂区域均观测到强健的磁场增强与重入超导行为，这归因于 Eu²⁺ 局域磁矩在外场下的极化产生内部交换场，部分补偿外场所致；仅靠 Jaccarino-Peter 效应不足以完全解释该现象，暗示存在其他机制。在刚高于起始超导转变温度的低温区，霍尔电阻呈现非线性特征，且未伴随明显的磁滞，这可能源于磁性杂质散射。这些结果突出了磁性稀土 Eu²⁺ 离子在赋予无限层镍氧化物奇异物理属性中的关键作用。

材料

• Nd_1-xEu_xNiO₂

方法

• PLD
• Topotactic reduction with CaH₂
• X-ray diffraction
• Resistivity measurements
• Hall effect measurements
• Magneto-transport measurements
• STEM
• Electron energy-loss spectroscopy
• Energy-dispersive X-ray spectroscopy

关键词

• superconducting dome
• field enhanced superconductivity
• re entrant superconductivity
• jaccarino peter effect
• nonlinear hall effect
• magnetic impurity scattering

亮点

• PLD-grown films exhibit a wider superconducting dome than those grown by MBE, comparable to chemical synthesis.
• Field-enhanced superconductivity is observed in the underdoped regime, where the T_c under 8 T in-plane field exceeds the zero-field value.
• Re-entrant superconductivity emerges at the overdoped edge (x=0.5).
• Nonlinear Hall resistance appears across a wide doping range at low temperatures, attributed to magnetic impurity scattering.
• The crucial role of magnetic rare-earth Eu₂+ ions in producing exotic physical properties is highlighted.

结论

• Pulsed laser deposition is an effective approach for fabricating high-quality, high-T_c Nd_1-xEu_xNiO₂ superconducting films with a wide superconducting dome (0.2 ≤ x ≤ 0.5).
• The optimal T_c of ≈31 K at x=0.3 is higher than those from other vacuum epitaxial techniques.
• Robust field-enhanced and re-entrant superconductivity are observed in both underdoped and overdoped regimes.
• The Jaccarino-Peter effect alone cannot fully explain the field-enhanced superconductivity; additional mechanisms are indicated.
• Nonlinear Hall effect in the low-temperature normal state may originate from magnetic impurity scattering.

主要论断

• PLD-synthesized Nd_1-xEu_xNiO₂ films exhibit a superconducting dome in 0.2≤x≤0.5, wider than MBE and comparable to chemical synthesis, with optimal T_c ≈31 K at x=0.3.
  • 证据: Resistivity measurements reveal a superconducting dome within the doping range 0.2≤x≤0.5 (Fig. 2).,The x=0.3 PLD film exhibits the optimal superconducting transition temperature T_c≈31 K (Fig. 2b).,The PLD films exhibit a much wider superconducting dome than those grown by MBE and comparable to those obtained via chemical synthesis (Fig. 5b).
• Field-enhanced and re-entrant superconductivity are observed, and the Jaccarino-Peter effect alone cannot fully explain the field-enhanced behavior, suggesting additional mechanisms.
  • 证据: For Nd₀.8Eu₀.2NiO₂ in an in-plane field, the kink at ≈3T is pronounced and the superconducting transition temperature T_c at 8 T even exceeds the zero-field value.,This indicates that the Jaccarino-Peter effect alone cannot fully explain the field-enhanced superconductivity, pointing to additional mechanisms.,Re-entrant superconductivity appears at the overdoped edge (x=0.5) (Fig. 4c).
• The nonlinear Hall resistance observed at low temperatures in both underdoped and overdoped regimes likely originates from magnetic impurity scattering.
  • 证据: At low temperatures just above the onset T_c, the Hall resistance exhibits nonlinear behavior (Fig. 3b-g).,No appreciable hysteresis is observed in either Rx_x or Rx_y in the low-temperature normal state.,Early studies on paramagnetic systems established that magnetic impurity scattering can generate an anomalous Hall voltage.

研究流程

• sample_preparation — Successful synthesis of infinite-layer Nd_1-xEu_xNiO₂ thin films with x up to 0.7
  • 材料: Nd(NO₃)₃·6H₂O; Eu(NO₃)₃·6H₂O; Ni(NO₃)₂·6H₂O; citric acid; nitric acid; LSAT substrate; CaH₂ powder; Al foil; quartz tube
  • 方法: sol-gel synthesis of polycrystalline Nd_1-xEu_xNiO₃ targets; pulsed laser deposition (PLD) of perovskite precursor films on LSAT substrate; topotactic reduction with CaH₂ at 290°C in sealed quartz tube
  • 观察: XRD shows shift of (001) and (002) reflections confirming formation of infinite-layer phase
• structural_characterization — Epitaxial, fully strained infinite-layer films with doping-dependent structural features
  • 材料: as-grown thin films
  • 方法: X-ray diffraction (XRD) θ–2θ scans; reciprocal space maps (RSM); scanning transmission electron microscopy (STEM) with HAADF, EELS, EDS
  • 观察: c-axis lattice constant varies non-monotonically with x; films are epitaxially coherent and fully strained to LSAT substrate; Ruddlesden-Popper stacking faults confirmed by STEM
• transport_measurements — Comprehensive electrical transport data reveal superconducting dome, anomalous Hall effect, and field-induced phenomena
  • 材料: Nd_1-xEu_xNiO₂ films with wire-bonded Al contacts
  • 方法: four-probe resistivity; six-probe Hall effect; magneto-transport in magnetic fields up to 9 T (and up to 35.6 T for high-field)
  • 观察: resistivity: superconducting transitions for x=0.2-0.5, metallic behavior for x=0.3-0.5, upturns for underdoped/overdoped; Hall coefficient: sign reversal for x>0.3, nonlinear Hall resistance at low T without hysteresis; upper critical field: field-enhanced superconductivity kinks for x=0.2,0.4; re-entrant for x=0.5; optimal film x=0.3 has T_c onset ≈31.3 K, zero ≈24.2K
• analysis — PLD-fabricated Nd_1-xEu_xNiO₂ films exhibit a broad superconducting dome with enhanced T_c, establishing PLD as an effective route
  • 材料: transport data
  • 方法: linear fit for Hall coefficient; resistivity exponent α extraction; phase diagram compilation; comparison with literature data
  • 观察: superconducting dome: 0.2≤x≤0.5, optimal T_c≈31 K at x=0.3; strange-metal regime extends across dome, persists to overdoped edge; asymmetric tail in overdoped region suggests hidden quantum critical point; PLD dome wider than MBE, comparable to chemical synthesis; optimal T_c higher than other vacuum methods
• interpretation — Field-enhanced superconductivity likely involves additional mechanisms beyond J-P effect; magnetic rare-earth Eu₂+ ions play a crucial role
  • 方法: theoretical consideration of Jaccarino-Peter effect; comparison with (Sm,Eu,Ca,Sr)NiO₂ system
  • 观察: Field-enhanced superconductivity for x=0.2: T_c at 8 T in-plane exceeds zero-field value; J-P effect alone cannot explain the phenomenon