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

Summary

Researchers successfully synthesized a series of infinite-layer Nd_1-xEu_xNiO₂ thin films with doping extended to x = 0–0.7 using pulsed laser deposition combined with calcium hydride topotactic reduction. Electrical transport measurements reveal a superconducting dome in the range 0.2 ≤ x ≤ 0.5, whose doping width is larger than that of samples fabricated by molecular beam epitaxy and comparable to that achieved by chemical solution methods. The film with x = 0.3 exhibits an optimal superconducting transition temperature of about 31 K, significantly higher than values obtained by other vacuum epitaxy techniques, indicating that pulsed laser deposition is an effective route for preparing high-quality, high-transition-temperature nickelate superconducting thin films. Magnetotransport experiments observe robust magnetic-field-enhanced and re-entrant superconductivity in both underdoped and overdoped regions, attributed to the polarization of Eu²⁺ local magnetic moments under an external field that generates an internal exchange field partially compensating the applied field; the Jaccarino-Peter effect alone cannot fully explain this phenomenon, suggesting the existence of additional mechanisms. In the low-temperature region just above the onset superconducting transition temperature, the Hall resistance exhibits a nonlinear character without noticeable magnetic hysteresis, which may arise from magnetic impurity scattering. These results highlight the critical role of magnetic rare-earth Eu²⁺ ions in imparting exotic physical properties to infinite-layer nickelates.

Materials

• Nd_1-xEu_xNiO₂

Methods

• 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

Keywords

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

Highlights

• 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.

Conclusions

• 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.

Main claims

• 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.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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.

Workflow

• sample_preparation — Successful synthesis of infinite-layer Nd_1-xEu_xNiO₂ thin films with x up to 0.7
  • Materials: Nd(NO₃)₃·6H₂O; Eu(NO₃)₃·6H₂O; Ni(NO₃)₂·6H₂O; citric acid; nitric acid; LSAT substrate; CaH₂ powder; Al foil; quartz tube
  • Methods: 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
  • Observations: 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
  • Materials: as-grown thin films
  • Methods: X-ray diffraction (XRD) θ–2θ scans; reciprocal space maps (RSM); scanning transmission electron microscopy (STEM) with HAADF, EELS, EDS
  • Observations: 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
  • Materials: Nd_1-xEu_xNiO₂ films with wire-bonded Al contacts
  • Methods: 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)
  • Observations: 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
  • Materials: transport data
  • Methods: linear fit for Hall coefficient; resistivity exponent α extraction; phase diagram compilation; comparison with literature data
  • Observations: 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
  • Methods: theoretical consideration of Jaccarino-Peter effect; comparison with (Sm,Eu,Ca,Sr)NiO₂ system
  • Observations: 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