Evolution of the Superfluid Density in Infinite-Layer Nickelates

Summary

This paper systematically measures the superfluid density of the infinite-layer nickel-based superconductor Nd_1-xSr_xNiO₂ within the doping superconducting dome using the mutual inductance method. The results show that the superfluid stiffness is weak and exhibits an approximate square-root relationship with the superconducting transition temperature T_c. Additionally, a strong interaction between the Nd 4f magnetic moments and the superfluid is observed, leading to a significant suppression of the superfluid density at low temperatures, with an effect far beyond simple paramagnetic explanations, suggesting a coupling between magnetic order and the superconducting phase. These findings indicate that superconducting phase fluctuations play an important role in limiting T_c and reveal an unexpectedly strong coupling between rare-earth magnetic ions and the superfluid.

Materials

• Nd_1-xSr_xNiO₂

Methods

• mutual inductance technique
• complex conductance measurement
• Berezinskii-Kosterlitz-Thouless (BKT) analysis
• superfluid density analysis
• penetration depth measurement

Keywords

• superfluid density
• superfluid stiffness
• phase fluctuations
• bkt transition
• nd magnetism
• superfluid suppression
• square root scaling
• uemura plot

Highlights

• The low-temperature suppression of superfluid density is substantially larger than in cuprates and does not show Curie-Weiss behavior, suggesting magnetic ordering of Nd moments.
• The superfluid stiffness is weak and the correlation between T_c and superfluid density follows a square-root dependence across the dome, similar to cuprates in the low-T_c, low-superfluid-density corner.

Conclusions

• We observe a weak superfluid stiffness that exhibits an approximately square-root correlation with T_c.
• We also find a strong interplay between Nd magnetism and the superconducting phase, manifested as a substantial low-temperature suppression of superfluid density.
• These observations highlight the importance of superconducting phase fluctuations in limiting T_c and unexpectedly strong coupling between the Nd 4f moments and the superfluid.

Main claims

• The superfluid stiffness is weak and exhibits an approximately square-root correlation with T_c across the doping dome.
  • Evidence: abstract: 'weak superfluid stiffness that exhibits an approximately square-root correlation with T_c',full_text Fig. 3b,c: T_c ∝ (λ₀^-2)₀.43±0.03
• Strong interplay between Nd 4f moments and superfluid causes substantial low-temperature suppression of superfluid density, beyond simple paramagnetic effects.
  • Evidence: abstract: 'strong interplay between Nd magnetism and the superconducting phase, manifested as a substantial low-temperature suppression of superfluid density',full_text Fig. 1b shows clear suppression below ≈4 K; Fig. 2a shows doping-dependent T(λ_min)
• Superconducting phase fluctuations play an important role in limiting T_c, as T_Θ/T_c approaches unity near the dome edges.
  • Evidence: abstract: 'importance of superconducting phase fluctuations in limiting T_c',full_text Fig. 2e shows T_c drops rapidly as T_Θ/T_c approaches 1
• The square-root scaling of T_c with superfluid density in nickelates mirrors the behavior in cuprates at low T_c/low superfluid density, suggesting a common origin.
  • Evidence: full_text Fig. 4 comparison with LSCO and YBCO shows similar scaling,full_text: 'the square-root scaling emerges near the low-T_c, low-superfluid-density corner'

Workflow

• sample_preparation — Samples span the entire doping-dependent dome.
  • Materials: Nd_1-xSr_xNiO₂ thin films (x = 0.12 to 0.25); LSAT substrates with STO capping
  • Methods: pulsed laser deposition (PLD); topochemical reduction
  • Observations: High-quality doping series across the superconducting dome
• mutual_inductance_measurements — Systematic mapping of superfluid density across the dome.
  • Materials: Nd_1-xSr_xNiO₂ films
  • Methods: two-coil mutual inductance technique; measure complex conductance σ₁ - iσ₂
  • Observations: Superfluid density vs temperature curves; BKT transition identified; Low-temperature suppression of superfluid density
• analysis — T_c is limited by superfluid stiffness and phase fluctuations.
  • Materials: mutual inductance data
  • Methods: BKT renormalization group fits; quadratic fits to extract λ₀^-2; power-law scaling analysis
  • Observations: λ₀^-2 ≈ 1-2.5 µm^-2; T_c ∝ √λ₀^-2 scaling; T_Θ/T_c ratio indicates phase fluctuation importance