Evidence for Clean d-wave Superconductivity in Samarium Nickelates

Summary

Using ultrafast terahertz spectroscopy, we performed optical-pump terahertz-probe experiments on an infinite-layer samarium nickel oxide thin film with T_c = 20 K to measure the temperature-dependent photoconductivity. Under weak excitation, the photoinduced destruction of the superfluid density is proportional to the equilibrium superfluid density and decreases linearly with increasing temperature, consistent with clean-limit d-wave pairing. From this linear relationship, the superconducting gap was extracted to be 2.5 meV, yielding 2Δ/kTc ≈ 3, indicating the system is in the weak coupling regime. Furthermore, independent estimates of the ratio of the mean free path to the coherence length (l/ξ) give approximately 1.5, further confirming clean-limit behavior. These results demonstrate that nickel oxide superconductors can realize a clean superconducting state and reveal a close similarity in pairing mechanism to cuprate high-temperature superconductors.

Materials

• Sm₀.75Ca₀.05Eu₀.2NiO₂

Methods

• ultrafast terahertz spectroscopy
• optical pump-THz probe spectroscopy

Keywords

• d wave superconductivity
• nickelate superconductors
• superfluid density
• pairing symmetry
• clean limit
• weak coupling
• quasiparticle recombination
• infinite layer nickelates

Highlights

• Our results demonstrate that nickelates can realize a clean superconducting regime, establishing a closer parallel to cuprates.
• The ability of time-resolved THz spectroscopy to selectively probe the superfluid response provides a broadly applicable approach for investigating superconductivity in systems where equilibrium measurements are hindered by low superfluid density or strong normal-state contributions.
• This is the first evidence for clean-limit d-wave superconductivity in nickelates.

Conclusions

• Temperature dependent measurements provide evidence for d-wave pairing symmetry in the clean limit, closely mirroring the behavior in cuprate superconductors.
• The superfluid density decreases linearly with rising temperature, consistent with clean-limit d-wave pairing, and the superconducting gap is extracted to be 2.5 ± 0.1 meV, yielding a gap-to-T_c ratio of ≈3, indicating weak coupling.
• The ratio of the mean free path to the coherence length is determined to be ≈1.5, confirming clean-limit behavior.

Main claims

• The temperature dependence of the superfluid density is linear, indicating clean-limit d-wave pairing symmetry.
  • Evidence: The superfluid density decreases linearly with rising temperature (Fig. 4),Linear temperature dependence of ∆ρs(T)/∆ρs(0) from near T_c down to 5 K,Previous studies show clean-limit d-wave leads to linear dependence
• The superconductor is in the weak-coupling regime with 2∆/kTc ≈ 3.
  • Evidence: Extracted superconducting gap ∆(0)=2.5±0.1 meV,Gap-to-T_c ratio 2∆(0)/kTc ≈ 3
• The sample is in the clean limit with l/ξ ≈ 1.5.
  • Evidence: Ratio derived from scattering rate and gap: l/ξ = π∆(0)/ℏτ ≈ 1.5,Scattering rate τ = 8.3 THz from two-fluid fits
• Nickelate superconductors can realize a clean superconducting state, establishing strong parallels to cuprates.
  • Evidence: All above evidence: linear superfluid density, weak coupling, clean limit,Previous nickelates often in dirty limit, so this is first demonstration of clean limit

Workflow

• sample_preparation — An infinite-layer samarium nickelate thin film with T_c=20 K was grown.
  • Materials: Sm₀.75Ca₀.05Eu₀.2NiO₂; LSAT substrate
  • Methods: pulsed laser deposition
  • Observations: electrical resistance drops to zero at 20 K (T_c)
• measurement — Transient THz conductivity reveals Cooper pair breaking and quasiparticle recovery dynamics.
  • Materials: ultrafast laser (1.55 eV pump); terahertz probe
  • Methods: time-resolved optical pump-THz probe spectroscopy
  • Observations: photoinduced change in THz field shows spectral weight transfer from superfluid to Drude response; recovery kinetics follow bimolecular recombination
• analysis — Linear temperature dependence of superfluid density and l/ξ ratio confirm clean-limit d-wave pairing.
  • Methods: two-fluid model fitting; Rothwarf-Taylor model for recovery kinetics; linear fitting of superfluid density vs temperature
  • Observations: ∆ρs(T)/∆ρs(0) shows linear temperature dependence from near T_c down to 5K; mean free path to coherence length ratio l/ξ ≈ 1.5; extracted superconducting gap ∆(0)=2.5±0.1 meV, gap-to-T_c ratio 2∆(0)/kTc ≈ 3
• interpretation — The samarium nickelate superconductor exhibits clean-limit d-wave pairing symmetry, closely paralleling cuprates.
  • Methods: comparison with cuprate superconductor behavior
  • Observations: linear decrease consistent with d-wave nodes; weak-coupling regime indicated by gap ratio; clean-limit behavior l > ξ