Evidence for Clean d-wave Superconductivity in Samarium Nickelates

摘要

利用超快太赫兹光谱技术，对T_c为20 K的无限层钐镍氧化物薄膜进行了光学泵浦-太赫兹探测实验，测量光致电导随温度的变化。在弱激发下，光致破坏的超流密度与平衡态超流密度成正比，其随温度升高呈线性下降，符合干净极限d波配对特征。从该线性关系提取出超导能隙为2.5 meV，2Δ/kTc约等于3，表明体系处于弱耦合区间。同时，独立估算得到平均自由程与相干长度之比(l/ξ)约为1.5，进一步确认干净极限行为。这些结果证明镍氧化物超导体可实现干净超导态，并揭示其配对机制与铜氧化物高温超导体存在密切相似性。

材料

• Sm₀.75Ca₀.05Eu₀.2NiO₂

方法

• ultrafast terahertz spectroscopy
• optical pump-THz probe spectroscopy

关键词

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

亮点

• 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.

结论

• 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.

主要论断

• The temperature dependence of the superfluid density is linear, indicating clean-limit d-wave pairing symmetry.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: All above evidence: linear superfluid density, weak coupling, clean limit,Previous nickelates often in dirty limit, so this is first demonstration of clean limit

研究流程

• sample_preparation — An infinite-layer samarium nickelate thin film with T_c=20 K was grown.
  • 材料: Sm₀.75Ca₀.05Eu₀.2NiO₂; LSAT substrate
  • 方法: pulsed laser deposition
  • 观察: electrical resistance drops to zero at 20 K (T_c)
• measurement — Transient THz conductivity reveals Cooper pair breaking and quasiparticle recovery dynamics.
  • 材料: ultrafast laser (1.55 eV pump); terahertz probe
  • 方法: time-resolved optical pump-THz probe spectroscopy
  • 观察: 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.
  • 方法: two-fluid model fitting; Rothwarf-Taylor model for recovery kinetics; linear fitting of superfluid density vs temperature
  • 观察: ∆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.
  • 方法: comparison with cuprate superconductor behavior
  • 观察: linear decrease consistent with d-wave nodes; weak-coupling regime indicated by gap ratio; clean-limit behavior l > ξ