Thermodynamic phase transition, pairing symmetry and Fermi surface topology in Ruddlesden-Popper nickelate films

摘要

研究人员利用角分辨光电子能谱研究了Ruddlesden-Popper相镍酸盐异质结La₂PrNi₂O₇/NdAlO₃的电子结构，首次在无赝能隙状态下观测到在超导转变温度T_C处打开的超导能隙，伴随显著的相干峰，直接获得约16 meV的无节点超导序参量。通过动量积分谱重提取电子态密度，观测到T_C附近态密度陡降及电子比热跳变，为镍酸盐超导相变提供了缺失的热力学证据。费米面由α、β和γ三个多轨道口袋构成；不同外延应变下的薄膜中γ口袋均存在，表明其与超导并无一一对应关系，但陡峭的γ带色散及显著的β口袋变化提示费米面拓扑与超导涌现之间存在关联。上述结果确立了无节点配对对称性（支持s波而非d波），揭示费米面拓扑、应变与超导电性之间的关系，为理解RP镍酸盐高温超导机理提供了直接实验依据。

材料

• La₂PrNi₂O₇
• La₂PrNi₂O₇/NdAlO₃
• La₂PrNi₂O₇/SrLaAlO₄
• La₂PrNi₂O₇/LaAlO₃
• La₂PrNi₂O₇/(La、Sr)(Al、Ta)O₃

方法

• ARPES
• DFT
• Resistivity measurement

关键词

• superconductivity
• nodeless superconducting gap
• s wave pairing symmetry
• thermodynamic phase transition
• electronic specific heat jump
• fermi surface topology
• epitaxial strain
• hole doping
• multi orbital
• pseudogap absence

亮点

• First observation of a superconducting gap opening at T_c without a pseudogap in Ruddlesden-Popper nickelate films, enabling direct measurement of the order parameter.
• Microscopic extraction of electronic specific heat from momentum-integrated ARPES spectra reveals a specific heat jump, providing the previously missing thermodynamic evidence for the superconducting transition.
• Nodeless superconducting gap unambiguously established through coherence peaks and temperature-dependent spectra, supporting s-wave (s±) pairing.
• Strain-dependent ARPES demonstrates that the γ Fermi pocket exists in all films irrespective of superconductivity, refuting its sole requirement but indicating a contributing role when combined with other factors.
• The combination of steep γ-band dispersion induced by hole doping and strain-tuned β-band shifts provides new insight into the Fermi surface conditions necessary for high-T_c superconductivity in nickelates.

结论

• The superconducting gap in La₂PrNi₂O₇ thin films is nodeless with coherence peaks, indicating an s-wave pairing symmetry rather than d-wave.
• The temperature evolution of the gap and the extracted electronic specific heat show a jump at T_c, providing thermodynamic evidence for the superconducting phase transition.
• The absence of a pseudogap allows direct measurement of the superconducting order parameter and microscopic extraction of the electronic specific heat coefficient.
• The γ Fermi pocket (dz2 orbital) is present in both superconducting and non-superconducting films under different strains, ruling out a one-to-one correspondence with superconductivity but suggesting it participates when superconductivity appears.
• Epitaxial strain significantly alters the β pocket size, while hole doping shifts the γ band to a steep dispersion near EF, influencing Fermi surface topology and likely affecting the emergence of superconductivity.

主要论断

• The superconducting order parameter is nodeless, consistent with s-wave symmetry, and opens at T_c without a pseudogap.
  • 证据: Symmetrized EDC at k_F shows a finite gap of ≈16 meV with prominent coherence peaks.,Gap closes above T_c with no signature of pseudogap.,Temperature-dependent ARPES along the BZ diagonal confirms a nodeless gap.
• A thermodynamic superconducting phase transition is demonstrated by an electronic specific heat jump at T_c extracted from ARPES data.
  • 证据: Momentum-integrated DOS exhibits a sudden drop at the Fermi level below T_c.,Normalized DOS at EF drops and its temperature derivative peaks at ~T_c.,Specific heat coefficient γ_s calculated both from DOS and directly from the measured gap Δ shows a jump at T_c.
• The γ Fermi pocket (dz2 orbital) is present in all films but does not directly correlate with superconductivity; rather, strain-driven changes in the β pocket and other coupling mechanisms are likely more important.
  • 证据: γ pocket observed in superconducting films on NdAlO₃ and SrLaAlO₄ and in non-superconducting films on LaAlO₃ and LSAT.,β pocket size varies substantially with substrate strain, whereas γ pocket changes only moderately.,DFT calculations incorporating hole doping reproduce the steep γ dispersion and strain sensitivity of β band.

研究流程

• sample_preparation — Superconducting and non-superconducting La₂PrNi₂O₇ thin films are grown on various substrates with different epitaxial strain.
  • 材料: La₀.67Pr₀.33O_x target; NiO_x target; NdAlO₃(001) substrate; SrLaAlO₄(001) substrate; LaAlO₃(001) substrate; (La,Sr)(Al,Ta)O₃(001) substrate
  • 方法: energy-switchable ozone-assisted atomic-layer-by-layer epitaxy (pulsed laser deposition); in situ reflection high-energy electron diffraction (RHEED) monitoring
  • 观察: High-quality epitaxial thin films with controlled stoichiometry and superlattice structure
• transport_measurement — The film shows robust superconductivity with an onset T_c of 60 K.
  • 材料: La₂PrNi₂O₇/NdAlO₃ film
  • 方法: temperature-dependent resistivity measurement
  • 观察: onset of superconductivity at ≈60 K, zero resistance at ≈33K
• arpes_measurement — ARPES directly resolves a nodeless superconducting gap that opens at T_c and characterizes the multiorbital Fermi surface.
  • 材料: La₂PrNi₂O₇ thin films on various substrates
  • 方法: laser-based ARPES (photon energy 6.994 eV, resolution ≈2.5 meV); synchrotron-based ARPES (photon energies 63 eV, 103 eV)
  • 观察: Fermi surface maps show α, β, γ pockets; photoelectron intensity along BZ diagonal reveals a superconducting gap; symmetrized EDCs at k_F exhibit coherence peaks; gap magnitude ≈16 meV at 10K; gap closes above T_c without pseudogap; γ band observed in all films with steep dispersion
• data_analysis — The electronic specific heat jump provides direct thermodynamic evidence for the superconducting phase transition.
  • 材料: ARPES spectra from La₂PrNi₂O₇/NdAlO₃
  • 方法: momentum integration of photoemission spectra to obtain electron density of states (DOS); temperature derivative analysis of DOS at EF; extraction of electronic specific heat coefficient γ_s using BCS density of states with measured Δ(T)
  • 观察: DOS at EF drops suddenly across T_c (≈60 K); temperature derivative of normalized DOS peaks at T_c; γ_s from both methods shows a jump at T_c
• strain_dependent_comparison — The existence of the γ pocket alone does not guarantee superconductivity; strain-induced changes in the β pocket and other factors like interlayer coupling or electron-boson coupling are likely critical.
  • 材料: La₂PrNi₂O₇ films on NdAlO₃, SrLaAlO₄, LaAlO₃, LSAT
  • 方法: comparative ARPES measurements along high-symmetry directions; density functional theory (DFT) calculations with Wannierization
  • 观察: γ pocket is present in all films regardless of superconductivity; β pocket size changes significantly with strain; hole doping is inferred compared to bulk La₃Ni₂O₇; steep dispersion of γ band near EF in films
• interpretation — These results collectively provide the missing thermodynamic evidence for the superconducting phase transition in nickelates, establish nodeless s-wave pairing symmetry, and clarify the role of Fermi surface topology and epitaxial strain in the emergence of superconductivity.