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

Summary

Using angle-resolved photoemission spectroscopy, the electronic structure of the Ruddlesden-Popper phase nickelate heterostructure La₂PrNi₂O₇/NdAlO₃ was investigated, and for the first time a superconducting gap opening at T_c was observed in the absence of a pseudogap, accompanied by pronounced coherent peaks, directly yielding a nodeless superconducting order parameter of approximately 16 meV. B_y reconstructing the electronic density of states from momentum-integrated spectra, a sharp drop and an electronic specific heat jump near T_c were revealed, providing the previously missing thermodynamic evidence for the superconducting phase transition in nickelates. The Fermi surface consists of three multiorbital pockets, α, β, and γ; while the γ pocket persists under various epitaxial strains and thus shows no one-to-one correspondence with superconductivity, its steep dispersion and the significant changes of the β pocket suggest a link between Fermi surface topology and the onset of superconductivity. These results establish a nodeless pairing symmetry (supporting s-wave rather than d-wave) and clarify the interplay among Fermi surface topology, strain, and superconductivity, offering direct experimental insight into the mechanism of high-temperature superconductivity in RP nickelates.

Materials

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

Methods

• ARPES
• DFT
• Resistivity measurement

Keywords

• 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

Highlights

• 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.

Conclusions

• 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.

Main claims

• The superconducting order parameter is nodeless, consistent with s-wave symmetry, and opens at T_c without a pseudogap.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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.

Workflow

• sample_preparation — Superconducting and non-superconducting La₂PrNi₂O₇ thin films are grown on various substrates with different epitaxial strain.
  • Materials: La₀.67Pr₀.33O_x target; NiO_x target; NdAlO₃(001) substrate; SrLaAlO₄(001) substrate; LaAlO₃(001) substrate; (La,Sr)(Al,Ta)O₃(001) substrate
  • Methods: energy-switchable ozone-assisted atomic-layer-by-layer epitaxy (pulsed laser deposition); in situ reflection high-energy electron diffraction (RHEED) monitoring
  • Observations: 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.
  • Materials: La₂PrNi₂O₇/NdAlO₃ film
  • Methods: temperature-dependent resistivity measurement
  • Observations: 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.
  • Materials: La₂PrNi₂O₇ thin films on various substrates
  • Methods: laser-based ARPES (photon energy 6.994 eV, resolution ≈2.5 meV); synchrotron-based ARPES (photon energies 63 eV, 103 eV)
  • Observations: 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.
  • Materials: ARPES spectra from La₂PrNi₂O₇/NdAlO₃
  • Methods: 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)
  • Observations: 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.
  • Materials: La₂PrNi₂O₇ films on NdAlO₃, SrLaAlO₄, LaAlO₃, LSAT
  • Methods: comparative ARPES measurements along high-symmetry directions; density functional theory (DFT) calculations with Wannierization
  • Observations: 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.