Summary
This study addresses the discrepancy between bulk La3Ni2O7 with high transition temperatures under high pressure and its thin-film counterpart, which superconducts at ambient pressure but with a reduced Tc, by developing a symmetry-based phenomenological approach combined with DFT+U calculations and experimentally determined Tc and structural symmetries to analyze the superconducting gap structure. The results reveal that both systems exhibit s±-wave pairing symmetry and two-band superconductivity, but the dominant microscopic pairing configurations differ: in pressurized bulk, superconductivity is primarily governed by out-of-plane pairing of Ni-dz2 orbitals, whereas in thin films, in-plane pairing of Ni-dx2-y2 orbitals dominates. The reduction in Tc is attributed to a decreased ratio of interlayer to intralayer hopping in the film, which shifts the dominant pairing type from out-of-plane to in-plane. These findings highlight the crucial role of symmetry in unconventional superconductivity, and the developed methodology is expected to be extendable to other unconventional superconductors.
Materials
Methods
- DFT
- Bogoliubov–de Gennes theory
- Symmetry analysis
- BCS mean-field theory
Keywords
- s± wave pairing
- two gap superconductivity
- interlayer pairing
- orbital selectivity
- fermi surface reconstruction
- pairing symmetry
Highlights
- Developed a phenomenological symmetry-based method that can be generalized to other unconventional superconductors.
- Calculated spectra show close agreement with ARPES and STM/STS experimental measurements.
- The dominant pairing type transition explains the approximately halved Tc in thin films relative to pressurized bulk.
Conclusions
- Both pressurized bulk and thin-film La3Ni2O7 exhibit s±-wave pairing symmetry and two-gap superconductivity.
- In pressurized bulk, superconductivity is dominated by out-of-plane pairing of Ni-dz2 orbitals; in thin films, in-plane pairing of Ni-dx2-y2 orbitals dominates.
- The reduction in Tc in thin films is attributed to a decreased ratio of inter-layer to intra-layer hoppings, shifting the dominant pairing type.
Main claims
- Both pressurized bulk and thin-film La3Ni2O7 exhibit s±-wave pairing symmetry and two-gap superconductivity.
- Evidence: Symmetry analysis and free energy minimization identify s± pairing; gap function on Fermi surfaces shows sign change between pockets
- In pressurized bulk, superconductivity is dominated by out-of-plane pairing of Ni-d_z2 orbitals; in thin film, in-plane pairing of Ni-dx2-y2 orbitals prevails.
- Evidence: Free energy comparison shows different leading pairing types; parameter analysis shows decreased interlayer-to-intralayer hopping ratio in thin film
- The reduced T_c in thin film (half of bulk) is due to transition of dominant pairing type driven by decreased interlayer/intralayer hopping ratio.
- Evidence: Calculations show dominant pairing type shift leads to lower T_c; consistent with experimental dependence on lattice constants
Workflow
- model_construction — Effective tight-binding model for La3Ni2O7.
- Materials: DFT+U calculations
- Methods: density functional theory with Hubbard U correction; Wannier downfolding
- Observations: two-orbital model with d_z2 and dx2-y2 orbitals
- symmetry_analysis — s±-wave pairing symmetry is preferred.
- Materials: crystal symmetry information
- Methods: irreducible representations of point group D4h; BdG generators
- Observations: identification of symmetry-allowed pairing types
- numerical_calculation — Dominant pairing configurations differ between bulk and thin film.
- Materials: tight-binding parameters
- Methods: self-consistent BCS gap equation; free energy minimization; differential evolution
- Observations: out-of-plane pairing dominates in bulk; in-plane dominates in thin film
- comparison_with_experiment — Calculated gap structure matches experimental observations.
- Materials: ARPES, STM/STS data
- Methods: projected gap function on Fermi surfaces
- Observations: agreement with RPA calculations and experimental spectra