A Unified Understanding of the Experimental Controlling of the Tc of La₃Ni₂O₇

Summary

Based on the previously proposed effective dx2-y2 orbital bilayer t-J∥-J⊥ model with model parameters input from first-principles calculations, this paper provides a unified explanation for a series of experiments on the regulation of the superconducting transition temperature (T_c) in La₃Ni₂O₇ via oxygen stoichiometry, elemental substitution, pressure, or strain, using slave-boson mean-field and density matrix renormalization group methods. The model reveals that, due to the near quarter-filling of the dx2-y2 orbital, its T_c tuning behavior resembles that of hole-doped overdoped cuprates. In terms of doping dependence, the system exhibits particle-hole asymmetry: hole doping suppresses T_c by making the system more overdoped, while electron doping has the opposite effect, explaining the T_c suppression caused by excess oxygen or Ca/Sr substitution for La, as well as the “half-dome” behavior in oxygen stoichiometry tuning. Regarding interaction dependence, T_c varies with the interlayer antiferromagnetic superexchange interaction J⊥, accounting for the enhancement of bulk T_c by Sm/Nd substitution for La, the “right-triangle” shape of pressure-dependent bulk T_c, and the enhancement of T_c under compressive strain in thin films. Compared with weak-coupling theory (where T_c depends mainly on the density of states) and the dz2 orbital-dominated pairing mechanism (where T_c is proportional to the dz2 hole density), this model provides a more natural and unified explanation. The paper further proposes that T_c can be increased through electron doping that does not introduce disorder, such as substituting La with higher-valent elements.

Materials

• La₃Ni₂O₇
• La₂PrNi₂O₇

Methods

• Slave-boson mean-field theory (SBMF)
• Density matrix renormalization group (DMRG)
• First-principles DFT calculations

Keywords

• overdoped cuprates analogy
• particle hole asymmetry
• interlayer superexchange j⊥
• hole doping suppresses tc
• electron doping enhances tc
• half dome behavior

Highlights

• Provides a unified understanding of diverse experiments (oxygen stoichiometry, element substitution, pressure, strain) within a single d-orbital bilayer model.
• Predicts that electron doping without inducing disorder (e.g., substituting La with higher-valence elements) can enhance T_c.

Conclusions

• T_c is determined by the pairing temperature in the overdoped regime, increasing with filling fraction and interlayer superexchange J⊥.
• Hole doping suppresses T_c by moving the system further into the overdoped regime, while electron doping enhances T_c.
• Pressure and strain dependence of T_c are mediated by changes in J⊥, explaining the dome-shaped pressure-T_c relation and the enhancement under compressive strain.
• The half-dome behavior in oxygen stoichiometry tuning arises from competing effects of electron doping and oxygen vacancy disorder.

Main claims

• The T_c of La₃Ni₂O₇ is controlled by the filling fraction of the dx²-y² orbital and the interlayer antiferromagnetic superexchange J⊥, analogous to hole-doped overdoped cuprates.
  • Evidence: SBMF and DMRG calculations show T_c increases with filling fraction and J⊥ in the overdoped regime
• Hole doping (via over-oxidization or Ca/Sr substitution) suppresses T_c, while electron doping enhances T_c.
  • Evidence: Calculations show particle-hole asymmetry: hole doping decreases DOS and T_c; electron doping increases DOS and T_c; consistent with experimental half-dome and substitution experiments
• The enhancement of T_c by Nd/Sm substitution of La, the right-triangle pressure dependence, and the enhancement by compressive strain are all due to variation of J⊥ with experimental conditions.
  • Evidence: DFT calculations show J⊥ increases with Nd substitution, with pressure up to 30 GPa then decreases, and with compressive strain; SBMF calculates corresponding T_c that matches experimental trends

Workflow

• model_development — A minimal model for La₃Ni₂O₇ superconductivity.
  • Materials: first-principles DFT calculations
  • Methods: effective dx²-y² orbital bilayer t-J∥-J⊥ model
  • Observations: model captures near quarter-filling of dx²-y² orbital
• theoretical_calculations — T_c behavior analogous to hole-doped overdoped cuprates.
  • Materials: model parameters
  • Methods: slave-boson mean field theory; DMRG
  • Observations: T_c is controlled by filling fraction and interlayer superexchange J⊥
• comparison_with_experiments — Unified understanding of T_c control experiments.
  • Materials: experimental data on T_c control
  • Methods: systematic variation of parameters to match experiments
  • Observations: model reproduces half-dome oxygen stoichiometry dependence, enhancement by Nd/Sm substitution, right-triangle pressure dependence, and compressive strain enhancement
• comparison_with_alternative_theories — Strong-coupling dx²-y² model provides more natural understanding.
  • Materials: RPA and d_z²-orbital dominated pairing calculations
  • Methods: same model parameters for RPA
  • Observations: RPA fails to explain most experiments; d_z² mechanism also fails