Effect of Rare-Earth Element Substitution in Superconducting R3Ni2O7 under Pressure

Recently, high temperature (T_c ≈ 80 K) superconductivity (SC) has been discovered in La₃Ni₂O₇ (LNO) under pressure. This raises the question of whether the superconducting transition temperature T_c could be further enhanced under suitable conditions. One possible route for achieving higher T_c is element substitution. Similar SC could appear in the Fmmm phase of rare-earth (RE) R₃Ni₂O₇ (RNO, R = RE element) material series under suitable pressure. The electronic properties in the RNO materials are dominated by the Ni 3d orbitals in the bilayer NiO₂ plane. In the strong coupling limit, the SC could be fully characterized by a bilayer single 3d_x²–y²-orbital t–J_∥–J_⊥ model. With RE element substitution from La to other RE element, the lattice constant of the Fmmm RNO material decreases, and the resultant electronic hopping integral increases, leading to stronger superexchanges between the 3d_x²–y² orbitals. Based on the slave-boson mean-field theory, we explore the pairing nature and the evolution of T_c in RNO materials under pressure. Consequently, it is found that the element substitution does not alter the pairing nature, i.e., the inter-layer s-wave pairing is always favored in the superconducting RNO under pressure. However, the T_c increases from La to Sm, and a nearly doubled T_c could be realized in SmNO under pressure. This work provides evidence for possible higher T_c R₃Ni₂O₇ materials, which may be realized in further experiments.