The generalized Hubbard model applied to triplet p-wave pairing in Sr1−xKxFe2As2

Abstract

Nowadays several superconductors with singlet pairing of d-wave symmetry are known, but very few triplet-pairing superconductors have been found. It is believed that Sr₂RuO₄ and some uranium compounds such as UTe₂, with very low critical temperatures (T_c) around 1 K, could have Cooper pairs in a triplet state. Other possible candidates that could present triplet pairing are magnetic compounds based in Fe, for example Sr_1−xK_xFe₂As₂, which have optimal doping around x = 0.5 with maximum critical temperature T_c−max=37K. On the other hand, p-wave pairing in a slightly distorted square lattice, described by a generalized Hubbard model, has been investigated considering that the next-nearest neighbor correlated hopping interactions along the two lattice diagonals are slightly different. In this work, we investigate the appearance of triplet-pairing p-wave superconducting states in square lattices where the electron dynamics is described by the generalized Hubbard model. An optimal electron density (n_op), where the maximum critical temperature occurs, was found for each value of the ratio between the next-nearest-neighbor and the nearest-neighbor one-electron hoppings (t'/t), and a search of Hamiltonian parameters was performed to fit T_c as a function of x in the compound Sr_1−xK_xFe₂As₂. An optimal electron density (n_op), where the maximum critical temperature occurs, was found for each value of the ratio t'/t . In particular, the set of Hamiltonian parameters adjusting the fitting is ∆t = 0.5eV , ∆t₃ = 0.05eV and δ₃ = 0.08eV , and the ratio −t′/t = 0.29 adjust in order to obtain a maximum of the critical temperature of T_c−max ≈ 36K at n_op = 0.50, which agrees with the maximum critical temperature (≈ 37K) of the compound  Sr_0.5K_0.5Fe₂As₂. With this set of Hamiltonian parameters, other superconducting properties were calculated, such as the amplitude of the p-wave superconducting gap, the ground state and condensation energies, and the electronic specific heat as a function of the temperature and the jump of this at T_c. Additionally, a comparison between the ground states energies of the d- and p-wave superconducting states was made, finding that the p-wave superconducting state has the lowest energy and it can be considered as the ground state. Hence, the compound Sr_1−xK_xFe₂As₂ can be studied under the assumption of triplet pairing with a p-wave superconducting gap symmetry, and probably other similar pnictides, such as Ba_0.4K_0.6Fe₂As₂ with T_c−max = 38K, may present a p-wave triplet pairing.