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

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 (Tc) 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 Sr1−xKxFe2As2, which have optimal doping around x = 0.5 with maximum critical temperature Tc−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 (nop), 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 Tc as a function of x in the compound Sr1−xKxFe2As2. An optimal electron density (nop), 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 Tc−max ≈ 36K at nop = 0.50, which agrees with the maximum critical temperature (≈ 37K) of the compound Sr0.5K0.5Fe2As2. 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 Tc. 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 Sr1−xKxFe2As2 can be studied under the assumption of triplet pairing with a p-wave superconducting gap symmetry, and probably other similar pnictides, such as Ba0.4K0.6Fe2As2 with Tc−max = 38K, may present a p-wave triplet pairing.