Comparative study of the electronic and magnetic properties of BaFe_2 As_2 and BaMn_2 As_2 using the Gutzwiller approximation

摘要

To elucidate the role played by the transition-metal ion in the pnictide materials, we compare the electronic and magnetic properties of BaFe_2As_2 with BaMn_2As_2. To this end we employ the LDA + Gutzwiller method to analyze the mass renormalizations and the size of the ordered magnetic moment of the two systems. We study a model that contains all five transition-metal 3d orbitals together with the Ba 5d and As 4p states (ddp-model) and compare these results with a downfolded model that consists of Fe/Mn d states only (d-model). Electronic correlations are treated using the multiband Gutzwiller approximation. The paramagnetic phase has also been investigated using the LDA + Gutzwiller method with electron density self-consistency. The renormalization factors for the correlated Mn 3d orbitals in the paramagnetic phase of BaMn_2As_2 are shown to be generally smaller than those of BaFe_2As_2, which indicates that BaMn_2As_2 has stronger electron correlation effect than BaFe_2As_2- The screening effect of the main As Ap electrons to the correlated Fe/Mn 3d electrons is evident by the systematic shift of the results to the larger Hund's rule coupling J side from the ddp-model compared with those from the d-model. A gradual transition from paramagnetic state to the antiferromagnetic ground state with increasing J is obtained for the models of BaFe_2 AS_2 which has a small experimental magnetic moment, while a rather sharp jump occurs for the models of BaMn_2As_2, which has a large experimental magnetic moment. The key difference between the two systems is shown to be the d-level occupation. BaMn_2As_2, with approximately five d electrons per Mn atom, is for the same values of the electron correlations closer to the transition to a Mott insulating state than BaFe_2As_2. Here an orbitally selective Mott transition, required for a system with close to six electrons, only occurs at significantly larger values for the Coulomb interactions.