Physics – Condensed Matter – Strongly Correlated Electrons
Scientific paper
2011-05-02
Phys. Rev. B 84, 155107 (2011)
Physics
Condensed Matter
Strongly Correlated Electrons
5 pages, 5 figures. accepted by prb. new section on effective Heisenberg model from orbital order
Scientific paper
10.1103/PhysRevB.84.155107
We investigate the effects of the $\sqrt{5}\times\sqrt{5}$ Fe vacancy ordering on the orbital and magnetic order in (K,Tl,Cs)$_y$Fe$_{2-x}$Se$_2$ using a three-orbital ($t_{2g}$) tight-binding Hamiltonian with generalized Hubbard interactions. We find that vacancy order enhances electron correlations, resulting in the onset of a block antiferromagnetic phase with large moments at smaller interaction strengths. In addition, vacancy ordering modulates the kinetic energy differently for the three $t_{2g}$ orbitals. This results in a breaking of the degeneracy between the $d_{xz}$ and $d_{yz}$ orbitals on each Fe site, and the onset of orbital order. Consequently, we obtain a novel inverse relation between orbital polarization and the magnetic moment. We predict that a transition from high-spin to low-spin states accompanied by a crossover from orbitally-disordered to orbitally-ordered states will be driven by doping the parent compound with electrons, which can be verified by neutron scattering and soft X-ray measurements.
Lee Wei-Cheng
Lv Weicheng
Phillips Philip
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