Vacancy Driven Orbital and Magnetic Order in (K,Tl,Cs)${}_y$Fe${}_{2-x}$Se${}_2$

Abstract

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.

5 pages, 5 figures. accepted by prb. new section on effective Heisenberg model from orbital order

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• The Unexpected Properties of Alkali Metal Iron Selenide Superconductors
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• Structure, magnetic order and excitations in the 245 family of Fe-based superconductors
• Non-Fermi Liquid due to Orbital Fluctuations in Iron Pnictide Superconductors
• Unconventional Mott Transition in KxFe2-ySe2
• Novel Insulating Magnetism in Vacancy-Ordered K2Fe4Se5
• Polar State induced by Block-type Lattice Distortions in BaFe2Se3 with Quasi-One-Dimensional Ladder Structure
• Evolution of the nuclear and magnetic structures of TlFe1.6Se2 with temperature
• Physics picture from neutron scattering study on Fe-based superconductors
• Orbital Resonance Mode in Superconducting Iron Pnictides
• Magnetic anisotropic energy gap and low energy spin wave excitation in antiferromagnetic block phase of K2Fe4Se5
• Low-energy bound states at interfaces between superconducting and block antiferromagnet regions in KxFe{2-y}Se2