Structural and metal-insulator transitions in rhenium based double perovskites via orbital ordering

Alex Taekyung Lee; Chris A. Marianetti

doi:10.1103/PhysRevB.97.045102

paper

Structural and metal-insulator transitions in rhenium based double perovskites via orbital ordering

arXiv:1708.07798 · doi:10.1103/PhysRevB.97.045102

Abstract

Re-based double perovskites (DPs) exhibit a complex interplay of structural and metal-insulator transitions. Here we systematically study the ground state electronic and structural properties for a family of Re-based DPs $A_{2} B$ ReO $_{6}$ ( $A$ =Sr, Ca and $B$ =Cr, Fe), which are related by a common low energy Hamiltonian, using density functional theory + $U$ calculations. We show that the on-site interaction $U$ of Re induces orbital ordering (denoted COO), with each Re site having an occupied $d_{x y}$ orbital and a C-type alternation among $d_{x z} / d_{y z}$ , resulting in an insulating state consistent with experimentally determined insulators Sr $_{2}$ CrReO $_{6}$ , Ca $_{2}$ CrReO $_{6}$ , and Ca $_{2}$ FeReO $_{6}$ . The threshold value of $U_{R e}$ for orbital ordering is reduced by inducing $E_{g}$ octahedral distortions of the same C-type wavelength (denoted COD), which serves as a structural signature of the orbital ordering; octahedral tilting also reduces the threshold. The COO, and the concomitant COD, are a spontaneously broken symmetry for the Sr based materials (i.e. $a^{0} a^{0} c^{-}$ tilt pattern), while not for the Ca based systems (i.e. $a^{-} a^{-} b^{+}$ tilt pattern). Spin-orbit coupling does not qualitatively change the physics of the COO/COD, but can induce relevant quantitative changes. We prove that a single set of $U_{C r}$ , $U_{F e}$ , $U_{R e}$ capture the experimentally observed metallic state in Sr $_{2}$ FeReO $_{6}$ and insulating states in the other three systems. We predict that the COO is the origin of the insulating state in Sr $_{2}$ CrReO $_{6}$ , and that the concomitant COD may be experimentally observed at sufficiently low temperatures (ie. space group P $4_{2} / m$ ). Additionally, given our prescribed values of $U$ , we show that the COO induced insulating state in Ca $_{2}$ CrReO $_{6}$ will survive even if the COD amplitude is suppressed (e.g. due to thermal fluctuations).

29 pages, 20 figures, 7 tables

References in corpus (23)

Cited by in corpus (1)

Electronic structure of Fe and magnetism in the $3 d /5 d$ double perovskites Ca $_{2}$ FeReO $_{6}$ and Ba $_{2}$ FeReO $_{6}$