Strong correlation effects of the Re 5d electrons on the metal-insulator transition in Ca₂FeReO₆

Abstract

We have investigated the electronic structure of polycrystalline Ca₂FeReO₆ using photoemission spectroscopy and band-structure calculations within the local-density approximation+U (LDA+U) scheme. In valence-band photoemission spectra, a double-peak structure which is characteristic of the metallic double-perovskite series has been observed near the Fermi level (E_F), although it is less distinct compared to the Sr₂FeMoO₆ case. The leading near-E_F structure has a very weak spectral weight at E_F above the metal-insulator transition (MIT) temperature T_MI of ~140 K, and it loses the E_F weight below T_MI, forming a small energy gap. To reproduce this small energy gap in the calculation, we require a very large effective U (U_eff) for Re (4 eV) in addition to a relatively large U_eff for Fe (4 eV). Although the most of the experimental features can be interpreted with the help of the band theory, the overall agreement between the theory and experiment was not satisfactory. We demonstrate that the effective transfer integral between Fe and Re is actually smaller than that between Fe and Mo in Ca₂FeMoO₆, which can explain both the MIT and very high ferrimagnetic transition temperature.