Comparison of the electronic structure of LnBaCo2O5+δ (Ln=Gd, Dy; Ln-112) and LnBaCo4O7 (Ln=Yb; Ln-114) single-crystal surfaces using resonant photoemission

Abstract

A comparison of the electronic structure of LnBaCo2O5+( (Ln = Gd, Dy; Ln-112) and LnBaCo4O7+δ (Ln = Yb; Ln-114) single-crystal surfaces has been made using synchrotron photoemission spectroscopy. Resonant photoemission is used to identify the atomic parentage of the valence band states of Ln-114. The states close to the Fermi energy are found to be of mixed Co 3d/O 2p character. Comparison of the photoemission results for the two systems allows unambiguous identification of the spectral signal due to low spin octahedral Co3+ in Ln-112. High resolution valence band spectra taken as a function of temperature reveal the presence of the metal–insulator (MI) transition in Ln-112 in the 300–400 K temperature range. The gradual changes in the spectral profile of the low energy states with temperature rule out a sudden ‘high spin–low spin’ switch as the mechanism of the MI transition. They are instead consistent with a gradually shifting equilibrium between three states—low spin, intermediate spin and high spin.