Charge and spin ordering process in the mixed-valence system LuFe₂O₄: Charge ordering

The charge ordering process in a mixed valence system LuFe₂O₄ is investigated both experimentally and theoretically. The experimental results using various types of diffraction techniques reveal the following unique features. (i) The system undergoes sequential transitions: disorder→2D-CDW→3D-CDW. (ii) The structure stabilized in the lowest temperature is characterized by an incommensurate charge-density-wave (CDW) state. (iii) In the intermediate temperature range, there appear strong diffuse streaks running along the rhombohedral unique axis which show subtle zig-zag modulation. These experimental results are analyzed based on the assumption that the ordering charges are essentially localized on Fe sites. The energy between the electron pairs up to the nearest-neighbor hexagonal double layers are taken into account to discuss the stable charge configuration and the properties of charge fluctuations. It has been shown that the experimentally observed unique features of the charge ordering process are satisfactorily explained only when the intercharge interaction between the neighboring layers is taken to be attractive.