Manifestation of charge/orbital order and charge transfer in temperature-dependent optical conductivity of single-layered Pr_0.5Ca_1.5MnO₄

Half-doped single-layered manganite, Pr_0.5Ca_1.5MnO₄ has shown a charge/orbital order of the eg electrons and CE-type spin order of the t2g electrons of the Mn ions. A previous experimental study on that system, supported by a simple modelling, has suggested that the charge/orbital ordering play an important role in governing the temperature dependence of optical conductivity of a broad peak around 0.7-0.8 eV. In addition, another peak around 3.5 eV, which is less sensitive to temperature, has been attributed to the charge transfer from O-p to Mn-e_g orbitals. Nevertheless, the theoretical explanation was incomplete as the role of O-p orbitals was not considered in the model. In this paper, we propose to improve the model by incorporating both Mn-e_g and O-p orbitals. We assume the existence of charge/orbital ordering and investigate how this ordering as well as the charge-transfer phenomenon control the temperature dependence of the optical conductivity. Our results reveal the charge/orbitalordering peak in the region 0.7-1.2 eV, which is blue-shifted with decreasing temperature, and the charge-transfer peak around 3.5 eV, which is less sensitive to temperature. The capability of our model to capture the general profile and temperature dependence of the optical conductivity suggests the validity of our theory.