Interpretation of optical conductivity in the ferromagnetic metallic state of La0.7Ca0.3MnO3 manganites

Abstract

Calculations of the optical conductivity σ(ω) for hole doped La0.7Cao.3MnO3 manganites have been made within the two-component scheme: one is the coherent Drude free carrier excitation. The other is incoherent motion of carriers from one site to other leading to a polaron formation, originated from inter-band transitions etween the Hund rule split bands (Mn-Mn) and due to charge-transfer transitions etween the O 2p and the Mn eg ands. The model successfully accounts for the anomalies reported in the optical measurements for the ferromagnetic metallic state. The frequency dependent relaxation rates are expressed in terms of memory functions and the coherent Drude carriers from the effective interaction potential leads to a sharp peak at zero frequency and a long tail at higher frequencies, i.e. in the infrared region. While to that the hopping of carriers from Mn site to Mn and Mn site to O (incoherent motion of doped carriers) yields two-peak value around 0.4 and 4.0 eV in the optical conductivity centred at mid-infrared region. Both the Drude and hopping carriers in the manganites contribute to the optical process of conduction in the Mn-O planes and shows similar results on optical conductivity in the mid-infrared as well as infrared frequency regions as those revealed from experiments.