EXPLANATION OF OPTICAL CONDUCTIVITY IN THE FERROMAGNETIC METALLIC STATE OF La0.7Ca0.3MnO3 MANGANITES

Abstract

Calculations of the optical conductivity σ(ω) for hole-doped cubic La 0.7 Ca 0.3 MnO 3 manganites have been made within the two-component scheme. One is the coherent Drude free carrier excitations. The other is the incoherent motion of carriers from one site to another leading to a polaron formation. This originated from interband transitions between the Hund rule split bands ( Mn – Mn ) and is due to charge-transfer transitions between the O 2p and the Mn eg bands. 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. The hopping of carriers from Mn site to Mn and Mn site to O (incoherent motion of doped carriers) yields two peak values around 0.4 and 4.0 eV in the optical conductivity centered in the 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.