Optical study of continuous thin films and island films of Ir: Interband absorption and localization of conduction electrons of Ir

For continuous thin Ir films and Ir island films consisting of Ir particles larger than about 25 Å in diameter, optical absorption has been measured in the photon energy range of 0.5–6.5 eV. Interband absorption does not exist at about 1.1 eV. Absorption at about 3.1 eV is identified as interband absorption because its particle-size dependent change is similar to that of interband absorption of Cu particles. Interband absorption at about 4.4 eV is suggested to arise from transitions between bands 2 and 5 near L in the direction Q. Absorption due to plasma oscillations of conduction electrons is absent in the island films, showing that the conduction electrons are localized by a strong correlation interaction between the conduction electrons having strong d character. Strong d character based on large hybridization of s and d bands is responsible for the strong correlation interaction.