Infrared absorption in defect induced anharmonic solids

Abstract

The infrared absorption in isotopically disordered anharmonic crystals is theoretically investigated using double time thermal Green’s function technique. The Hamiltonian used in the study includes the cubic and quartic anharmonicities alongwith the terms associated with the crystal defects. The total i.r. absorption in impurity induced anharmonic crystals is obtained as the sum of diagonal and non-diagonal terms. The diagonal terms can be further separated into defect dependent, anharmonic and impurity-anharmonicity cross term contributions, the non-diagonal terms chiefly depend on mass change parameters and vanishes in the absence of impurities. The impurity-anharmonicity interactions involve defect and anharmonic parameters simultaneously and contribute significantly to the absorption coefficient. A brief description of impurity modes, phonon life times and strength of absorption is given with special reference to impurity-anharmonicity interactions to study the effects of temperature, frequency and impurity concentration on optical absorption.