T. Held et al., INFRARED-ABSORPTION IN ANHARMONIC CRYSTALS WITH IMPURITIES - AN APPLICATION TO A DIATOMIC LINEAR KI-CHAIN, Physica status solidi. b, Basic research, 186(1), 1994, pp. 101-117
In expansion of well-known results of dynamic and optical properties o
f anharmonic ideal crystals, a perturbation theory, using Matsubara fu
nctions and diagrammatic treatment is carried out to describe anharmon
ic crystals with impurities. Taking explicity into account anharmonic
contributions of the potential expansion up to fourth order, mass chan
ges as well as force constant changes between impurity and host-lattic
e particles, it is shown that the anharmonic phonon propagator can be
written as a sum of terms, which are added up within a Dyson equation.
The corresponding proper self-energy includes impurity-activated harm
onic as well as impurity influenced anharmonic contributions. As an ap
plication the infrared absorption of a diatomic linear KI-chain with n
ext-neighbour interaction, a single mass defect, and consideration of
anharmonicities up to second order of the van Hove parameter (O(alpha(
2))) is chosen. Numerical calculations of temperature-dependent linesh
apes and shifts of defect and main absorption lines are presented and
discussed together with results of defect-induced in-band absorption.