Gap modes in defective diatomic chains

Abstract

Conditions are investigated for the occurence of gap modes in diatomic linear chains with substitutional impurities and associated changes in force constants for the interactions between the impurity and its nearest neighbours and between first and second neighbours. Particular attention is given to pocket modes in which displacements of the second neighbours are dominant. The results are obtained by Green's function analysis guided by direct calculations with finite chains. Comparisons are made with existing observations of gap and pocket modes in KI with substitutional impurities.