Prediction and observation of pocket vibrational modes in crystals

Abstract

A perturbed-shell-model calculation for the impurity system KI:${\mathrm{Ag}}^{+}$ predicts three nearly-degenerate localized gap modes of different symmetry and unusual amplitude patterns: The displacements of the defect's fourth-nearest neighbors are more than an order of magnitude larger than those of the impurity or its nearest neighbors. Far-infrared isotope-effect measurements directly confirm the existence of these localized pocket modes.