Impurity Modes in an Induced Moment System and Hydrogen-Bonded Ferroelectrics

Abstract

Impurity modes arising from the presence of an isolated impurity in an induced moment system and in KH2PO4-type ferroelectrics have been investigated. These two systems can be characterized by a parameter Δ′ which is the ratio of the energy difference between the lowest ionic states (or the tunneling integral in KDP) to the exchange (or interaction) constant. In the absence of external fields it is found that (i) impurity modes can exist at the top or bottom (where they give rise to scattering at low temperatures) of the excitation band, (ii) localized modes can appear in the energy gap below the band, and (iii) no modes exist for Δ′ larger than some critical value (except for modes at the impurity). A magnetic (or electric, in the case of KDP) field can cause modes to appear or vanish and can move the modes in and out of the band, thus localizing a resonance.