`Jellyfish' fluctuations of atoms in solids

Abstract We have extended the concept of jellyfish fluctuations, which was originally developed for the ac conductivity of ionic solids at low-frequency and low-temperature ( σ LF–LT ), to describe their microwave conductivity at room temperature and above ( σ MW–HT ). The results for a series of potassium germanate glass confirm the previous correlation between the two regions of conductivity of a silicate glass, and indicate a common origin of σ LF–LT and σ MW–HT . A detailed analysis of the data in terms of a rather simplistic, phenomenological, asymmetric double well potential (ADWP) model shows that the nature of jellyfish is modified once the diffusive motion of potassium ions becomes significant above ∼200 K. Limitation of the ADWP model is noted again for describing the conductivity in the two regions.