Spins and pseudospins: NMR and soft collective lattice modes in disordered solids

Abstract

The analogies and parallelisms between spin dynamics and lattice dynamics of disordered solids, where some collective excitations can be described as pseudospin waves, are discussed. It is shown that the concepts developed in the theory of magnetic resonance are very helpful in understanding the nature of collective “pseudospin-wave”-like excitations in disordered solids and that, vice versa, the pseudospin concept is important in the interpretation of NMR results in such systems. As an example, NMR studies of soft and “central” modes in H-bonded ferroelectrics are discussed.