This project entails nuclear magnetic resonance (NMR) investigations of incommensurate systems, i.e. solids which have perfect long range order but no lattice periodicity and are thus intermediate between crystal lattices and disordered systems like glasses and biological systems. Incommensurate solids are characterized by at least two sublattices the periodicities of which can not be described as the ratio of two integers. These systems exhibit a number of new phenomena not found in perfect crystals or disordered systems: an sliding mass density wave distinct from lattice vibrations in normal crystal lattices. Excitations of the modulation waves are known as phasons and can be studied in a unique fashion by NMR techniques. In addition, these modulation waves can become pinned in a manner similar to vortex pinning in superconductors. The current resaerch will study pinning and depinning processes in a range of insulators, including examples on incommensurate lattices in 1, 2 and 3 dimensions. The results of the investigations described in this proposal should be of basic importance for the whole field of solid-state "superfluidity" in crystals, since the modulation wave in an incommensurate solid may be thought of a solid-state superfluid which may be either charged or uncharged.
