Neutron observations of phasons and amplitudons in incommensurate phases

Abstract

Abstract We present the first observations of phase modes in low-temperature incommensurate phases. The experiments were performed using thermal and cold neutron triple-axis-spectrometers at I.L.L on biphenyl and thorium bromide. Biphenyl is a molecular crystal exhibiting two distinct low-temperature incommensurate phases but no lock-in transition. Despite its structural complexity two low-lying phonon branches corresponding to torsional motions of the biphenyl molecule can be well studied. In the high-temperature phase one of these modes becomes soft and condenses at an incommensurate lattice position near the zone boundary. Within the incommensurate phases this mode splits into a phason with a linear dispersion and a soft-mode-like amplitudon. The other mode also splits into two distinct branches in the incommensurate phase. Thorium bromide is a structurally much simpler system which displays an incommensurate phase below 95 K. Here too a high-temperature soft mode is clearly observed, below T c it splits into a phason and an amplitudon branch.