Phasons and amplitudons in an n = 4 incommensurate structure: Phase II of biphenyl under pressure

Abstract

We report neutron scattering study of the dispersion of phasons and amplitudons in the n = 4 incommensurate phase II of Biphenyl under hydrostatic pressure. Due to the nature of the molecular instability which is at the origin of the different phase transitions in Biphenyl, pressure lowers transition temperatures. Phase II disappears at a rather low pressure (1.85 kbar) which gives us the opportunity to study its excitations at very low temperature, where their damping constants are small. We measured the frequencies of three well separated modes and their dispersions at different pressure values. In the framework of the Ginzburg-Landau theory, these modes are unambiguously identified as phasons and amplitudons of a four ray star incommensurate structure.