Neutron observation of phase and amplitude modes in ann=4incommensurate system

Abstract
We report neutron-scattering measurements of phase and amplitude modes in the n=4 incommensurate phase II of biphenyl under hydrostatic pressure. Because of a strong lowering of the temperature range of existence of this phase with pressure, the damping of the modes is considerably reduced. It is therefore possible to measure the frequencies of three well-separated branches and their dispersion at different pressure values. These modes are clearly identified as phase and amplitude modes of a four-arm-star incommensurate structure and the results are discussed in the framework of the Landau theory.