Structural modulation and low-frequency excitations in the incommensurate phases of biphenyl

The incommensurate phases IC II and IC III of biphenyl are investigated by means of NMR. It is shown that the structural modulation appears along one direction, and can be described by a periodic twisting of the phenyl rings about the molecular axis. The temperature dependence of the maximum twist angle below the N − IC phase transition follows a power law with a critical exponent . A two−dimensional NMR exchange spectrum in the IC II phase of biphenyl is recorded. It clearly excludes a `floating' of the modulation wave. The temperature and Larmor frequency dependences of the NMR spin − lattice relaxation rates in the IC phases of biphenyl are measured. A distinct dependence of on the Larmor frequency is found and analysed on the basis of a relaxatory dynamics of the order parameter (OP). A reliable estimate of about 5 MHz for the characteristic frequencies of the OP dynamics is derived. In the IC III phase, the spectra as well as the spin − lattice relaxation rates indicate that the structure of biphenyl is not a truly incommensurate on