STRUCTURAL MODULATION AND LOW-FREQUENCY EXCITATIONS IN THE INCOMMENSURATE PHASES OF BIPHENYL

The incommensurate phases IC II and IC III of biphenyl are investigate d by means of H-2 NMR. It is shown that the structural modulation appe ars along one direction, and can be described by a periodic twisting o f the phenyl rings about the molecular axis. The temperature dependenc e of the maximum twist angle below the N-IC phase transition follows a power law with a critical exponent beta = 0.35 +/- 0.03. A two-dimens ional NMR exchange spectrum in the RC II phase of biphenyl is recorded . It clearly excludes a 'floating' of the modulation wave. The tempera ture and Larmor frequency dependences of the 2H NMR spin-lattice relax ation rates 1/T-1 in the IC phases of biphenyl are measured. A distinc t dependence of 1/T-1 on the Larmor frequency is found and analysed on the basis of a relaxatory dynamics of the order parameter (OF). A rel iable estimate of about 5 MHz for the characteristic frequencies of th e OP dynamics is derived. In the IC III phase, the spectra as well as the spin-lattice relaxation rates indicate that the structure of biphe nyl is not a truly incommensurate one.