L. Vonlaue et al., STRUCTURAL MODULATION AND LOW-FREQUENCY EXCITATIONS IN THE INCOMMENSURATE PHASES OF BIPHENYL, Journal of physics. Condensed matter, 8(22), 1996, pp. 3977-3992
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.