Evidence for nonclassical critical behavior of a one-dimensional incommensurately modulated crystal:: A 35Cl NQR study of bis(4-chlorophenyl)sulfone

The temperature dependence of Cl-35 nuclear quadrupole resonance (NQR) spectra and the respective spin-lattice relaxation times have been measured between room temperature and 120 K on high quality single crystals of bis(4-chlorophenyl)sulfone [(C6H4Cl)(2)SO2]. Spectra with the typical incommensurate edge singularities were found at temperatures below the transition temperature Ti into the incommensurate (IC) phase. This holds in particular for temperatures very close to T-i. The exponent describing the temperature dependence of the distance between the edge singularities of the incommensurate spectrum is near T-i about 0.35 +/- 0.03 which is identified with the critical exponent beta of the order parameter. Above T-i a considerable critical contribution to the spin-lattice relaxation rate 1/T-1 is detected which can be described by some critical exponent zeta of about 0.69+/-0.02. The data are interpreted in terms of the three-dimensional (3D) XY model. In the IC phase T-1 stays low indicating a considerable influence of the phason excitations on every part of the IC spectrum. Contrary to the behavior of other systems the minimum of T-1 does not occur at T-i. Rather, it is shifted some degrees from T-i to lower temperatures. 2D Cl-35 NQR exchange spectra show no indication for large scale fluctuations of the modulation wave above about 500 Hz.