F. Decker et J. Petersson, Evidence for nonclassical critical behavior of a one-dimensional incommensurately modulated crystal: A Cl-35 NQR study of bis(4-chlorophenyl)sulfone, PHYS REV B, 61(13), 2000, pp. 8993-9000
The temperature dependence of Cl-35 nuclear quadrupole resonance (NQR) spec
tra and the respective spin-lattice relaxation times have been measured bet
ween room temperature and 120 K on high quality single crystals of bis(4-ch
lorophenyl)sulfone [(C6H4Cl)(2)SO2]. Spectra with the typical incommensurat
e edge singularities were found at temperatures below the transition temper
ature Ti into the incommensurate (IC) phase. This holds in particular for t
emperatures very close to T-i. The exponent describing the temperature depe
ndence 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 crit
ical exponent beta of the order parameter. Above T-i a considerable critica
l 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 d
ata 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 e
xcitations on every part of the IC spectrum. Contrary to the behavior of ot
her 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 abo
ve about 500 Hz.