Dynamics of the Pinned Modulation Wave in Incommensurate bis (4-chlorophenyl) sulfone (BCPS)

Abstract

We show that both the anomalously huge resonance-frequency dependence of the (35)Cl nuclear quadrupole resonance (NQR) spin-lattice relaxation time in BCPS, reported here for the first time, and its anomalous temperature dependence can be explained by large-scale fluctuations of the pinned modulation wave instead of small-scale fluctuations (phasons and amplitudons). The results were obtained by measuring the laboratory (T(1Q)) and rotating frame (T(1Q,rho)) (35)Cl relaxation times. This is the first time that an effective resonance frequency dependence of the spin-lattice relaxation rate was measured in pure NQR.

Phason and amplitudon dynamics in the incommensurate phase of bis(4-chlorophenyl)sulphone

“…The further decrease of 35 Cl NQR spinlattice relaxation time in the IC phase 20,21 does not appear in our high-field measurements. Hence, it is a frequencydependent effect in relation with the NQR relaxation rate measurements in zero field and in the presence of weak rf fields 21 and in 35 Cl NMR measurements at lower Larmor frequencies but not a contradiction to the predominance of phason and amplitudon fluctuations below T i .…”

“…20, this curious behavior of the relaxation time below T i cannot be understood within the standard theory of the spin relaxation of the incommensurate systems. Subsequently these results were confirmed by Blinc et al 21 and in an attempt to explain this incongruous behavior, they suggested that the spin-lattice relaxation is induced by a dominating influence of "large scale" fluctuations ͑␦ 2 ӷ 1͒ of the modulation wave, contrary to the small phase ͑␦ 2 Ӷ 1͒ and amplitude ͑␦〈 2 Ӷ 1͒ fluctuations usually assumed. This situation shows that several points in the previous 35 Cl NQR studies need further consideration and suggest to perform 35 Cl NMR measurements on BCPS.…”

“…The task is to check whether the model of amplitudon and phason modes is applicable or whether large-scale phase fluctuations of the modulation wave dominate the collective motions in the IC phase of BCPS crystals as suggested in Ref. 21. In our previous 35 Cl NMR T 1 measurements, the typical deviations, as observed in the NQR spin-lattice relaxation rates, were also found below T i when the Larmor frequencies in the NMR measurements were comparable to the quadrupole frequencies.…”

“…[19][20][21] were measured near the phase-transition temperature and in the IC phase below 150 K. In the 35 Cl-NQR spin-lattice relaxation rate measurements on BCPS in the normal phase, a strong critical contribution was found in the proximity of the N-IC phase transition. 20,21 The variation of the 35 Cl spin-lattice relaxation rate over the incommensurate NQR frequency distribution in BCPS has been measured at various temperatures. 20 The characteristic features of the NQR spin-lattice relaxation rate 1 / T 1 in the IC phase could be explained in terms of competing phason and amplitudon contributions.…”

A Molecular Dynamics Study of an Incommensurate Modulation Compound: 4, 4′-Dichlorobiphenyl Sulphone

“…1) exhibits a phase transition from a normal phase to an incommensurate (IC) modulated phase occurring at 150 K [1]. This observation has created a new interest in this compound , which has since then been extensively studied by X-ray diffraction [2,3], neutron scattering [4], NMR [5], NQR and Raman spectroscopy [6,7]. Elastic neutron measurements, Cl NQR and calorimetric measurements have indicated that the incommensurate phase is stable down to very low temperature (4.5 K) with the presence of high-order satellite reflections and intensities increasing on cooling [8,9].…”