Large phase fluctuations near TI and phase pinning in incommensurate ThBr4 : Gd3+ through E.S.R. measurements

Abstract

2014 Récemment la coexistence de raies « comme normales » et incommensurables dans les spectres R.M.N. de 87Rb, a été observée au-dessous de T, dans Rb2ZnBr4, et a été interprétée en termes d'ondes stationnaires de fluctuations de phase entre des défauts épingleurs. Nous analysons un effet identique dans les spectres R.P.E. de ThBr4 : Gd3+. Nous excluons un effet extrinsèque de la sonde E.P.R. qui ne manifeste aucune tendance à accrocher la phase et nous montrons que des fluctuations de phase inhomogènes peuvent rendre compte des observations. Une application sommaire du modèle développé par Blinc indique que le « pinning » apparait probablement sur les dislocations.Abstract. 2014 Recently, the coexistence of « normal-like » and incommensurate 87Rb N.M.R. lines have been observed in Rb2ZnBr4 below TI, and has been interpreted in terms of a standing wave regime of phase fluctuations between pinning defects. We analyse a similar effect in the E.P.R. spectra of ThBr4 : Gd3+. We rule out the possibility of an extrinsic pinning effect due to the E.P.R. probe and we show that inhomogeneous phase fluctuations can account for the observation. A crude application of the model developed by Blinc indicates that the average distance between pinning centres is compatible with their being dislocations.

Phase and amplitude fluctuations near TI in the incommensurate phase of Rb2ZnCl4 : Mn2+ through E.P.R. investigations

“…Much more complicated line shapes are obtained generally. They have been discussed [13] and experimentally obtained [7] elsewhere.…”

“…[6] or E.P.R. [7] line below T, or for an anomalously high value of the effective critical exponent p [8], and also for a particular behaviour of the relaxation time T 1,41' which monitors the spectral density J 1,41 forw = COL -108 s-1 in N.M.R. spectroscopy [9].…”

ESR INVESTIGATIONS OF INCOMMENSURATE Rb_{2}ZnCl_{4}:Mn^{2+}

“…Nevertheless, the tendency of high field singularity temperature behaviour (figure 1) allows us to assume that at lower temperatures "non-local" effects would become noticeable. One may suppose that superposition of rotation and twist of chloride tetrahedra would result in "non-locality" of interaction [9].…”

Observation of phason dispersion in a ferroelectric liquid crystal by light scattering

“…(o$=At + K 2 (q-q 0 y (1) NMR, 5,10 EPR, 6 and neutron-scattering data 3,4 have indeed shown that A^ is of the order of 10 10 Hz in solid IC dielectrics.…”

“…Whereas the presence of acousticlike phason modes 1,2 in structurally incommensurate (IC) crystals has been by now clearly demonstrated by inelastic neutron scattering, 3,4 NMR, 5 EPR, 6 and a variety of other techniques, no truly gapless phason seems to have been observed so far, and the question of the possible existence of such a mode is still open. The phason represents the sliding of the IC modulation wave recovering the broken translational high-temperature periodicity of the phase.…”