Finite-size effects on the incommensurate phase transition of bis(4-chlorophenyl)sulphone studied byCl35NQR

Abstract

The temperature behavior of the 35 Cl nuclear quadrupolar resonance ͑NQR͒ spectrum of thin bis͑4-chlorophenyl͒sulphone films grown on fine powder samples of TiO 2 was studied. In order to characterize the structural and dynamical changes, a comparative study of the NQR parameters of the film with respect to the bulk samples was performed in the temperature range 80-370 K. A shift of the incommensurate phasetransition temperature from 150 in bulk to 156 K in film was detected. Also, the observed NQR spectra in the incommensurate phase suggest qualitative changes in the modulation. The presence of a central intense peak below the transition temperature indicates important contributions to the NQR spectra from periodic domains in the film. ͓S0163-1829͑96͒09129-1͔

Incommensurate Phase Transition in a Thin Film

“…Such a general tendency was also observed in solutions for ferroelectric and antiferroelectric thin films [10,16] and in approximate solutions for incommensurate thin films [12,13]. However, in contrast to monotone changes in the phase transition temperature with decreasing the film thickness obtained in [10,12,13,16], the exact boundary value problem (4), (6) leads to more complex size dependence of T i which is shown in Fig. 4.…”

“…It is easy to see that any solution of the boundary value problem (4), (6) can be continued to [−L, 0] by the symmetry condition η 0 (x) = η 0 (−x) yielding a solution of the initial boundary value problem (4), (5). The surface extrapolation length δ in (6) describes the effect of the surface on the order parameter distribution over the film and can be positive or negative depending on the film neighborhood.…”

“…In particular, experiments on incommensurate thin films were made in [6]. It is generally believed that the alterations in phase transition temperatures and critical susceptibility are primarily induced by size reduction while environment conditions are also of great importance.…”

Performance of frequency-modulated pulses in NQR

“…Frequently in NQR of solids one is dealing with broad lines of a few or hundreds of kilohertz as in the disordered samples [1], incommensurate samples [2] or high-Te superconducting samples [3]. Also chemically inequivalent sites in the crystal lattice lead to multiple lines in the spectrum [4].…”