NMR Studies on Crystals with Structurally Incommensurately Modulated Phases

Abstract

The dynamics of the incommensurate (IC) modulation is investigated for several one-dimensionally incommensurately modulated crystals near the transition to the normal high temperature phase at the temperature T _i . Using nuclei with nuclear quadrupole moments as local probe, nuclear magnetic resonance (NMR) line shape and spin-lattice relaxation have shown to be very sensitive to study the order parameter dynamics and slow elementary excitations in crystals with IC phases. All results can be described consistently in terms of a static modulation in the IC phase without any indication for “floating” or large scale fluctuations of the modulation wave. The critical exponents derived from the NMR line shape and the relaxation times T₁ very nicely fit to the universality class of the 3d-XY model. Recently, the validity of this conclusion was also confirmed for the particular IC system of bis(4-chlorophenyl)sulphone ((ClC₆H₄)₂2SO₂). For Rb₂ZnBr₄ and Rb₂ZnCl₄ crystals the characteristic frequency of the critical dynamics of the order parameter (OP) slows down to a frequency range which has been estimated to lie between 2 … 10 MHz. In this particular case one can derive the characteristic frequency of the critical dynamics of the OP above T _i and of the phason below T _i from the Larmor frequency dependence of ⁸⁷Rb NMR relaxation times T₁. The relatively broad temperature range where the critical dynamics was observed may be explained.

Keywords:

• Incommensurate
• phase transition
• NMR
• NQR
• spin-lattice-relaxation
• line shape

PACS:

• 68.35.Rh
• 61.18.Fs
• 64.60.Ht
• 64.70.Rh
• 76.60.Gv
• 61.44.Fw
• 61.50.Ah
• 64.60.-i

Acknowledgement

The contributions and suggestions to this work by Dr. F. Decker, Dr. U. Häcker, Dr. K.-P. Holzer, Dr. M. Irsch, Dr. P. Mischo, Prof. G. Völkel and Prof. J. Wesselinowa are kindly acknowledged. The authors are deeply indebted to Mr. Gert Klotzsche (Leipzig) for his help during the difficult ³⁵Cl NMR relaxation time and line shape measurements at lower temperatures by means of the high field NMR spectrometer AVANCE 750. The support of the Deutsche Forschungsgemeinschaft (DFG) is greatly acknowledged. One of authors (A. T.) is grateful to the University of Leipzig for a personal funds.