K.-P. Holzer

CRITICAL DYNAMICS ABOVE THE INCOMMENSURATE PHASE TRANSITION OF RB2ZNCL4 STUDIED BY NMR

Order parameter (OP) fluctuations are studied in the normal (N) phase of the incommensurately modulated (IC) system Rb2ZnCl4 by measuring the 87Rb NMR spin-lattice relaxation time T1. The results are analysed on the basis of dynamical-scaling laws. Since both the temperature and the frequency dependence of T1 reveal the slowing-down of the OP dynamics below the Larmor frequency near the N-IC phase transition, the critical dynamics could be studied quantitatively. As an upper bound for the phason frequency gap a value of 100 MHz is estimated.

Dynamics of incommensurate crystals studied by NMR

Abstract The order parameter (OP) dynamics above and below the incommensurate (IC) phase transition of Rb2ZnCl4 and BCCD is investigated by means of nuclear spin-lattice relaxation measurements. The results are interpreted on the basis of a general formalism describing the spin-lattice relaxation rate in terms of the OP normal modes. The consequences for the interpretation of spin-lattice relaxation data in the IC phase both in the plane-wave limit and the soliton regime are pointed out. In particular, at the IC phase transition of Rb2ZnCl4 the critical slowing down to frequencies below 100 MHz is observed.

Symmetry of the normal and low-temperature phase of betaine calciumchloride dihydrate as revealed by 35Cl NMR

Abstract Quadrupolar perturbed 35Cl NMR spectra are measured in the high-temperature normal (N) phase and in the low-temperature (LT) phase of betaine calciumchloride dihydrate (BCCD) single crystals. The electric field gradient (EFG) tensors at the chlorine sites are precisely determined for both phases. In the N phase, the symmetry of the tensors is in agreement with the space group Pnma known from X-ray studies. The symmetry of the tensors in the LT phase allows to exclude the P11a symmetry proposed previously and confirms the space group Pn21a derived theoretically on the assumption that one order parameter drives the phase transitions in BCCD.

NMR evidence for an exchange process of the D2O deuterons in betaine calcium chloride dihydrate

Abstract Quadrupolar perturbed 2 H NMR spectra were measured in the high-temperature (N) phase of deuterated betaine calcium chloride dihydrate (BCCD). Several results clearly demonstrate a thermally activated exchange process of the deuterons within the D 2 O molecules. The motionally averaged electric field gradient tensor (EFG) of the D 2 O deuterons is determined. The results are discussed briefly in relation to the origin of the spontaneous polarization in the low-temperature phase.

Comparative study of critical behavior in several one-dimensionally modulated incommensurate phases by quadrupolar perturbed NMR

Abstract Critical behavior of the one-dimensionally modulated incommensurate phases of Rb2ZnCl4, Rb2ZnBr4, (NH4)2ZnCl4, and BCCD was studied by measuring the temperature dependence of the first order quadrupolar perturbation of the NMR-frequencies of suitable nuclei. In each case, results are in accordance with the non-classical critical exponents predicted by the 3d XY model, thus confirming the universality hypothesis. No deviations from the power laws as reported sometimes in literature and attributed to the influence of phase fluctuations could be observed.

Study of Excitations in Structurally Incommensurately Modulated Solids by Means of Nuclear Magnetic Resonance

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 by means of quadrupolar perturbed nuclear magnetic resonance (NMR) spectra and nuclear spin-lattice relaxation. 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 T 1 very nicely fit to the universality class of the 3d-XY model. For Rb2ZnBr4 and Rb2ZnCl4 crystals the characteristic frequency of the critical dynamics of the order parameter (OP) slows down below the Larmor frequency used in the 87Rb NMR relaxation time measurements. 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 T 1.

NMR-evidence for absence of floating in structurally incommensurate crystals

Abstract The dynamics of the incommensurate (IC) modulation is investigated for several IC systems near the transition to the normal high temperature phase by means of quadrupolar perturbed NMR spectra using first-order effects. For the special case of the IC phase of RB2ZnCl4 also two-dimensional 87Rb NMR exchange spectra are reported. 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.