Physical Review B - Vol 77 No 21-24

Intrinsic origin of spin echoes in dipolar solids generated by strong pi pulses
Dale Li, Yanqun Dong, R. G. Ramos, J. D. Murray, K. MacLean, A. E. Dementyev, and S. E. Barrett
In spectroscopy, it is conventional to treat pulses much stronger than the linewidth as delta functions. In NMR, this assumption leads to the prediction that pi pulses do not refocus the dipolar coupling. However, NMR spin echo measurements in dipolar solids defy these conventional expectations when more than one pi pulse is used. Observed effects include a long tail in the CPMG echo train for short delays between pi pulses, an even-odd asymmetry in the echo amplitudes for long delays, an unusual fingerprint pattern for intermediate delays, and a strong sensitivity to pi-pulse phase. Experiments that set limits on possible extrinsic causes for the phenomena are reported. We find that the action of the system's internal Hamiltonian during any real pulse is sufficient to cause the effects. Exact numerical calculations, combined with average Hamiltonian theory, identify terms that are sensitive to parameters such as pulse phase, dipolar coupling, and system size. Visualization of the entire density matrix shows a unique flow of quantum coherence from nonobservable to observable channels when applying repeated pi pulses.

NMR study of the impurity induced ordered state in the doped Haldane chain compound SrNi1.93Mg0.07 V2O8
B. Pahari, K. Ghoshray, R. Sarkar, and A. Ghoshray
We report the effect of Mg (S=0) substitution at the Ni site of S=1 Haldane chain compound SrNi2V2O8 from magnetic susceptibility and 51V NMR studies. The magnetic-susceptibility results in presence of different external fields (H=0.1, 1.198, 3, and 7 T) in SrNi1.93Mg0.07V2O8 indicate a strongly field dependent nature of the antiferromagnetic (AF) ordering in this compound, with the ordering being suppressed at H3 T. A comparison of these results with those reported in isostructural compound PbNi1.76Mg0.24V2O8 indicates a stronger interchain exchange in the Sr compound. In order to probe the local magnetic properties of SrNi1.93Mg0.07V2O8 both in the AF ordered state and also when the ordering is suppressed, the 51V NMR studies were performed in presence of the magnetic fields of H=1.198 and 7.04 T. The resonance line shape in presence of both the fields remains almost unaltered in the temperature range of 50–300 K, as was reported in the pure system. However, the linewidth increased appreciably in the range 4T20 K in presence of both the fields. The temperature dependence of the first moment (M1) and the second moment (M2) of the NMR spectra indicates the effect of the development of three-dimensional (3D) correlations among the staggered spins below 20 K when H=1.198 T. Whereas, the behavior at H=7.04 T indicates the more dominance of the short-range correlations among the staggered spins within the chain in the same temperature range. The behavior of the spin-lattice relaxation rate and the dynamic susceptibility in the same temperature range further support these findings. Moreover it is seen that the intrachain exchange (J=106 K) and the spin gap (=25 K) for the uninterrupted chain remain almost unchanged after Mg substitution. Finally the present results suggest the coexistence of spin gapped phase of the uninterrupted chain together with the 3D correlated impurity induced staggered spins at H=1.198 T in SrNi1.93Mg0.07V2O8.