Decay of highly-correlated spin states in a dipolar-coupled solid

TL;DR

This study investigates how highly-correlated spin states decay in dipolar-coupled solids, revealing dependence on cluster size and coherence number, with Gaussian decay patterns observed across different systems and conditions.

Contribution

It provides new insights into the decay behavior of multiple quantum coherences in both three-dimensional and one-dimensional spin systems, under various Hamiltonian conditions.

Findings

Decay rates depend on cluster size and coherence number in calcium fluoride.

Gaussian decay functions with standard deviation scaling linearly with coherence number.

Coherence growth is restricted in one-dimensional systems but still involves increasing correlated spins.

Abstract

We have measured the decay of NMR multiple quantum coherence intensities both under the internal dipolar Hamiltonian as well as when this interaction is effectively averaged to zero, in the cubic calcium fluoride (CaF2) spin system and the pseudo one-dimensional system of fluoroapatite. In calcium fluoride the decay rates depend both on the number of correlated spins in the cluster, as well as on the coherence number. For smaller clusters, the decays depend strongly on coherence number, but this dependence weakens as the size of the cluster increases. The same scaling was observed when the coherence distribution was measured in both the usual Zeeman or z basis and the x basis. The coherence decay in the one dimensional fluoroapatite system did not change significantly as a function of the multiple quantum growth time, in contrast to the calcium fluoride case. While the growth of coherence orders is severely restricted in this case, the number of correlated spins should continue to grow, albeit more slowly. All coherence intensities were observed to decay as Gaussian functions in time. In all cases the standard deviation of the observed decay appeared to scale linearly with coherence number.