Comment on "Generating Unexpected Spin Echoes in Dipolar Solids with Pi Pulses" arXiv:0705.0667

TL;DR

This paper critiques a previous study on NMR pulse sequences in solids, arguing that the assumptions and parameters used to explain long-lived signals are incorrect, thus challenging the original interpretation of the experimental results.

Contribution

It provides a critical analysis of the assumptions and parameters used in the original work, clarifying the interpretation of long-lived signals in NMR experiments.

Findings

The long tails are not due to coherent dipolar phenomena during finite pulses.

Incorrect assumptions led to misinterpretation of experimental signals.

The critique clarifies the conditions under which long-lived signals can occur.

Abstract

The present work is a comment on PRL 98, 190401 (2007), arXiv:0705.0667 . The authors reported NMR measurements in solids with different pulse sequences. In some multiple $\pi$ pulse echo trains they observed coherent signals of several seconds, well beyond the Hahn echo decay, $T_{2HE}$ (few milliseconds). They ascribed the long tails to coherent phenomena arising on the dipolar couplings during any real finite NMR pulse. Their statement is based on some assumptions that are incorrect and parameters in the calculations, that do not represent the experimental conditions.