Evidence of several dipolar quasi-invariants in Liquid Crystals

TL;DR

This study provides experimental evidence of multiple dipolar quasi-invariants in small spin clusters in liquid crystals using NMR, revealing complex correlated spin states and their dynamics.

Contribution

It demonstrates the existence of several dipolar quasi-invariants in NMR experiments, extending previous knowledge beyond Zeeman and dipolar orders.

Findings

Identification of short-time dipolar quasi-invariants

Observation of echo-like signals at longer times

Correlation of states with multiple quantum coherence

Abstract

In a closed quantum system of N coupled spins with magnetic quantum number I, there are about (2I + 1)^N constants of motion. However, the possibility of observing such quasi-invariant (QI) states in solid-like spin systems in Nuclear Magnetic Resonance (NMR) is not a strictly exact prediction. The aim of this work is to provide experimental evidence of several QI, in the proton NMR of small spin clusters, besides those already known Zeeman, and dipolar orders (strong and weak). We explore the spin states prepared with the Jeener-Broekaert pulse sequence by analyzing the time-domain signals yielded by this sequence as a function of the preparation times, in a variety of dipolar networks. We observe that the signals can be explained with two dipolar QIs only within a range of short preparation times. At longer times the time-domain signals have an echo-like behaviour. We study their multiple quantum coherence content on a basis orthogonal to the z-basis and see that such states involve a significant number of correlated spins. Then we show that the whole preparation time-scale can only be reconstructed by assuming the occurrence of multiple QI which we isolate experimentally.