Decoupling Dipolar Interactions in Dense Spin Ensembles

TL;DR

This paper evaluates various pulse sequences for suppressing dipolar interactions in dense spin ensembles, combining simulations and NMR experiments to understand their effectiveness and the influence of experimental imperfections.

Contribution

It provides a comprehensive comparison of known pulse sequences using simulations and experiments, highlighting the effects of control errors and local disorder on decoupling performance.

Findings

Disagreements between theory and experiment often due to control errors.

Finite pulse widths and rotation errors significantly impact sequence performance.

Local disorder influences the efficiency of decoupling sequences.

Abstract

Dense spin ensembles in solids present a natural platform for studying quantum many-body dynamics. Multiple-pulse coherent control can be used to manipulate the magnetic dipolar interaction between the spins to engineer their dynamics. Here, we investigate the performance of a series of well-known pulse sequences that aim to suppress inter-spin dipolar couplings. We use a combination of numerical simulations and solid-state nuclear magnetic resonance (NMR) experiments on adamantane to evaluate and compare sequence performance. We study the role of sequence parameters like inter-pulse delays and resonance offsets. Disagreements between experiments and theory are typically explained by the presence of control errors and experimental non-idealities. The simulations allow us to explore the influence of factors such as finite pulse widths, rotation errors, and phase transient errors. We also investigate the role of local disorder and establish that it is, perhaps unsurprisingly, a distinguishing factor in the decoupling efficiency of spectroscopic sequences (that preserve Hamiltonian terms proportional to $S_z$) and time-suspension sequences (which refocus all terms in the internal Hamiltonian). We discuss our findings in the context of previously known analytical results from Average Hamiltonian Theory. Finally, we explore the ability of time-suspension sequences to protect multi-spin correlations in the system.