Generating Unexpected Spin Echoes in Dipolar Solids with Pi Pulses

TL;DR

This paper demonstrates that multiple pi pulses in NMR can unexpectedly either preserve or accelerate signal decay in dipolar solids, due to many-body effects during finite pulses, challenging conventional understanding.

Contribution

It introduces a new understanding of how finite pulse effects influence dipolar spin echoes, supported by theoretical and quantum calculations.

Findings

Multiple pi pulses can either freeze or accelerate echo decay.

The phase of pi pulses determines the echo behavior.

Dipolar coupling exhibits many-body effects during finite pulses.

Abstract

NMR spin echo measurements of C-13 in C60, Y-89 in Y2O3, and Si-29 in silicon are shown to defy conventional expectations when more that one pi pulse is used. Multiple pi-pulse echo trains may either freeze our or accelerate the decay of the signal, depending on the pi-pulse phase. Average Hamiltonian theory, combined with exact quantum calculations, reveals an intrinsic cause for these coherent phenomena: the dipolar coupling has a many-body effect during any real, finite pulse.