Attenuation of polarization echoes in NMR: A study of the emergence of dynamical irreversibility in many-body quantum systems

TL;DR

This study investigates how polarization echoes decay in NMR experiments on molecular spin systems, revealing a transition from exponential to Gaussian attenuation linked to increasing dynamical irreversibility.

Contribution

It demonstrates the crossover from exponential to Gaussian decay in polarization echoes, highlighting the role of many-body spin dynamics in irreversibility.

Findings

Exponential decay observed in cymantrene

Gaussian decay observed in ferrocene

Decay behavior linked to spin dynamics and irreversibility

Abstract

The reversal of the time evolution of the local polarization in an interacting spin system involves a sign change of the effective dipolar Hamiltonian which refocuses the 'spin diffusion' process generating a polarization echo. Here, the attenuation of these echo amplitudes as a function of evolution time is presented for cymantrene and ferrocene polycrystalline samples, involving one and two five spin rings per molecule respectively. We calculate the fraction of polarization which is not refocused because only the secular part of the dipolar Hamiltonian is inverted. The results indicate that, as long as the spin dynamics is restricted to a single ring, the non-inverted part of the Hamiltonian is notable by itself to explain the whole decay of the polarization echoes. A cross over from exponential (cymantrene) to Gaussian (ferrocene) attenuation is experimentally observed. This is attributed to an increase of the relative importance of the spin dynamics, as compared with irreversible interactions, which favors dynamical irreversibility.