Decay of nuclear hyperpolarization in silicon microparticles

TL;DR

This study examines how nuclear hyperpolarization in silicon microparticles relaxes at room temperature, revealing different relaxation mechanisms in doped versus undoped particles and modeling surface-related relaxation processes.

Contribution

It provides a detailed analysis of nuclear spin relaxation times in silicon microparticles, introducing a model for surface-mediated relaxation and distinguishing effects of doping.

Findings

Undoped particles show two relaxation time scales, T_1s and T_1f.

Doped particles exhibit a single relaxation time scale.

Surface relaxation dominates in undoped particles, bulk mechanisms in doped ones.

Abstract

We investigate the low-field relaxation of nuclear hyperpolarization in undoped and highly doped silicon microparticles at room temperature following removal from high field. For nominally undoped particles, two relaxation time scales are identified for ambient fields above 0.2 mT. The slower, T_1s, is roughly independent of ambient field; the faster, T_1f, decreases with increasing ambient field. A model in which nuclear spin relaxation occurs at the particle surface via a two-electron mechanism is shown to be in good agreement with the experimental data, particularly the field-independence of T_1s. For boron-doped particles, a single relaxation time scale is observed. This suggests that for doped particles, mobile carriers and bulk ionized acceptor sites, rather than paramagnetic surface states, are the dominant relaxation mechanisms. Relaxation times for the undoped particles are not affected by tumbling in a liquid solution.