Rapid Diffusion of dipolar order enhances dynamic nuclear polarization

TL;DR

This study demonstrates that rapid diffusion of dipolar order can significantly enhance dynamic nuclear polarization by cooling the dipolar reservoir faster than the Zeeman reservoir, leading to increased nuclear spin polarization.

Contribution

It reveals that exploiting the fast dipolar spin diffusion can rapidly polarize nuclear spins, improving DNP efficiency beyond previous methods.

Findings

Dipolar reservoir cooled to 15.5 micro-K during DNP.

Equilibration increased NMR signal by 50%.

Dipolar spin diffusion is faster than Zeeman diffusion.

Abstract

In a dynamic nuclear polarization experiment on a 40 mM solution of 4-amino-TEMPO in a 40:60 water/glycerol mixture, we have observed that the bulk dipolar reservoir is cooled to a spin temperature of 15.5 micro-K, following microwave irradiation for 800 s. This is significantly cooler than the 35 mK spin temperature of the Zeeman reservoir. Equilibration of the two reservoirs results in a 50 % increase in the NMR signal intensity, corresponding to a Zeeman spin temperature of 23 mK. In order to achieve this polarization directly, it was necessary to irradiate the sample with microwaves for 1500 s. Cooling of the dipolar reservoir occurs during polarization transport through the magnetic field gradient around the paramagnetic impurity, and is rapidly communicated to the bulk by dipolar spin diffusion. As dipolar spin diffusion is significantly faster than Zeeman spin diffusion, the bulk dipolar reservoir cools faster than the Zeeman reservoir. This process can be exploited to rapidly polarize the nuclear spins, by repeatedly cooling the dipolar system and transferring the polarization to the Zeeman reservoir.