Electron spin relaxation of N@C60 in CS2

TL;DR

This study investigates electron spin relaxation in N@C60 and N@C70 molecules in CS2, revealing an Arrhenius temperature dependence for N@C60 and identifying an additional relaxation mechanism in N@C70, with the longest T2 observed for such spins.

Contribution

It demonstrates that spin relaxation in N@C60 is driven by an Orbach process, challenging the previously assumed fluctuating ZFS mechanism, and reports the longest T2 for molecular electron spins.

Findings

N@C60 relaxation follows an Arrhenius law, indicating an Orbach process.

N@C70 exhibits an additional relaxation mechanism due to ZFS reorientation.

Longest observed T2 for molecular electron spin is 0.25 ms at 170K.

Abstract

We examine the temperature dependence of the relaxation times of the molecules N@C60 and N@C70 (which comprise atomic nitrogen trapped within a carbon cage) in liquid CS2 solution. The results are inconsistent with the fluctuating zero field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for S > 1/2 spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find a clear Arrhenius temperature dependence for N@C60, indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric N@C70 molecule, which has a permanent non-zero ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time (T2) ever observed for a molecular electron spin, being 0.25 ms at 170K.