Binding Sites, Vibrations and Spin-Lattice Relaxation Times in Europium(II)-based Metallofullerene Spin Qubits

TL;DR

This study investigates how structural differences in europium(II)-based metallofullerenes influence spin relaxation times, revealing that low-energy vibrations critically mediate quantum coherence, with findings showing a universal temperature dependence of T1 across different complexes.

Contribution

It demonstrates that metal-cage binding site variations affect vibrational properties and spin relaxation, providing insights for designing more coherent molecular spin qubits.

Findings

Low-energy metal-based vibrations govern spin-lattice relaxation.

Universal temperature dependence of T1 across different EMFs.

Structural differences influence vibrational rigidity and spin-vibration coupling.

Abstract

To design molecular spin qubits with enhanced quantum coherence, a control of the coupling between the local vibrations and the spin states is crucial, which could be realized in principle by engineering molecular structures via coordination chemistry. To this end, understanding the underlying structural factors that govern the spin relaxation is a central topic. Here, we report the investigation of the spin dynamics in a series of chemically-designed europium(II)-based endohedral metallofullerenes (EMFs). By introducing a unique structural difference, i.e. metal-cage binding site, while keeping other molecular parameters constant between different complexes, these manifest the key role of the three low energy metal-based vibrations in mediating the spin-lattice relaxation times (T1). The temperature dependence of T1 can thus be normalized by the frequencies of these low energy vibrations to show an unprecedentedly universal behavior for EMFs in frozen CS2 solution. Our theoretical analysis indicates that this structural difference determines not only the vibrational rigidity but also spin-vibration coupling in these EMF-based qubit candidates.