Insights on the coupling between vibronically active molecular vibrations and lattice phonons in molecular nanomagnets

TL;DR

This study investigates how local molecular vibrations interact with lattice phonons in molecular nanomagnets, revealing that certain vibrational modes do not significantly couple with phonons, thus affecting spin relaxation mechanisms.

Contribution

It introduces a simple model to analyze vibrational-phonon coupling in molecular nanomagnets and explores implications for spin relaxation on different substrates.

Findings

Vibrational mode at low temperature does not couple significantly to lattice phonons.

Further intramolecular energy transfer is necessary for spin relaxation.

Methodology can estimate effects of different surfaces on spin relaxation.

Abstract

Spin-lattice relaxation is a key open problem to understand the spin dynamics of single-molecule magnets and molecular spin qubits. While modelling the coupling between spin states and local vibrations allows to determine the more relevant molecular vibrations for spin relaxation, this is not sufficient to explain how energy is dissipated towards the thermal bath. Herein, we employ a simple and efficient model to examine the coupling of local vibrational modes with long-wavelength longitudinal and transverse phonons in the clock-like spin qubit [Ho(W$_5$O$_{18}$)$_2$]$^{9-}$. We find that in crystals of this polyoxometalate the vibrational mode previously found to be vibronically active at low temperature does not couple significantly to lattice phonons. This means that further intramolecular energy transfer via anharmonic vibrations is necessary for spin relaxation in this system. Finally, we discuss implications for the spin-phonon coupling of [Ho(W$_5$O$_{18}$)$_2$]$^{9-}$ deposited on a MgO (001) substrate, offering a simple methodology that can be extrapolated to estimate the effects on spin relaxation of different surfaces, including 2D materials.