Collective Spin Dynamics in the "Coherence Window" for Quantum Nanomagnets

TL;DR

This paper investigates the conditions under which collective coherent spin dynamics can occur in nanomagnetic insulators within a narrow 'coherence window', providing a framework to identify suitable systems for observing quantum coherence.

Contribution

It offers a theoretical analysis of multispin correlations and identifies parameter regions for coherent cluster formation in dipole-coupled nanomagnetic systems.

Findings

Identification of a 'coherence window' where decoherence is minimized

Determination of parameter regimes for coherent cluster emergence

Application to specific molecular nanomagnet systems

Abstract

The spin coherence phenomena and the possibility of their observation in nanomagnetic insulators attract more and more attention in the last several years. Recently it has been shown that in these systems in large transverse magnetic field there can be a fairly narrow "coherence window" for phonon and nuclear spin-mediated decoherence. What kind of spin dynamics can then be expected in this window in a crystal of magnetic nanomolecules coupled to phonons, to nuclear spin bath and it to each other via dipole-dipole interactions? Studying multispin correlations, we determine the region of parameters where "coherent clusters" of collective spin excitations can appear. Although two particular systems, namely crystals of $Fe_8$-triazacyclonane and $Mn_{12}$-acetate molecules, are used in this work to illustrate the results, here we are not trying to predict an existence of collective coherent dynamics in some particular system. Instead, we discuss the way how any crystalline system of dipole-dipole coupled nanomolecules can be analyzed to decide whether this system is suitable for attempts to observe coherent dynamics. The presented analysis can be useful in the search for magnetic systems showing the spin coherence phenomena.