Comment on 'Quantum Coherence between High Spin Superposition States of Single Molecule Magnet Ni4'

TL;DR

This paper comments on recent experimental studies of Ni4 molecular magnets, emphasizing that their technique measures phonon-bottleneck times rather than direct quantum coherence, making conclusions about coherence preliminary.

Contribution

It clarifies the limitations of the experimental method used in prior studies, highlighting that it does not directly measure quantum coherence or spin-lattice relaxation times.

Findings

The technique measures phonon-bottleneck times, not T1.

Absorption line-widths provide upper bounds on decoherence rates.

Conclusions about quantum coherence are preliminary.

Abstract

In a recent paper, cond-mat/0405331 (Ref. 1), del Barco et al. reported experimental studies on a Ni4 molecular system. They used an experimental method (combining microwave spectroscopy with high sensitivity magnetic measurements) that we have introduced before, cond-mat/0404410 (see also Ref. 2 and 3). Among other things, our technique allows us to monitor spin-state populations in the presence of microwave magnetic fields. Absorption line-widths give rough 'upper bounds' on the decoherence rate similar to 'standard' high frequency electron paramagnetic resonance (HF-EPR) techniques. In the case of quasi continuous radiation our technique does NOT give directly the spin-lattice relaxation time T1. For measurements like those of del Barco et al., it simply gives access to the phonon-bottleneck time, a parameter that is many orders of magnitude longer than the spin-lattice relaxation time. Any conclusion concerning quantum coherence is preliminary.