Determine whether an unmodeled fluctuating spin bath contributes to decoherence

Determine whether any overlooked spin species in the paramagnetic bath contributes to the low-temperature decoherence and resonant-field dependence of the Tb^3+ Hahn-echo coherence time in CaWO4, and, if so, characterize the concentration and spectral-diffusion dynamics required to account for the observations.

Background

The paper explains the observed low-temperature saturation and monotonic decrease of the Tb^3+ coherence time with resonant field by introducing dynamical angular jitter of the crystal c-axis. A parameter-free model incorporating known spin-lattice relaxation, instantaneous diffusion, and spectral diffusion from quantified impurities fails to reproduce the measurements, motivating the angular-jitter mechanism.

The authors nevertheless acknowledge that an incompletely characterized paramagnetic environment could provide an alternative explanation. They state that an overlooked spin species remains possible, although the concentration and field- and temperature-dependent spectral-diffusion behavior required to reproduce the data make this hypothesis improbable. Thus, whether an additional fluctuating spin bath exists and contributes materially remains unresolved.