Strongly anisotropic non-Kramers electron spin as a quantum coherence probe of angular fluctuations

Rare-earth ions with non-Kramers ground states exhibit strong g-factor anisotropy and a vanishing transverse g-factor, making their spin transition frequencies highly sensitive to the orientation of the applied magnetic field. This paper investigates whether this sensitivity, while useful for certain applications, acts as a significant decoherence channel in pulsed electron paramagnetic resonance (EPR) experiments, and whether it can be exploited to sense mechanical angular fluctuations.

The researchers performed pulsed EPR measurements on Tb3+ ions in a native-doped CaWO4 crystal, mapping the Hahn-echo coherence time (T2) across a wide range of temperatures (2–10 K) and resonant magnetic fields (10^3–10^4 G). They compared these experimental results against a comprehensive parameter-free model that accounted for spin-lattice relaxation, instantaneous diffusion, and spectral diffusion from all identified paramagnetic impurities. When this standard model failed to explain the observed T2 values and their field dependence, the authors introduced a two-parameter extension to account for dynamical angular fluctuations of the crystal axis.

The standard spin-bath model significantly overestimated T2 at low temperatures and failed to predict the observed monotonic decrease of T2 with increasing resonant field. The extended model, which incorporates both global mechanical vibrations and local phonon-driven angular jitter, successfully reproduced the entire dataset. Controlled experiments—such as switching mechanical pumps on and off and adjusting the mechanical tightness of the cryostat insert—confirmed that the dominant decoherence source is indeed mechanical in origin. The study demonstrates that these anisotropic spins can serve as sensitive angular probes, with a resolution of approximately 36 n°/√Hz in the kilohertz frequency band.