Thorium-229 as a Phonomagnetometer

Abstract

Thorium-229 possesses the only known low-energy nuclear transition suitable for spectroscopy with narrowband VUV lasers. While previous experiments have focused on application as a nuclear clock, this transition also offers a route to high-accuracy magnetometry. Experimental observations of magnetic fields generated by phonons carrying angular momentum remain inconclusive, highlighting the need for quantitative tests of the underlying physical mechanisms. In this article, we propose doping thorium-229 into a solid-state host to probe phonomagnetic fields in situ. We identify Na$_2$ThF$_6$, a chiral stoichiometric thorium compound, as a promising host material and evaluate its spectroscopic sensitivity to magnetic interactions under two excitation schemes: driving degenerate phonon modes with a circularly polarized laser, and applying a temperature gradient. Density functional theory simulations combined with quantitative estimates suggest that the temperature-gradient scheme yields a magnetic signal that appears too weak to be measurable, while a circularly polarized, high-intensity THz/VUV pump-probe driven phonomagnetic response may approach the shot-noise-limited detection threshold. While experimental challenges remain, the unique suitability of thorium-229 provides a testable pathway toward detecting phonomagnetic fields in a solid-state platform.