arXiv subjects

Ricky Elwell

Publications and source records attributed to Ricky Elwell.

3 recordsLinked to original sources

arXiv2026

Defect assignment of the clock site in $^{229}\text{Th:CaF}_2$

The performance of solid-state $^{229}\text{Th}$ nuclear clocks depends sensitively on the microscopic environment of the thorium nucleus in the host crystal. Here we reassess the dominant quadrupole-split thorium site in $^{229}\text{Th:CaF}_2$, which has been assigned to a thorium dimer in recent spectroscopic work. Thermodynamic estimates, density functional theory calculations, and electric-field-gradient comparisons instead favor an isolated $\text{Th}^{4+}$ substitution on a $\text{Ca}^{2+}$ site charge-compensated by two nearby fluorine interstitials in a relaxed $90^\circ$ motif. The same calculation identifies a higher-energy mixed-shell interstitial motif as a plausible minor site. The clock-active quadrupole-split site is therefore controlled by local fluoride compensation rather than unavoidable thorium aggregation. This defect assignment also has implications for achievable linewidths and provides a microscopic basis for reducing broadening in solid-state nuclear clocks.

physics.atom-ph↗

arXiv2025

$^{229}$Th Nuclear Spectroscopy in an Opaque Material: Laser-Based Conversion Electron M\"ossbauer Spectroscopy of $^{229}$ThO$_2$

Here, we report the first demonstration of laser-induced conversion electron M\"{o}ssbauer spectroscopy of the $^{229}$Th nuclear isomeric state, which provides the ability to probe the nuclear transition in a material that is opaque to light resonant with the nuclear transition. Specifically, we excite the nuclear transition in a thin ThO$_2$ sample whose band gap ($\sim$ 6 eV) is considerably smaller than the nuclear isomeric state energy (8.4 eV). As a result, the excited nucleus can quickly decay by internal conversion, resulting in the ejection of electrons from the surface. By collecting these conversion electrons, nuclear spectroscopy can be recorded. Unlike fluorescence spectroscopy, this technique is compatible with materials whose work function is less than the nuclear transition energy, opening a wider class of systems to study. Further, because ThO$_2$ can be made from spinless isotopes and the internal conversion decay process reduces the isomeric state lifetime to only $\sim$10 $\mu$s, allowing $\sim$10$^8$ relative reduction in clock interrogation time, a conversion-electron-based nuclear clock could lead to a $\sim$10$^4$ reduction in clock instability.

physics.atom-ph↗

arXiv2025

Investigation into thorium sulfate as a spinless crystal for a high-performance solid-state $^{229}Th$ nuclear clock

The $^{229}Th$ isotope has a laser-accessible nuclear transition allowing the construction of a solid-state nuclear clock. Solid-state $^{229}Th$ nuclear clock development has focused on $^{229}Th$-doped $CaF_2$ and other metal fluorides as the solid material. However, the accuracy of any fluoride-based clock will be reduced by nuclear magnetic dipole coupling to $^{19}F$ and by inhomogeneities in the thorium defect environments. Here we explore thorium sulfate, $Th(SO_4)_2$, as a solution to both problems. Strong bonding and charge delocalization in the $SO_4^{2-}$ anion may give $Th(SO_4)_2$ the wide band gap needed for a good clock. In this work we synthesize $Th(SO_4)_2$ and probe it as a nuclear clock material spectroscopically, finding that it is opaque to the 148 nm radiation that excites the $^{229}Th$ nucleus. Theoretical analysis of the optical spectra shows that charge transfer excitons are responsible for the absorption. We give guidelines for future material development and assess the prospects for $Th(SO_4)_2$ as a clock material.

cond-mat.mtrl-sci↗