arXiv subjects

H. W. T. Morgan

Publications and source records attributed to H. W. T. Morgan.

9 recordsLinked to original sources

arXiv2026

Ab initio calculations of $^{229}$Th band-to-band internal conversion rate in $^{229}$ThO$_2$

We present an ab initio calculation of the band-to-band internal-conversion rate of the $\hbarω_{\rm nuc} \approx 8.35$ eV isomeric transition in $^{229}$ThO$_2$. Because the nuclear transition energy exceeds the electronic band gap of ThO$_2$, the isomer can decay nonradiatively by resonantly promoting a valence electron into the conduction band. We formulate this process as a Brillouin-zone sum over vertical interband transitions weighted by local Th-centered hyperfine matrix elements, which are evaluated directly from all-electron full-potential linearized augmented-plane-wave Bloch spinors. A finite nuclear magnetization model is included to regularize the short-range hyperfine interaction and to account for the Bohr-Weisskopf effect. After applying scissor shifts to span the experimentally reported ThO$_2$ band gaps, we find calculated internal-conversion lifetimes in the range of $1-16~μ{\rm s}$. The lifetime increases strongly as the band gap approaches $ω_{\rm nuc}$ because the resonant interband phase space at the nuclear transition energy is reduced. For the larger reported ThO$_2$ gaps, the calculated lifetime is comparable to the measured conversion-electron Mössbauer lifetime [Nature 648, 300 (2025)]. Our analysis implies that choosing solid-state hosts with band-gap values slightly lower than $ω_{\rm nuc}$ can optimize solid-state nuclear clock performance with internal-conversion electron readout.

nucl-th↗

arXiv2026

Electric field gradient in accurate quantum chemical calculations

The electric field gradients (EFGs) at the (non-spherical) nucleus contribute to atomic and molecular hyperfine structure and govern Nuclear Quadrupole Resonance (NQR) and Mössbauer spectra. EFGs provide a highly sensitive probe of local bonding, symmetry, and crystal defect geometry and electronic structure. The EFGs can be obtained from electronic structure calculations and can also be extracted from spectroscopic measurements, thus linking electronic structure theory and spectroscopic observables. In this work, we present a methodological study of EFGs for a range of molecules and crystalline materials, using both periodic boundary conditions and embedded cluster models, and compare the results with reported experimental data. We analyze the sensitivity of EFG values to details of the calculations, such as the selection of the model Hamiltonians, basis sets, and the geometries of molecules and crystals. We also address persistent differences in EFG sign conventions and tensor definitions employed in the literature and in widely used quantum chemistry codes. While the EFG sign does not affect zero B-field NQR spectra, they can become critical in Mossbauer spectroscopy or when the quadrupolar interactions are combined with other interactions of the nucleus with the environment. Together, our systematic study results provide practical guidelines for computing, interpreting, and exploiting EFGs as quantitative descriptors of electronic structure and chemical environment.

physics.chem-ph↗

arXiv2025

Host-dependent frequency offsets in $^{229}$Th nuclear clockwork

Recent advances in laser excitation of the low-energy nuclear isomer transition in $^{229}$Th have opened avenues for developing nuclear clocks, a novel quantum technology with exceptional performance and sensitivity to exotic physics. Here we explore the host-dependence of the nuclear clock frequency, focusing on the isomer shift induced by the difference in the nuclear charge distribution between the ground and excited nuclear states. We combine relativistic many-body methods of atomic structure with periodic density functional theory to evaluate the isomer shifts in solid-state hosts. We elucidate the critical importance of the ``relaxation'' effect in evaluating the isomer shifts. Our analysis predicts nuclear clock frequencies for various solid-state and trapped ion platforms: $ ω_\text{clk}(\text{solid state}) = 2,020,407,384(40) \, \text{MHz}$, $ω_\text{clk}(^{229}\text{Th}^{4+}) = 2,020,407,648(70) \, \text{MHz}$, and $ ω_\text{clk}(^{229}\text{Th}^{3+}) = 2,020,407,114(70) \, \text{MHz}$. We also determine the nuclear transition energy for the bare $^{229}$Th nucleus to be $ω_\text{nuc} = 8.272(22) \,\text{eV}$. Our calculated valence-band isomer shifts for different host materials constrain the nuclear transition frequencies to an 80 MHz-wide frequency window, aiding experimental searches for the $^{229}$Th nuclear transition in novel materials.

physics.atom-ph↗

arXiv2025

Theory of internal conversion of the thorium-229 nuclear isomer in solid-state hosts

Laser excitation of thorium-229 nuclei in doped wide bandgap crystals has been demonstrated recently, opening the possibility of developing ultrastable solid-state clocks and sensitive searches for new physics. We develop a quantitative theory of the internal conversion of isomeric thorium-229 in solid-state hosts. The internal conversion of the isomer proceeds by resonantly exciting a valence band electron to a defect state, accompanied by multi-phonon emission. We demonstrate that, if the process is energetically allowed, it generally quenches the isomer on timescales much faster than the isomer's radiative lifetime, despite thorium being in the +4 charge state in the valence band.

physics.atom-ph↗

arXiv2024

Photo-Induced Quenching of the 229Th Isomer in a Solid-State Host

The population dynamics of the 229Th isomeric state is studied in a solid-state host under laser illumination. A photoquenching process is observed, where off-resonant vacuum-ultraviolet (VUV) radiation leads to relaxation of the isomeric state. The cross-section for this photoquenching process is measured and a model for the decay process, where photoexcitation of electronic states within the material bandgap opens an internal conversion decay channel, is presented and appears to reproduce the measured cross-section.

physics.atom-ph↗

arXiv2024

229Th-doped nonlinear optical crystals for compact solid-state clocks

The recent laser excitation of the 229Th isomeric transition in a solid-state host opens the door for a portable solid-state nuclear optical clock. However, at present the vacuum-ultraviolet laser systems required for clock operation are not conducive to a fieldable form factor. Here, we propose a possible solution to this problem by using 229Th-doped nonlinear optical crystals, which would allow clock operation without a vacuum-ultraviolet laser system and without the need of maintaining the crystal under vacuum.

physics.optics↗

arXiv2024

$^{229}\mathrm{ThF}_4$ thin films for solid-state nuclear clocks

After nearly fifty years of searching, the vacuum ultraviolet $^{229}$Th nuclear isomeric transition has recently been directly laser excited [1,2] and measured with high spectroscopic precision [3]. Nuclear clocks based on this transition are expected to be more robust [4,5] than and may outperform [6,7] current optical atomic clocks. They also promise sensitive tests for new physics beyond the standard model [5,8,9]. In light of these important advances and applications, a dramatic increase in the need for $^{229}$Th spectroscopy targets in a variety of platforms is anticipated. However, the growth and handling of high-concentration $^{229}$Th-doped crystals [5] used in previous measurements [1-3,10] are challenging due to the scarcity and radioactivity of the $^{229}$Th material. Here, we demonstrate a potentially scalable solution to these problems by demonstrating laser excitation of the nuclear transition in $^{229}$ThF$_4$ thin films grown with a physical vapor deposition process, consuming only micrograms of $^{229}$Th material. The $^{229}$ThF$_4$ thin films are intrinsically compatible with photonics platforms and nanofabrication tools for integration with laser sources and detectors, paving the way for an integrated and field-deployable solid-state nuclear clock with radioactivity up to three orders of magnitude smaller than typical \thor-doped crystals [1-3,10]. The high nuclear emitter density in $^{229}$ThF$_4$ also potentially enables quantum optics studies in a new regime. Finally, we describe the operation and present the estimation of the performance of a nuclear clock based on a defect-free ThF$_4$ crystal.

physics.atom-ph↗

arXiv2024

Radiative Decay of the $^{229m}$Th Nuclear Clock Isomer in Different Host Materials

A comparative vacuum ultraviolet spectroscopy study conducted at ISOLDE-CERN of the radiative decay of the $^{229m}$Th nuclear clock isomer embedded in different host materials is reported. The ratio of the number of radiative decay photons and the number of $^{229m}$Th embedded are determined for single crystalline CaF$_2$, MgF$_2$, LiSrAlF$_6$, AlN, and amorphous SiO$_2$. For the latter two materials, no radiative decay signal was observed and an upper limit of the ratio is reported. The radiative decay wavelength was determined in LiSrAlF$_6$ and CaF$_2$, reducing its uncertainty by a factor of 2.5 relative to our previous measurement. This value is in agreement with the recently reported improved values from laser excitation.

nucl-ex↗

arXiv2024

Laser excitation of the $^{229}$Th nuclear isomeric transition in a solid-state host

LiSrAlF$_6$ crystals doped with $^{229}$Th are used in a laser-based search for the nuclear isomeric transition. Two spectroscopic features near the nuclear transition energy are observed. The first is a broad excitation feature that produces red-shifted fluorescence that decays with a timescale of a few seconds. The second is a narrow, laser-linewidth-limited spectral feature at $148.38219(4)_{\textrm{stat}}(20)_{\textrm{sys}}$ nm ($2020407.3(5)_{\textrm{stat}}(30)_{\textrm{sys}}$ GHz) that decays with a lifetime of $568(13)_{\textrm{stat}}(20)_{\textrm{sys}}$ s. This feature is assigned to the excitation of the $^{229}$Th nuclear isomeric state, whose energy is found to be $8.355733(2)_{\textrm{stat}}(10)_{\textrm{sys}}$ eV in $^{229}$Th:\thor:LiSrAlF$_6$.

physics.atom-ph↗