arXiv subjects

C. Cheung

Publications and source records attributed to C. Cheung.

12 recordsLinked to original sources

arXiv2025

Polarizabilities of low-lying states of silver

Assembly of ultracold polar molecules containing silver (Ag) from laser-cooled atoms requires knowledge of the dynamic polarizabilities of Ag at convenient laser wavelengths. We present calculations and analysis of the energies and electric-dipole dc and ac polarizabilities of the low-lying states of neutral Ag. Calculations of the properties of the 4d^{10}x states, where x=5s,6s,7s,5p,6p,7p,5d,6d, and 4f, are performed using the linearized coupled cluster single-double method. The properties of the 4d^9 5s^2 ^2D_{5/2,3/2} states are obtained within the framework of configuration interaction with 11 and 17 electrons in the valence field. We analyze the different contributions to the polarizabilities and estimate the uncertainties of our predictions.

physics.atom-ph↗

arXiv2024

Pr10+ as a candidate for a high-accuracy optical clock for tests of fundamental physics

We propose In-like Pr10+ as a candidate for the development of a high-accuracy optical clock with high sensitivity to a time variation of the fine-structure constant, (\dot alpha}/alpha, as well as favorable experimental systematics. We calculate its low-lying energy levels by combining the configuration interaction and the coupled cluster method, achieving uncertainties as low as 0.1%, and improving previous work. We benchmark these results by comparing our calculations for the (5s^2 5p 2P_1/2) - (5s^2 5p 2P_3/2) transition in Pr10+ with a dedicated measurement and for Pr9+ with a recent experiment, respectively. In addition, we report calculated hyperfine-structure constants for the clock and logic states in Pr10+.

physics.atom-ph↗

arXiv2022

State-Insensitive Trapping of Alkaline-Earth Atoms in a Nanofiber-Based Optical Dipole Trap

Neutral atoms trapped in the evanescent optical potentials of nanotapered optical fibers are a promising platform for developing quantum technologies and exploring fundamental science, such as quantum networks and quantum electrodynamics. Building on the successful advancements with trapped alkali atoms, here we demonstrate a state-insensitive optical dipole trap for strontium-88, an alkaline-earth atom, using the evanescent fields of a nanotapered optical fiber. Leveraging the low laser-cooling temperatures of $\sim\!\!1~\mu$K readily achievable with strontium, we demonstrate trapping in record low trap depths corresponding to $\sim\!\!3~\mu$K. Further, employing a double magic wavelength trapping scheme, we realize state-insensitive trapping on the kilohertz-wide $5s^{2}\;^{1}\!S_{0}-5s5p\;^{3}\!P_{1,|m|=1}$ cooling transition, which we verify by performing near-surface high-resolution spectroscopy of the atomic transition. This allows us to experimentally find and verify the state insensitivity of the trap nearby a theoretically predicted magic wavelength of 435.827(25) nm. Given the non-magnetic ground state and low collisional scattering length of strontium-88, this work also lays the foundation for developing versatile and robust matter-wave atomtronic circuits over nanophotonic waveguides.

physics.atom-ph↗

arXiv2022

Calculation of energies and hyperfine structure constants of 233U^+ and 233U

We carried out calculations of the energies and magnetic dipole hyperfine structure constants of the low-lying states of 233U^+ and 233U using two different approaches. With six valence electrons and a very heavy core, uranium represents a major challenge for precision atomic theory even using large-scale computational resources. The first approach combines configuration interaction (CI) with a method allowing us to include core-valence correlations to all orders of the perturbation theory over residual Coulomb interaction. The second approach is a pure CI method which allows the use of different initial approximations. We present a detailed analysis of all calculated properties and discuss the advantages and disadvantages of each of these methods. We report a preliminary value of the U nuclear magnetic moment and outline the need for further experiments.

physics.atom-ph↗

arXiv2021

Laser Spectroscopy of the y$^7$P$_J^{\circ}$ states of Cr I

Here we report measured and calculated values of decay rates of the 3d$^4$($^5$D)4s4p($^3$P$^{\rm{o}}$)\ y$^7$P$^{\rm{o}}_{2,3,4}$ states of Cr I. The decay rates are measured using time-correlated single photon counting with roughly 1% total uncertainty. In addition, the isotope shifts for these transitions are measured by laser induced fluorescence to roughly 0.5% uncertainty. The decay rate calculations are carried out by a hybrid approach that combines configuration interaction and the linearized coupled cluster method (CI+all-order method). The measurements provide a much needed precision benchmark for testing the accuracy of the CI+all-order approach for such complicated systems with six valence electrons, allowing to significantly expand its applicability. These measurements also demonstrate operation of a cryogenic buffer gas beam source for future experiments with MgF molecules toward quantum blackbody thermometry.

physics.atom-ph↗

arXiv2021

Low-lying energy levels of ^{229}Th35+ and the electronic bridge process

The nuclear transition between the ground and the low-energy isomeric state in the ^{229}Th nucleus is of interest due to its high sensitivity to a hypothetical temporal variation of the fundamental constants and a possibility to build a very precise nuclear clock, but precise knowledge of the nuclear clock transition frequency is required. In this work we estimate the probability of an electronic bridge process in ^{229}Th^35+, allowing to determine the nuclear transition frequency and reduce its uncertainty. Using configuration interaction methods we calculated energies of the low-lying states of Th^35+ and determined their uncertainties.

physics.atom-ph↗

arXiv2021

Scalable codes for precision calculations of properties of complex atomic systems

High precision atomic data is indispensable for experiments involving studies of fundamental interactions, astrophysics, atomic clocks, plasma science, and others. We develop new parallel atomic structure codes and explore the difficulties of load-balancing in these codes. Efficient load-balancing of matrix elements for many-electron systems is very difficult due to the intrinsic nature of the computational methods used to compute them. By arithmetically selecting determinants for each core, we achieve very even workload distribution, and attain near-perfect linear scalability and efficiency with the number of cores. We also implement dynamic memory allocation to minimize memory usage and remove the need for users to set certain array parameters. Our newly developed codes enable computations that were not possible before due to lack of memory or prohibitive computation times, and allow a broader range of correlations to be investigated in a shorter period of time. This includes calculations correlating all 60 electrons in the highly charged Ir$^{17+}$ ion and calculations predicting the $3C/3D$ line intensity ratio in Fe$^{16+}$. Our new code package will also be used to produce large volumes of high precision atomic data for a new online portal being developed at the University of Delaware.

physics.atom-ph↗

arXiv2020

Observation of an electric quadrupole transition in a negative ion: Experiment and Theory

The first direct experimental observation of an electric quadrupole ($\textit{E}$2) transition between bound states of an atomic negative ion has been made. The transition was observed in the negative ion of bismuth by resonant (1+1) photodetachment from Bi$^-$ $^3\textit{P}_2$ via excitation of the Bi$^-$ $^3\textit{P}_0$ fine structure state. The $\textit{E}$2 transition properties were independently calculated using a hybrid theoretical approach to account for the strong multi-level electron interactions and relativistic effects. The experimental and theoretical results are in excellent agreement, providing valuable new insights into this complex system and forbidden transitions in negative ions.

physics.atom-ph↗

arXiv2020

Predicting quasibound states of negative ions

We demonstrated the accurate prediction of a quasibound spectrum of a negative ion using a novel high-precision theoretical approach. We used La$^-$ as a test case due to a recent experiment that measured energies of 11 resonances in its photodetachment spectrum attributed to transitions to quasibound states [C. W. Walter et al., PRA, in press (2020); arXiv:2010.01122]. We identified all of the observed resonances, and predicted one more peak just outside the range of the prior experiment. Following the theoretical prediction, the peak was observed at the predicted wavelength, validating the identification. The same approach is applicable to a wide range of negative ions. Moreover, theory advances reported in this work can be used for massive generation of atomic transition properties for neutrals and positive ions needed for a variety of applications.

physics.atom-ph↗

arXiv2019

Accurate prediction of clock transitions in a highly charged ion with complex electronic structure

We have developed a broadly-applicable approach that drastically increases the ability to accurately predict properties of complex atoms. We applied it to the case of Ir$^{17+}$, which is of particular interest for the development of novel atomic clocks with high sensitivity to the variation of the fine-structure constant and dark matter searches. The clock transitions are weak and very difficult to identity without accurate theoretical predictions. In the case of Ir$^{17+}$, even stronger electric-dipole (E1) transitions eluded observation despite years of effort raising the possibility that theory predictions are grossly wrong. In this work, we provide accurate predictions of transition wavelengths and E1 transition rates in Ir$^{17+}$. Our results explain the lack of observation of the E1 transitions and provide a pathway towards detection of clock transitions. Computational advances demonstrated in this work are widely applicable to most elements in the periodic table and will allow to solve numerous problems in atomic physics, astrophysics, and plasma physics.

physics.atom-ph↗

arXiv2015

The Third Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope

The third catalog of active galactic nuclei (AGNs) detected by the Fermi-LAT (3LAC) is presented. It is based on the third Fermi-LAT catalog (3FGL) of sources detected between 100 MeV and 300 GeV with a Test Statistic (TS) greater than 25, between 2008 August 4 and 2012 July 31. The 3LAC includes 1591 AGNs located at high Galactic latitudes (|b|>10{\deg}), a 71% increase over the second catalog based on 2 years of data. There are 28 duplicate associations, thus 1563 of the 2192 high-latitude gamma-ray sources of the 3FGL catalog are AGNs. Most of them (98%) are blazars. About half of the newly detected blazars are of unknown type, i.e., they lack spectroscopic information of sufficient quality to determine the strength of their emission lines. Based on their gamma-ray spectral properties, these sources are evenly split between flat-spectrum radio quasars (FSRQs) and BL~Lacs. The most abundant detected BL~Lacs are of the high-synchrotron-peaked (HSP) type. About 50% of the BL~Lacs have no measured redshifts. A few new rare outliers (HSP-FSRQs and high-luminosity HSP BL~Lacs) are reported. The general properties of the 3LAC sample confirm previous findings from earlier catalogs. The fraction of 3LAC blazars in the total population of blazars listed in BZCAT remains non-negligible even at the faint ends of the BZCAT-blazar radio, optical and X-ray flux distributions, which is a clue that even the faintest known blazars could eventually shine in gamma rays at LAT-detection levels. The energy-flux distributions of the different blazar populations are in good agreement with extrapolation from earlier catalogs.

astro-ph.HE↗

arXiv2005

Stark-modulation spectroscopy of the B(1)$[^3Π]$ state of PbO

We report detailed spectroscopic measurements of the X(0)$[^3Σ^+]$(v=0) to B(1)$[^3Π]$ (v=5) transition in PbO. Using a Stark-modulated laser absorption technique, we have measured the hyperfine constant of $^{207}$PbO in the B(1) state, as well as B(1)(v=5) rotational constant, X-B isotope shifts, etc. The hyperfine structure of the B(1) state is of interest as a benchmark for calculations of PbO electronic structure, related to experiments to search for the electric dipole moment of the electron.

physics.atom-ph↗