arXiv subjects

U. C. Perera

Publications and source records attributed to U. C. Perera.

10 recordsLinked to original sources

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

Further steps towards next generation of covariant energy density functionals

The present study aims at further development of covariant energy density functionals (CEDFs) towards more accurate description of binding energies across the nuclear chart. For the first time, infinite basis corrections to binding energies in the fermionic and bosonic sectors of the covariant density functional theory have been taken into account in the fitting protocol within the covariant density functional theory. In addition, total electron binding energies have been used in the conversion of atomic binding energies into nuclear ones. Their dependence on neutron excess has been investigated for the first time across the nuclear chart within atomic approach. These factors have been disregarded in previous generation of covariant energy density functionals but their neglect leads to substantial global calculation errors for physical quantities of interest. For example, these errors for binding energies are of the order of 0.8 MeV or higher for the three major classes of covariant energy density functionals.

nucl-th↗

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↗

arXiv2023

Differential charge radii: self-consistency and proton-neutron interaction effects

The analysis of self-consistency and proton-neutron interaction effects in the buildup of differential charge radii has been carried out in covariant density functional theoretical calculations without pairing interaction. Two configurations of the $^{218}$Pb nucleus, generated by the occupation of the neutron $1i_{11/2}$ and $2g_{9/2}$ subshells, are compared with the ground state configuration in $^{208}$Pb. The interaction of added neutron(s) and the protons forming the $Z=82$ proton core is responsible for a major contribution to the buildup of differential charge radii. It depends on the overlaps of proton and neutron wave functions and leads to a redistribution of single-particle density of occupied proton states which in turn modifies the charge radii. Self-consistency effects affecting the shape of proton potential, total proton densities and the energies of the single-particle proton states provide only secondary contribution to differential charge radii. The buildup of differential charge radii is a combination of single-particle and collective phenomena. The former is due to proton-neutron interaction, the impact of which is state dependent, and the latter reflects the fact that all occupied proton single-particle states contribute to this process. The neglect of either one of these aspects of the process by ignoring proton-neutron interaction and self-consistency effects as it is done in macroscopic+microscopic approach or by introducing the core as in spherical shell model introduces uncontrollable errors and restricts the applicability of such approaches to the description of differential charge radii.

nucl-th↗

arXiv2022

Bubble nuclei: single-particle versus Coulomb interaction effects

The detailed investigation of microscopic mechanisms leading to the formation of bubble structures in the nuclei has been performed in the framework of covariant density functional theory. The main emphasis of this study is on the role of single-particle degrees of freedom and Coulomb interaction. In general, the formation of bubbles lowers the Coulomb energy. However, in nuclei this trend is counteracted by the quantum nature of the single-particle states: only specific single-particle states with specific density profiles can be occupied with increasing proton and neutron numbers. A significant role of central classically forbidden region at the bottom of the wine bottle potentials in the formation of nuclear bubbles (via primarily the reduction of the densities of the $s$ states at $r=0$) has been revealed for the first time. Their formation also depends on the availability of low-$l$ single-particle states for occupation since single-particle densities represent the basic building blocks of total densities. Nucleonic potentials disfavor the occupation of such states in hyperheavy nuclei and this contributes to the formation of bubbles in such nuclei. Additivity rule for densities has been proposed for the first time. It was shown that the differences in the densities of bubble and flat density nuclei follow this rule in the $A\approx 40$ mass region and in superheavy nuclei with comparable accuracy. This strongly suggests the same mechanism of the formation of central depression in bubble nuclei of these two mass regions. Nuclear saturation mechanisms and self-consistency effects also affect the formation of bubble structures. The detailed analysis of different aspects of bubble physics strongly suggests that the formation of bubble structures in superheavy nuclei is dominated by single-particle effects.

nucl-th↗

arXiv2021

Charge radii, moments and masses of mercury isotopes across the N = 126 shell closure

Combining laser spectroscopy in a Versatile Arc Discharge and Laser Ion Source, with Penning-trap mass spectrometry at the CERN-ISOLDE facility, this work reports on mean-square charge radii of neutron-rich mercury isotopes across the $N = 126$ shell closure, the electromagnetic moments of $^{207}$Hg and more precise mass values of $^{206-208}$Hg. The odd-even staggering (OES) of the mean square charge radii and the kink at $N = 126$ are analyzed within the framework of covariant density functional theory (CDFT), with comparisons between different functionals to investigate the dependence of the results on the underlying single-particle structure. The observed features are defined predominantly in the particle-hole channel in CDFT, since both are present in the calculations without pairing. However, the magnitude of the kink is still affected by the occupation of the $1i_{11/2}$ and $2g_{9/2}$ orbitals with a dependence on the relative energies as well as pairing.

nucl-ex↗

arXiv2021

Global performance of covariant density functional theory in description of charge radii and related indicators

A short review of existing efforts to understand charge radii and related indicators on a global scale within the covariant density functional theory (CDFT) is presented. Using major classes of covariant energy density functionals (CEDFs), the global accuracy of the description of experimental absolute and differential charge radii within the CDFT framework has been established. This assessment is supplemented by an evaluation of theoretical statistical and systematic uncertainties in the description of charge radii. New results on the accuracy of the description of differential charge radii in deformed actinides and light superheavy nuclei are presented and the role of octupole deformation in their reproduction is evaluated. Novel mechanisms leading to odd-even staggering in charge radii are discussed. Finally, we analyze the role of self-consistency effects in an accurate description of differential charge radii.

nucl-th↗

arXiv2021

Charge radii in covariant density functional theory: a global view

A systematic global investigation of differential charge radii has been performed within the CDFT framework for the first time. Theoretical results obtained with conventional covariant energy density functionals and separable pairing interaction are compared with experimental differential charge radii in the regions of the nuclear chart in which available experimental data crosses neutron shell closures at N = 28, 50, 82 and 126. The analysis of absolute differential radii of different isotopic chains and their relative properties indicate clearly that such properties are reasonably well described in model calculations in the cases when the mean-field approximation is justified. However, while the observed clusterization of differential charge radii of different isotopic chains is well described above the N=50 and N=126 shell closures, it is more difficult to reproduce it above the N=28 and N=82 shell closures because of possible deficiencies in underlying single-particle structure. The impact of the latter has been evaluated for spherical shapes and it was shown that the relative energies of the single-particle states and the patterns of their occupation with increasing neutron number have an appreciable impact on the evolution of the differential charge radii. It is shown that the kinks in the charge radii at neutron shell closures are due to the underlying single-particle structure and due to weakening or collapse of pairing at these closures. It is usually assumed that pairing is a dominant contributor to odd-even staggering (OES) in charge radii. Our analysis paints a more complicated picture. It suggests a new mechanism in which the fragmentation of the single-particle content of the ground state in odd-mass nuclei due to particle-vibration coupling provides a significant contribution to OES in charge radii.

nucl-th↗