vignette|Représentation schématique des niveaux fins et hyperfins de l’hydrogène. La structure hyperfine d’un niveau d’énergie dans un atome consiste en une séparation de ce niveau en états d’énergie très proches. Il s’observe essentiellement par une raie spectrale dans le domaine radio ou micro-onde, comme la raie à 21 centimètres de l’hydrogène atomique. La structure hyperfine s’explique en physique quantique comme une interaction entre deux dipôles magnétiques :

Le dipôle magnétique nucléaire résultant du spin nucléaire ;

Le dipôle magnétique électronique lié au moment cinétique orbital et au spin de l’électron.

S’y ajoutent des corrections prenant en compte le moment quadripolaire du noyau atomique. Historique La structure hyperfine optique était déjà observée en 1891 par Albert Abraham Michelson. Elle n’a, cependant, uniquement pu être expliquée qu’avec l’aide de la mécanique quantique lorsque Wolfgang Pauli a proposé l’existence d’un petit moment magnétique nuclé

Shape staggering of midshell mercury isotopes from in-source laser spectroscopy compared with density-functional-theory and Monte Carlo shell-model calculations

Neutron-deficient Hg177-185 isotopes were studied using in-source laser resonance-ionization spectroscopy at the CERN-ISOLDE radioactive ion-beam facility in an experiment combining different detection methods tailored to the studied isotopes. These include either alpha-decay tagging or multireflection time-of-flight gating for isotope identification. The endpoint of the odd-even nuclear shape staggering in mercury was observed directly by measuring for the first time the isotope shifts and hyperfine structures of Hg177-185. Changes in the meansquare charge radii for all mentioned isotopes, magnetic dipole, and electric quadrupole moments of the odd-A isotopes and arguments in favor of I = 7/2 spin assignment for Hg-177,Hg-179 were deduced. Experimental results are compared with density functional theory (DFT) and Monte Carlo shell model (MCSM) calculations. DFT calculations using Skyrme parametrizations predict a jump in the charge radius around the neutron N = 104 midshell, with an odd-even staggering pattern related to the coexistence of nearly degenerate oblate and prolate minima This near-degeneracy is highly sensitive to many aspects of the effective interaction, a fact that renders perfect agreement with experiments out of reach for current functionals. Despite this inherent difficulty, the SLy5s1 and a modified UNEDF1(SO) parametrization predict a qualitatively correct staggering that is off by two neutron numbers. MCSM calculations of states with the experimental spins and parities show good agreement for both electromagnetic moments and the observed charge radii. A clear mechanism for the origin of shape staggering within this context is identified: a substantial change in occupancy of the proton pi h(9/2) and neutron nu i(13/2) orbitals.

AMER PHYSICAL SOC2019

Magnetic Properties of Surface Adsorbed Metal Adatoms and Dimers

Dante Philippe Sblendorio

This thesis investigates the magnetic properties of single atoms and dimers adsorbed on graphene and oxide decoupling layers supported by single crystal metal substrates, using scanning tunneling microscopy (STM) and spin-polarized scanning tunneling microscopy (SP-STM). The goal is twofold: to use SP-STM to further advance the understanding of the interactions that determine the magnetic stability of Dy adatoms on graphene/Ir(111) and Ho adatoms on MgO/Ag(100), and to use insights from these systems to motivate the study of new systems-- mainly, IrCo heterodimers on graphene/Ir(111).For Dy adatoms on graphene/Ir(111), the characteristic lifetime of the spin system is measured as a function of temperature and tunnel bias, probing the available magnetization reversal pathways in the energy level diagram. The necessity of including the intra-atomic exchange to correctly describe spin lifetimes is demonstrated. In addition, naturally abundant Dy isotopes possess two possible nuclear spin values. Models of both nuclear spin cases of Dy are compared, and shown to produce similar behavior in the temperature and bias ranges probed by SP-STM. Furthermore, accounting for both nuclear spin cases allows for modeling of X-ray magnetic circular dichroism (XMCD) magnetization sweeps. For Ho adatoms on MgO/Ag(100), novel measurement protocols are used to determine the zero-field stability, and the correct ground state model. These protocols are also used to induce magnetic state reversal via Landau-Zener tunneling at avoided level crossings due to the hyperfine interaction. Finally, the study of the IrCo heterodimer adsorbed on graphene is motivated with stability considerations derived from the studies of Dy adatoms on graphene/Ir(111) and Ho adatoms on MgO/Ag(100), in addition to calculations based on density function theory (DFT). Strategies of engineering these heterodimers using statistical growth and atomic manipulation are successfully implemented. The heterodimer is highly mobile on graphene/Ir(111) and displays geometric instability, consistent with its predicted upstanding geometry. Despite this, measurement via STM and SP-STM is demonstrated. Observations are consistent with the robust magnetic stability expected from DFT studies, but additional investigation is needed to disentangle the measurements made thus far.

EPFL2022

First-principles modeling of paramagnetic Si dangling-bond defects in amorphous SiO2

Alfredo Pasquarello

We modeled paramagnetic Si dangling-bond defects in amorphous SiO2 using a generalized-gradient density-functional approach. By creating single oxygen vacancies in a periodic model of amorphous SiO2, we first generated several model structures in which the core of the defect consists of a threefold coordinated Si atom carrying a dangling bond. These model structures were then fully relaxed and the hyperfine parameters calculated. We found that the hyperfine parameters of such model defects, in both the neutral and positive charge states, reproduced those characteristic of the E', in accord with experimental observations for amorphous SiO2. By eliminating a second O atom in the nearest-neighbor shell of these defect centers, we then generated model defects in which the Si atom carrying the dangling bond forms bonds with two O atoms and one Si atom. In this defect, the spin density is found to delocalize over the Si-Si dimer bond, giving rise to two important hyperfine interactions. These properties match the characteristics of the hyperfine spectrum measured for the S center. Our results are complemented by the calculation of hyperfine interactions for small cluster models which serve the threefold purpose of comparing different electronic-structure schemes for the calculation of hyperfine interactions, estimating the size of core-polarization effects, and determining the reliability of cluster approximations used in the literature.

2002

Magnetic Properties of Surface Adsorbed Metal Adatoms and Dimers

Dante Philippe Sblendorio

EPFL2022

Structure hyperfine

Le dipôle magnétique nucléaire résultant du spin nucléaire ;

Le dipôle magnétique électronique lié au moment cinétique orbital et au spin de l’électron.

Shape staggering of midshell mercury isotopes from in-source laser spectroscopy compared with density-functional-theory and Monte Carlo shell-model calculations

Magnetic Properties of Surface Adsorbed Metal Adatoms and Dimers

First-principles modeling of paramagnetic Si dangling-bond defects in amorphous SiO2

Shape staggering of midshell mercury isotopes from in-source laser spectroscopy compared with density-functional-theory and Monte Carlo shell-model calculations

Magnetic Properties of Surface Adsorbed Metal Adatoms and Dimers

First-principles modeling of paramagnetic Si dangling-bond defects in amorphous SiO2