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Atomic and molecular structure
Learn about the atom, what it is, the people responsible for helping us understand it, and how it affects us in the world today.
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Atomic weights of the elements 2011 (IUPAC technical report)
The biennial review of atomic-weight determinations and other cognate data has resulted in changes for the standard atomic weights of five elements. The atomic weight of bromine has changed from 79.904(1) to the interval [79.901, 79.907], germanium from 72.63(1) to 72.630(8), indium from 114.818(3) to 114.818(1), magnesium from 24.3050(6) to the interval [24.304, 24.307], and mercury from 200.59(2) to 200.592(3). For bromine and magnesium, assignment of intervals for the new standard atomic weights reflects the common occurrence of variations in the atomic weights of those elements in normal terrestrial materials. © 2013 IUPAC.
Journal Article
Atomic weights of the elements 2013 (IUPAC Technical Report)
The biennial review of atomic-weight determinations and other cognate data has resulted in changes for the standard atomic weights of 19 elements. The standard atomic weights of four elements have been revised based on recent determinations of isotopic abundances in natural terrestrial materials:
cadmium to 112.414(4) from 112.411(8),
molybdenum to 95.95(1) from 95.96(2),
selenium to 78.971(8) from 78.96(3), and
thorium to 232.0377(4) from 232.038 06(2).
The Commission on Isotopic Abundances and Atomic Weights (ciaaw.org) also revised the standard atomic weights of fifteen elements based on the 2012 Atomic Mass Evaluation:
aluminium (aluminum) to 26.981 5385(7) from 26.981 5386(8),
arsenic to 74.921 595(6) from 74.921 60(2),
beryllium to 9.012 1831(5) from 9.012 182(3),
caesium (cesium) to 132.905 451 96(6) from 132.905 4519(2),
cobalt to 58.933 194(4) from 58.933 195(5),
fluorine to 18.998 403 163(6) from 18.998 4032(5),
gold to 196.966 569(5) from 196.966 569(4),
holmium to 164.930 33(2) from 164.930 32(2),
manganese to 54.938 044(3) from 54.938 045(5),
niobium to 92.906 37(2) from 92.906 38(2),
phosphorus to 30.973 761 998(5) from 30.973 762(2),
praseodymium to 140.907 66(2) from 140.907 65(2),
scandium to 44.955 908(5) from 44.955 912(6),
thulium to 168.934 22(2) from 168.934 21(2), and
yttrium to 88.905 84(2) from 88.905 85(2).
The Commission also recommends the standard value for the natural terrestrial uranium isotope ratio, N(²³⁸U)/N(²³⁵U)=137.8(1).
Journal Article
Radiative and opacity data obtained from large-scale atomic structure calculations and from statistical simulations for the spectral analysis of kilonovae in their photospheric and nebular phases: the sample case of Er III
This study is an overview of the atomic data and opacity computations performed by the Atomic Physics and Astrophysics Unit of Mons University in the context of kilonova emission following neutron star mergers, in both the photospheric and nebular phases. In this work, as a sample case, we focus on a specific lanthanide ion, namely Er III. As far as the LTE photospheric phase of the kilonova ejecta is concerned, we present our calculations using both a theoretical method (the pseudo-relativistic Hartree-Fock method, HFR) and a statistical approach (the Resolved Transition Array approach, RTA) to obtain the atomic data required to estimate the Er III expansion opacity for typical conditions expected in kilonova ejecta one day after the merger. In order to draw the limitations of both of our strategies, the results obtained using the latter are compared, and a calibration procedure of the HFR atomic data in this context is also discussed. Concerning the kilonova ejecta nebular phase, atomic parameters that characterize forbidden lines in Er III are calculated using HFR as well as another computational approach, namely the Multiconfiguration Dirac–Hartree–Fock (MCDHF) method. The potential detection of such lines in late-phase kilonova spectra is then discussed.
Graphical abstract
Journal Article
Direct comparison of two spin-squeezed optical clock ensembles at the 10−17 level
Building scalable quantum systems that demonstrate performance enhancement based on entanglement is a major goal in quantum computing and metrology. The main challenge arises from the fragility of entanglement in large quantum systems. Optical atomic clocks utilizing a large number of atoms have pushed the frontier of measurement science, building on precise engineering of quantum states and control of atomic interactions. However, state-of-the-art optical atomic clocks are limited by a fundamental source of noise stemming from fluctuations of the population of many atoms—the quantum projection noise. Here, we present an optical clock platform integrated with collective strong-coupling cavity quantum electrodynamics for quantum non-demolition measurements. Optimizing the competition between spin measurement precision and loss of coherence, we measure a metrological enhancement for a large ensemble of atoms beyond the initial coherent spin state. Furthermore, a movable lattice allows the cavity to individually address two independent subensembles, enabling us to spin squeeze two clock ensembles successively and compare their performance without the influence of clock laser noise. Although the clock comparison remains above the effective standard quantum limit, the performance directly verifies 1.9(2) dB clock stability enhancement at the 10
−17
level without subtracting any technical noise contributions.
Noise is a fundamental obstacle to the stability of atomic optical clocks. An experiment now realizes the design of a spin-squeezed clock that improves interrogation times and enables direct comparisons of performance between different clocks.
Journal Article
Visualization of moiré superlattices
Moiré superlattices in van der Waals heterostructures have given rise to a number of emergent electronic phenomena due to the interplay between atomic structure and electron correlations. Indeed, electrons in these structures have been recently found to exhibit a number of emergent properties that the individual layers themselves do not exhibit. This includes superconductivity1,2, magnetism3, topological edge states4,5, exciton trapping6 and correlated insulator phases7. However, the lack of a straightforward technique to characterize the local structure of moiré superlattices has thus far impeded progress in the field. In this work we describe a simple, room-temperature, ambient method to visualize real-space moiré superlattices with sub-5-nm spatial resolution in a variety of twisted van der Waals heterostructures including, but not limited to, conducting graphene, insulating boron nitride and semiconducting transition metal dichalcogenides. Our method uses piezoresponse force microscopy, an atomic force microscope modality that locally measures electromechanical surface deformation. We find that all moiré superlattices, regardless of whether the constituent layers have inversion symmetry, exhibit a mechanical response to out-of-plane electric fields. This response is closely tied to flexoelectricity wherein electric polarization and electromechanical response is induced through strain gradients present within moiré superlattices. Therefore, moiré superlattices of two-dimensional materials manifest themselves as an interlinked network of polarized domain walls in a non-polar background matrix.An electromechanical response to an out-of-plane electric field in van der Waals heterostructures enables direct visualization of moiré superlattices using piezoresponse force microscopy.
Journal Article