C. Cousins

The collective structure of low-lying states in the $N=50$ nucleus $^{92}\mathrm{Mo}$ $(Z=42)$ has been investigated through sub-barrier Coulomb excitation using the CHICO2 device coupled to the $\ensuremath{\gamma}$-ray energy tracking in-beam nuclear array (GRETINA). Quadrupole and octupole collective properties are measured for the ${2}_{1}^{+}$, ${2}_{2}^{+}$, ${2}_{3}^{+}$, ${2}_{4}^{+}$, and ${3}_{1}^{\ensuremath{-}}$ states. The spectroscopic quadrupole moment for the ${2}_{1}^{+}$ state is measured to be ${Q}_{s}=0.23(8)\phantom{\rule{0.28em}{0ex}}e\mathrm{b}$, showing a weakly deformed oblate nature in good agreement with recent state-of-the-art Shell-Model calculations. A modest reduced electric-octupole transition strength of $B(E3;{3}_{1}^{\ensuremath{-}}\ensuremath{\rightarrow}{0}_{1}^{+})=15(3)\phantom{\rule{0.28em}{0ex}}\mathrm{W}.\mathrm{u}.$ is obtained, in disagreement with the literature, revealing an emerging tension in the strength of octupole collectivity for the $N=50$ nuclei between historic $(e,{e}^{\ensuremath{'}})$ and $(p,{p}^{\ensuremath{'}})$ experiments and more recent experiments utilizing Coulomb excitation and lifetime plus branching-ratio data.

The electromagnetic properties of low-lying states in $^{70}$Ge were investigated via multi-step Coulomb excitation of a $^{70}$Ge beam impinging on a $^{208}$Pb target at the ATLAS facility of the Argonne National Laboratory. A total of 27 transitional elements and six diagonal matrix elements coupling 11 low-lying states, were extracted from the measured cross sections. These were used to calculate reduced transition probabilities, spectroscopic quadrupole moments, and rotational invariant shape parameters, providing enhanced precision and expanding on previous studies. The experimental data were compared within several theoretical frameworks, including the generalized triaxial rotor model, configuration interaction shell-model calculations, and computations within the combined frameworks of relativistic density functional theory and the five-dimensional collective Hamiltonian. The results demonstrate a good agreement with the experimental data and, in conjunction with calculations using a two-state mixing model, support significant triaxiality and strong mixing between the $0^+_1$ and $0^+_2$ states. This results in the magnitudes of their respective quadrupole deformations $[β_\text{rms}(0^+_1) = 0.228\,(3),\,β_\text{rms}(0^+_2) = 0.273\,(1)]$ being more similar than previously observed. The implications of these results for understanding the complex shape coexistence phenomena, the role of triaxiality, and shape evolution along the Ge isotopic chain are discussed.

Lying between ${}^{16}\mathrm{O}$ and ${}^{40}\mathrm{Ca}$, the $sd$ shell is well described by robust phenomenological and ab initio nuclear theories. In this work, however, we highlight an unexplained reduction in electric-quadrupole strength in the rare isotope ${}^{32}\mathrm{Si}$, studied through sub-barrier Coulomb excitation. It is found that the oblate nature of the deformation is well reproduced, while the absolute scale of quadrupole deformation, however, is inhibited by approximately a factor of 2 compared to theoretical predictions. Through comparison with shell-model and ab initio calculations, we present a number of possible explanations for this inhibited $E2$ strength. By comparing the results of these calculations to multiple observables, we conclude that there is a reduced role for out-of-space excitations in ${}^{32}\mathrm{Si}$, resulting in a reduction in the corrections normally applied to both models.

We postulate wide-band-gap forms of carbon that locally have a planar bonding configuration as in graphite and, in contrast to diamond, are promising candidates for $n$-type doping by nitrogen. The presence of localized $\pi$ bonds makes them as stable as fullerene ${\mathrm{C}}_{60}$ and causes large band gaps of $\approx 3$ eV to appear. The allotropes accept both nitrogen and boron as substitutional dopants, making them potentially extremely useful for high-power, high-temperature, and high-speed device applications.