Search results for "SHELL"
showing 10 items of 748 documents
Deformations of quasi-two-dimensional electron gas clusters
1998
Shell effects and Jahn-Teller deformations of quasi-two-dimensional jellium droplets are studied. Utilizing the ultimate jellium assumption, previously successfully used for three-dimensional systems, we calculate unrestricted shape relaxations and binding energies of the ground-state and the lowest isomers, using the methods of density-functional theory in the local spin-density approximation. Strong variations with particle number are found in the shape of the droplets. In particular, for certain magic electron numbers the shapes show triangular or circular symmetry, while for other electron numbers, more complicated symmetries are found. We finally show that from a more simple ``billiard…
Nucleon Pairing and Seniority
2007
Until now we have been dealing with particle and hole aspects of nuclear structure. In this second part of the book we go farther away from a closed major shell. Still near the beginning or end of a major shell we encounter vibrational, spherical open-shell nuclei that cannot be described in terms of a few particles or holes. Farther towards the middle of the shell the spherical shape will give way to permanent deformation signalled by rotational bands analogous to those of diatomic molecules. Microscopic description of such nuclei requires a deformed mean field as the starting point. Deformed nuclei will not be considered in any detail in this book.
Fission Barrier of Superheavy Nuclei and Persistence of Shell Effects at High Spin: Cases ofNo254andTh220
2014
We report on the first measurement of the fission barrier height in a heavy shell-stabilized nucleus. The fission barrier height of No-254 is measured to be B-f = 6.0 +/- 0.5 MeV at spin 15 (h) over bar and, by extrapolation, B-f = 6.6 +/- 0.9 MeV at spin 0 (h) over bar. This information is deduced from the measured distribution of entry points in the excitation energy versus spin plane. The same measurement is performed for Th-220 and only a lower limit of the fission barrier height can be determined: B-f (I) > 8 MeV. Comparisons with theoretical fission barriers test theories that predict properties of superheavy elements.
Co-existence and shell structures of several superfluids in trapped three-component Fermi mixtures
2006
We study the properties of a trapped interacting three component Fermi gas. We assume that one of the components can have a different mass from the other two. We calculate the different phases of the three component mixture and find a rich variety of different phases corresponding to different pairing channels, and simple ways of tuning the system from one phase to another. In particular, we predict co-existence of several different superfluids in the trap, forming a shell structure, and phase transitions from this mixture of superfluids to a single superfluid when the system parameters or temperature is varied. Such shell structures realize superfluids with a non-trivial spatial topology a…
Spins and Magnetic Moments ofK49andK51: Establishing the1/2+and3/2+Level Ordering BeyondN=28
2013
The ground-state spins and magnetic moments of $^{49,51}\mathrm{K}$ have been measured using bunched-beam high-resolution collinear laser spectroscopy at ISOLDE CERN. For $^{49}\mathrm{K}$ a ground-state spin $I=1/2$ was firmly established. The observed hyperfine structure of $^{51}\mathrm{K}$ requires a spin $Ig1/2$ and strongly suggests $I=3/2$. From its magnetic moment $\ensuremath{\mu}(^{51}\mathrm{K})=+0.5129(22){\ensuremath{\mu}}_{N}$ a spin-parity ${I}^{\ensuremath{\pi}}=3/{2}^{+}$ with a dominant $\ensuremath{\pi}1{d}_{3/2}^{\ensuremath{-}1}$ hole configuration was deduced. This establishes for the first time the reinversion of the single-particle levels and illustrates the prominen…
Tight-Binding Model for Spontaneous Magnetism of Quantum Dot Lattices
2003
We use a simple tight-binding model to study the magnetism of two-dimensional quantum dot lattices with 1 to 12 electrons per dot. The results show that in the middle of an electron shell the lattice favours antiferromagnetism while with nearly empty or full shells ferromagnetism is favoured. The size of the antiferromagnetic region increases with the coordination number of the dot. A one-dimensional dot lattice shows a spin-Peierls transition. The results for a square lattice are in good agreement with density functional calculations of Koskinen et al.
Challenge of magnetism in strongly correlated open-shell 2p systems.
2009
We report on theoretical investigations of the exotic magnetism in rubidium sesquioxide ${\mathrm{Rb}}_{4}{\mathrm{O}}_{6}$, a model correlated system with an open $2p$ shell. Experimental investigations indicated that ${\mathrm{Rb}}_{4}{\mathrm{O}}_{6}$ is a magnetically frustrated insulator. The frustration is explained here by electronic structure calculations that incorporate the correlation between the oxygen $2p$ electrons and deal with the mixed-valent oxygen. This leads to a physical picture where the symmetry is reduced because one third of the oxygen in ${\mathrm{Rb}}_{4}{\mathrm{O}}_{6}$ is nonmagnetic while the remaining two thirds assemble in antiferromagnetic arrangements. A d…
Size-consistent self-consistent configuration interaction from a complete active space : Excited states
1998
The self-consistent size consistent on a complete active space singly and doubly configuration interaction (SC)2CAS-SDCI method is applied to excited states. The (SC)2 correction is performed on a closed shell state, and the excited states are obtained by diagonalization of the dressed matrix. A theoretical justification of the transferability of the improvement concerning the dressing state to all roots of the matrix is presented. The method is tested by three tests on the spectrum of small molecules. sanchezm@uv.es ; nebot@uv.es
Spin-restricted open-shell coupled-cluster theory for excited states
2000
Using a linear-response approach, the recently introduced spin-restricted coupled-cluster (SR-CC) theory is extended to the treatment of excited states of high-spin open-shell molecules. Explicit equations are given within the usual singles and doubles approximation and our implementation (within an existing spin–orbital code) is described. It is shown that in SR-CC theory, due to spin constraints, the spin-expectation value for the excited states calculated as corresponding energy derivatives always corresponds to the exact value. In addition, the SR-CC singles and doubles (SR-CCSD) approach is extended to include also the so-called pseudotriple excitations (best described as double excita…
Spectrum-shape method and the next-to-leading-order terms of the β-decay shape factor
2017
Effective values of the axial-vector coupling constant ${g}_{\mathrm{A}}$ have lately attracted much attention due to the prominent role of ${g}_{\mathrm{A}}$ in determining the half-lives of double $\ensuremath{\beta}$ decays, in particular their neutrinoless mode. The half-life method, i.e., comparing the calculated half-lives to the corresponding experimental ones, is the most widely used method to access the effective values of ${g}_{\mathrm{A}}$. The present paper investigates the possibilities offered by a complementary method: the spectrum-shape method (SSM). In the SSM, comparison of the shapes of the calculated and measured $\ensuremath{\beta}$ electron spectra of forbidden nonuniq…