184 research outputs found
Exact isovector pairing in a shell-model framework: Role of proton-neutron correlations in isobaric analog states
We utilize a nuclear shell model Hamiltonian with only two adjustable
parameters to generate, for the first time, exact solutions for pairing
correlations for light to medium-mass nuclei, including the challenging
proton-neutron pairs, while also identifying the primary physics involved. In
addition to single-particle energy and Coulomb potential terms, the shell model
Hamiltonian consists of an isovector pairing interaction and an average
proton-neutron isoscalar interaction, where the term describes the
average interaction between non-paired protons and neutrons. This Hamiltonian
is exactly solvable, where, utilizing 3 to 7 single-particle energy levels, we
reproduce experimental data for 0 state energies for isotopes with mass
through exceptionally well including isotopes from He to Ge.
Additionally, we isolate effects due to like-particle and proton-neutron
pairing, provide estimates for the total and proton-neutron pairing gaps, and
reproduce (neutron) = (proton) irregularity. These results provide a
further understanding for the key role of proton-neutron pairing correlations
in nuclei, which is especially important for waiting-point nuclei on the
rp-path of nucleosynthesis.Comment: 10 pages, 4 figure
Solution to the problem of the poor cyclic fatigue resistance of bulk metallic glasses
The recent development of metallic glass-matrix composites represents a particular milestone in engineering materials for structural applications owing to their remarkable combination of strength and toughness. However, metallic glasses are highly susceptible to cyclic fatigue damage, and previous attempts to solve this problem have been largely disappointing. Here, we propose and demonstrate a microstructural design strategy to overcome this limitation by matching the microstructural length scales (of the second phase) to mechanical crack-length scales. Specifically, semisolid processing is used to optimize the volume fraction, morphology, and size of second-phase dendrites to confine any initial deformation (shear banding) to the glassy regions separating dendrite arms having length scales of ā2 Ī¼m, i.e., to less than the critical crack size for failure. Confinement of the damage to such interdendritic regions results in enhancement of fatigue lifetimes and increases the fatigue limit by an order of magnitude, making these ādesignedā composites as resistant to fatigue damage as high-strength steels and aluminum alloys. These design strategies can be universally applied to any other metallic glass systems
Fracture toughness and crack-resistance curve behavior in metallic glass-matrix composites
Nonlinear-elastic fracture mechanics methods are used to assess the fracture toughness of bulk metallic glass (BMG) composites; results are compared with similar measurements for other monolithic and composite BMG alloys. Mechanistically, plastic shielding gives rise to characteristic resistance-curve behavior where the fracture resistance increases with crack extension. Specifically, confinement of damage by second-phase dendrites is shown to result in enhancement of the toughness by nearly an order of magnitude relative to unreinforced glass
Efficacy of the SU(3) scheme for ab initio large-scale calculations beyond the lightest nuclei
We report on the computational characteristics of ab initio nuclear structure
calculations in a symmetry-adapted no-core shell model (SA-NCSM) framework. We
examine the computational complexity of the current implementation of the
SA-NCSM approach, dubbed LSU3shell, by analyzing ab initio results for 6Li and
12C in large harmonic oscillator model spaces and SU(3)-selected subspaces. We
demonstrate LSU3shell's strong-scaling properties achieved with highly-parallel
methods for computing the many-body matrix elements. Results compare favorably
with complete model space calculations and significant memory savings are
achieved in physically important applications. In particular, a well-chosen
symmetry-adapted basis affords memory savings in calculations of states with a
fixed total angular momentum in large model spaces while exactly preserving
translational invariance.Comment: 11 pages, 8 figure
Collective Modes in Light Nuclei from First Principles
Results for ab initio no-core shell model calculations in a symmetry-adapted
SU(3)-based coupling scheme demonstrate that collective modes in light nuclei
emerge from first principles. The low-lying states of 6Li, 8Be, and 6He are
shown to exhibit orderly patterns that favor spatial configurations with strong
quadrupole deformation and complementary low intrinsic spin values, a picture
that is consistent with the nuclear symplectic model. The results also suggest
a pragmatic path forward to accommodate deformation-driven collective features
in ab initio analyses when they dominate the nuclear landscape.Comment: 5 pages 3 figures, accepted to Physical Review Letter
Synergistic regulation of cerebellar Purkinje neuron development by laminin epitopes and collagen on an artificial hybrid matrix construct
Cataloged from PDF version of article.The extracellular matrix (ECM) creates a dynamic environment around the cells in the developing central nervous system, providing them with the necessary biochemical and biophysical signals. Although the functions of many ECM molecules in neuronal development have been individually studied in detail, the combinatorial effects of multiple ECM components are not well characterized. Here we demonstrate that the expression of collagen and laminin-1 (lam-1) are spatially and temporally correlated during embryonic and post-natal development of the cerebellum. These changes in ECM distribution correspond to specific stages of Purkinje neuron (PC) migration, somatic monolayer formation and polarization. To clarify the respective roles of these ECM molecules on PC development, we cultured cerebellar neurons on a hybrid matrix comprised of collagen and a synthetic peptide amphiphile nanofiber bearing a potent lam-1 derived bioactive IKVAV peptide epitope. By systematically varying the concentration and ratio of collagen and the laminin epitope in the matrix, we could demonstrate a synergistic relationship between these two ECM components in controlling multiple aspects of PC maturation. An optimal ratio of collagen and IKVAV in the matrix was found to promote maximal PC survival and dendrite growth, while dendrite penetration into the matrix was enhanced by a high IKVAV to collagen ratio. In addition, the laminin epitope was found to guide PC axon development. By combining our observations in vivo and in vitro, we propose a model of PC development where the synergistic effects of collagen and lam-1 play a key role in migration, polarization and morphological maturation of PCs. This journal is Ā© the Partner Organisations 2014
No-core Symplectic Model: Exploiting Hidden Symmetry in Atomic Nuclei
We report on recent developments within the framework of the no-core symplectic shell model (NCSpM) that complements the no-core shell model (NavrƔtil, Vary, and Barrett) by exploiting the algebraic features of the symplectic shell model (Rowe and Rosensteel) while also allowing for high-performance computing applications, but in highly truncated, physically relevant subspaces of the complete space. The leading symplectic symmetry typically accounts for 70% to 90% of the structure of the low-lying states, a result that is only moderately dependent on the details of the selected inter-nucleon interaction. Examples for6Li,12C,16O, and20Ne are shown to illustrate the efficacy the NCSpM, and as well the strong overlap with cluster-like and pairing configurations that dominate the dynamics of low-lying states in these nuclei
Avalanche precursors of failure in hierarchical fuse networks
We study precursors of failure in hierarchical random fuse network models
which can be considered as idealizations of hierarchical (bio)materials where
fibrous assemblies are held together by multi-level (hierarchical) cross-links.
When such structures are loaded towards failure, the patterns of precursory
avalanche activity exhibit generic scale invariance: Irrespective of load,
precursor activity is characterized by power-law avalanche size distributions
without apparent cut-off, with power-law exponents that decrease continuously
with increasing load. This failure behavior and the ensuing super-rough crack
morphology differ significantly from the findings in non-hierarchical
structures
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Characterization of fatigue-induced free volume changes in bulk metallic glass using positron annihilation spectroscopy
Depth-profiled Doppler broadening spectroscopy of positron annihilation on the cyclic fatigue-induced fracture surfaces of three amorphous ZrāāTiāāNiāāCuāāBeāā
metallic glass specimens reveals the presence of a 30ā50 nm layer of increased free volume that is generated by the propagating fatigue crack tip. The presence and character of this fatigue transformation zone is independent of the initial amount of bulk free volume, which was varied by structural relaxation via annealing, and the voids generated in the zone by intense cyclic deformation are distinct from those typical of the bulk.Keywords: fatigue cracks,
copper alloys,
Doppler broadening,
deformation,
beryllium alloys,
voids (solid),
annealing,
zirconium alloys,
titanium alloys,
nickel alloys,
metallic glasses,
positron annihilatio
Fracture toughness and crack-resistance curve behavior in metallic glass-matrix composites
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