3,167 research outputs found
Giant Resonances using Correlated Realistic Interactions: The Case for Second RPA
Lately we have been tackling the problem of describing nuclear collective
excitations starting from correlated realistic nucleon-nucleon (NN)
interactions. The latter are constructed within the Unitary Correlation
Operator Method (UCOM), starting from realistic NN potentials. It has been
concluded that first-order RPA with a two-body UCOM interaction is not capable,
in general, of reproducing quantitatively the properties of giant resonances
(GRs), due to missing higher-order configurations and long-range correlations
as well as neglected three-body terms in the Hamiltonian.
Here we report results on GRs obtained by employing a UCOM interaction based
on the Argonne V18 potential in Second RPA (SRPA) calculations. The same
interaction is used to describe the Hartree-Fock (HF) ground state and the
residual interactions. We find that the inclusion of second-order
configurations -- which effectively dress the underlying HF single-particle
states with self-energy insertions -- produces sizable corrections. The effect
appears essential for a realistic description of GRs when using the UCOM. We
argue that effects of higher than second order should be negligible. Therefore,
the UCOM-SRPA emerges as a promising tool for consistent calculations of
collective states in closed-shell nuclei. This is an interesting development,
given that SRPA can accommodate more physics than RPA (e.g., fragmentation).
Remaining discrepancies due to the missing three-body terms and
self-consistency issues of the present SRPA model are pointed out.Comment: 6 pages, incl. 1 figure; Proc. 26th Int. Workshop on Nuclear Theory,
June 2007, Rila mountains, Bulgari
Large-scale second RPA calculations with finite-range interactions
Second RPA (SRPA) calculations of nuclear response are performed and
analyzed. Unlike in most other SRPA applications, the ground state,
approximated by the Hartree-Fock (HF) ground state, and the residual couplings
are described by the same Hamiltonian and no arbitrary truncations are imposed
on the model space. Finite-range interactions are used and thus divergence
problems are not present. We employ a realistic interaction, derived from the
Argonne V18 potenial using the unitary correlation operator method (UCOM), as
well as the simple Brink-Boeker interaction. Representative results are
discussed, mainly on giant resonances and low-lying collective states. The
focus of the present work is not on the comparison with data, but rather on
technical and physical aspects of the method. We present how the large-scale
eigenvalue problem that SRPA entails can be treated, and demonstrate how the
method operates in producing self-energy corrections and fragmentation. The
so-called diagonal approximation is conditionally validated. Stability problems
are traced back to missing ground-state correlations.Comment: 13 pages, incl. 9 figures, 1 tabl
Crowd-sourcing with uncertain quality - an auction approach
This article addresses two important issues in crowd-sourcing: ex ante uncertainty about the quality and cost of different workers and strategic behaviour. We present a novel multi-dimensional auction that incentivises the workers to make partial enquiry into the task and to honestly report quality-cost estimates based on which the crowd-sourcer can choose the worker that offers the best value for money. The mechanism extends second score auction design to settings where the quality is uncertain and it provides incentives to both collect information and deliver desired qualities
Proton pygmy resonances: predictions for N=20 isotones
We study theoretically the low-energy electric-dipole response of N=20
isotones. We present results from a quasiparticle random-phase approximation
(QRPA) and a continuum random-phase approximation (CRPA), and we compare them
with results for the mirror Z=20 nuclei. According to our analysis, enhanced E1
strength is expected energetically well below the giant dipole resonance in the
proton-rich isotones. Large amounts of E1 strength in the asymmetric N=20
isotones are predicted, unlike their equally asymmetric Z=20 mirror nuclei,
pointing unambiguously to the role of structural effects such as loose binding.
A proton-skin oscillation could develop especially in 46Fe. The proper
description of non localized threshold transitions and the nucleon effective
mass in mean-field treatments may affect theoretical predictions. We call for
systematic theoretical investigations to quantify the role bulk-matter
properties, in anticipation of measurements of E1 transitions in proton-rich
nuclei.Comment: 10 pages, incl. 9 figures and 2 tables; v2: some rephrasing and
clarifications, corrected Fig.
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Thermal stability study of nitrogen functionalities in a graphene network
Catalyst-free vertically aligned graphene nanoflakes possessing a large
amount of high density edge planes were functionalized using nitrogen species
in a low energy N+ ion bombardment process to achieve pyridinic, cyanide and
nitrogen substitution in hexagonal graphitic coordinated units. The evolution
of the electronic structure of the functionalized graphene nanoflakes over the
temperature range 20-800^{\circ}C was investigated in situ, using high
resolution x-ray photoemission spectroscopy. We demonstrate that low energy
irradiation is a useful tool for achieving nitrogen doping levels up to 9.6
at.%. Pyridinic configurations are found to be predominant at room temperature,
while at 800^{\circ}C graphitic nitrogen configurations become the dominant
ones. The findings have helped to provide an understanding of the thermal
stability of nitrogen functionalities in graphene, and offer prospects for
controllable tuning of nitrogen doping in device applications.Comment: Corresponding author: [email protected]
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