28,790 research outputs found
Comment to: Corrections to the fine structure constant in the spacetime of a cosmic string from the generalized uncertainty principle
In the paper [F. Nasseri, Phys. Lett. B 632 (2006) 151--154], F. Nasseri
supposed that the value of the angular momentum for the Bohr's atom in the
presence of the cosmic string is quantized in units of ℏ. Using this
assumption it was obtained an incorrect expression for Bohr radius in this
scenario. In this comment I want to point out that this assumption is not
correct and present a corrected expression for the Bohr radius in this
background.Comment: This paper was published in Phys. Lett.
T-duality and -corrections
We construct an invariant universal formulation of the first-order
-corrections of the string effective actions involving the dilaton,
metric and two-form fields. Two free parameters interpolate between
four-derivative terms that are even and odd with respect to a -parity
transformation that changes the sign of the two-form field. The -symmetric
model reproduces the closed bosonic string, and the heterotic string effective
action is obtained through a -parity-breaking choice of parameters. The
theory is an extension of the generalized frame formulation of Double Field
Theory, in which the gauge transformations are deformed by a first-order
generalized Green-Schwarz transformation. This deformation defines a duality
covariant gauge principle that requires and fixes the four-derivative terms. We
discuss the structure of the theory and the (non-)covariance of the
required field redefinitions.Comment: 35 pages, v2: Minor corrections, published in JHE
Identification of fullerene-like CdSe nanoparticles from optical spectroscopy calculations
Semiconducting nanoparticles are the building blocks of optical nanodevices
as their electronic states, and therefore light absorption and emission, can be
controlled by modifying their size and shape. CdSe is perhaps the most studied
of these nanoparticles, due to the efficiency of its synthesis, the high
quality of the resulting samples, and the fact that the optical gap is in the
visible range. In this article, we study light absorption of CdSe
nanostructures with sizes up to 1.5 nm within density functional theory. We
study both bulk fragments with wurtzite symmetry and novel fullerene-like
core-cage structures. The comparison with recent experimental optical spectra
allows us to confirm the synthesis of these fullerene-like CdSe clusters
The planar-to-tubular structural transition in boron clusters from optical absorption
The optical response of the lowest energy isomers of the B_20 family is
calculated using time-dependent density functional theory within a real-space,
real-time scheme. Significant differences are found among the absorption
spectra of the clusters studied. We show that these differences can be easily
related to changes in the overall geometry. Optical spectroscopy is thus an
efficient tool to characterize the planar to tubular structural transition,
known to be present in these boron based systems
Clustering and Correlations at the Neutron Dripline
Some recent experimental studies of clustering and correlations within very
neutron-rich light nuclei are reviewed. In particular, the development of the
novel probes of neutron-neutron interferometry and Dalitz-plot analyses is
presented through the example of the dissociation of the two-neutron halo
system Be. The utility of high-energy proton radiative capture is
illustrated using a study of the He(p,) reaction. A new approach
to the production and detection of bound neutron clusters is also described,
and the observation of events with the characteristics expected for
tetraneutrons (n) liberated in the breakup of Be is discussed. The
prospects for future work, including systems beyond the neutron dripline, are
briefly outlined.Comment: Invited contribution to a topical issue on Exotic Nuclei of Les
Comptes Rendus de l'Academie des Sciences Paris, Serie IV. 29 pages,11
figures (format RevTex preprint
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