243,225 research outputs found
Mean Field Theoretical Structure of He and Be Isotopes
The structures of He and Be even-even isotopes are investigated using an
axially symmetric Hartree-Fock approach with a Skyrme-IIIls mean field
potential. In these simple HF calculations, He and Be isotopes appear to be
prolate in their ground states and Be isotopes have oblate shape isomeric
states. It is also shown that there exists a level crossing when the nuclear
shape changes from the prolate state to the oblate state. The single neutron
levels of Be isotopes exhibit a neutron magic number 6 instead of 8 and show
that the level inversion between 1/2- and 1/2+ levels occurs only for a largely
deformed isotope. Protons are bound stronger in the isotope with more neutrons
while neutron levels are somewhat insensitive to the number of neutrons and
thus the nuclear size and also the neutron skin become larger as the neutron
number increases. In these simple calculations with Skyrme-IIIls interaction no
system with a clear indication of neutron halo was found among He and Be
isotopes. Instead of it we have found 8He+2n, 2n+8He+2n, and 16Be+2n like chain
structures with clusters of two correlated neutrons. It is also shown that 8He
and 14Be in their ground states are below the neutron drip line in which all
nucleons are bound with negative energy and that 16Be in its ground state is
beyond the neutron drip line with two neutrons in positive energy levels.Comment: CM energy correction, 1 figure and more discussions adde
Method for reinforcing tubing joints
Joint repair technique uses a longitudinally split aluminum shield over the joint ferrule and immediately adjacent tubing to reseal or reinforce leaking or weak joints in small tubing. Epoxy resin coating on inside surfaces of the two shield halves provides a tightly sealed bond between shield and tubing
Charge Disproportionation and Spin Ordering Tendencies in Na(x)CoO2
The strength and effect of Coulomb correlations in the (superconducting when
hydrated) x~1/3 and ``enhanced'' x~2/3 regimes of Na(x)CoO2 are evaluated using
the correlated band theory LDA+U method. Our results, neglecting quantum
fluctuations, are: (1) allowing only ferromagnetic order, there is a critical
U_c = 3 eV, above which charge disproportionation occurs for both x=1/3 and
x=2/3, (2) allowing antiferromagnetic order at x=1/3, U_c drops to 1 eV for
disproportionation, (3) disproportionation and gap opening occur
simultaneously, (4) in a Co(3+)-Co(4+) ordered state, antiferromagnetic
coupling is favored over ferromagnetic, while below U_c ferromagnetism is
favored. Comparison of the calculated Fermi level density of states compared to
reported linear specific heat coefficients indicates enhancement of the order
of five for x~0.7, but negligible enhancement for x~0.3. This trend is
consistent with strong magnetic behavior and local moments (Curie-Weiss
susceptibility) for x>0.5 while there no magnetic behavior or local moments
reported for x<0.5. We suggest that the phase diagram is characterized by a
crossover from effective single-band character with U >> W for x>0.5 into a
three-band regime for x U_eff <= U/\sqrt(3) ~ W and
correlation effects are substantially reduced.Comment: 10 pages, 8 figures, corrected a few typos and changed reference
c-axis Raman Scattering in MgB2: Observation of a Dirty-Limit Gap in the pi-bands
Raman scattering spectra from the ac-face of thick MgB2 single crystals were
measured in zz, xz and xx polarisations. In zz and xz polarisations a threshold
at around 29 cm^{-1} forms in the below Tc continuum but no pair-breaking peak
is seen, in contrast to the sharp pair-breaking peak at around 100 cm^{-1} seen
in xx polarisation. The zz and xz spectra are consistent with Raman scattering
from a dirty superconductor while the sharp peak in the xx spectra argues for a
clean system. Analysis of the spectra resolves this contradiction, placing the
larger and smaller gap magnitudes in the sigma and pi bands, and indicating
that relatively strong impurity scattering is restricted to the pi bands.Comment: Revised manuscript accepted for publication in Physical Review
Letter
Gauge Theory of Gravity Requires Massive Torsion Field
One of the greatest unsolved issues of the physics of this century is to find
a quantum field theory of gravity. According to a vast amount of literature
unification of quantum field theory and gravitation requires a gauge theory of
gravity which includes torsion and an associated spin field. Various models
including either massive or massless torsion fields have been suggested. We
present arguments for a massive torsion field, where the probable rest mass of
the corresponding spin three gauge boson is the Planck mass.Comment: 3 pages, Revte
- …