123 research outputs found
Anomalous behavior of the Debye temperature in Fe-rich Fe-Cr alloys
Debye temperature, , of Fe-rich FeCr disordered
alloys with was determined from the temperature dependence of
the central shift of M\"ossbauer spectra recorded in the temperature range of
60 -- 300 K. Its compositional dependence shows a maximum at with
a relative increase of % compared to a pure iron. The composition at
which the effect occurs correlates well with that at which several other
quantities, e. g. the Curie temperature and the spin-wave stiffness
coefficient, , show their maxima, but the enhancement of is
significantly greater and comparable with the enhancement of the hyperfine
field (spin-density of itinerant -like electrons) in the studied system. The
results suggest that the electron-phonon interaction is important in this alloy
system
Random phase approximation for multi-band Hubbard models
We derive the random-phase approximation for spin excitations in general
multi-band Hubbard models, starting from a collinear ferromagnetic Hartree-Fock
ground state. The results are compared with those of a recently introduced
variational many-body approach to spin-waves in itinerant ferromagnets. As we
exemplify for Hubbard models with one and two bands, the two approaches lead to
qualitatively different results. The discrepancies can be traced back to the
fact that the Hartree-Fock theory fails to describe properly the local moments
which naturally arise in a correlated-electron theory.Comment: 25 pages, 2 figure
The 100 m Composite Ship?
Fibre-reinforced polymer (FRP) matrix composites are widely used in large marine structures, and in wind turbines where blade lengths are now over 100 m. Composites are the material of choice for small vessels due to ease of manufacture, high hull girder stiffness, buckling resistance, corrosion resistance and underwater shock resistance. Ships over 100 m are still built using traditional steel and/or aluminium, but so far not FRP. Composite ship lengths have increased over the past 50 years, but fundamental technical challenges remain for the 100 m composite ship. Preliminary studies suggest a possible 30% saving in structural weight, a 7–21% reduction in full load displacement, and a cost saving of 15%. However, economic considerations, design codes, manufacturing limits, safety and end of life scenarios need to be addressed before a 100 m ship is built. Innovative materials and structures, notably carbon fibre composite skinned sandwich construction, or aramid fibres with vinylester modified epoxy resin, should result in increased mechanical performance and consequent improvements in economics and manufacturing processes. A linear extrapolation of length vs. launch dates predicts the first 100 m ship would be launched in 2042.</jats:p
On the interpretation of spin-polarized electron energy loss spectra
We study the origin of the structure in the spin-polarized electron energy
loss spectroscopy (SPEELS) spectra of ferromagnetic crystals. Our study is
based on a 3d tight-binding Fe model, with constant onsite Coulomb repulsion U
between electrons of opposite spin. We find it is not the total density of
Stoner states as a function of energy loss which determines the response of the
system in the Stoner region, as usually thought, but the densities of Stoner
states for only a few interband transitions. Which transitions are important
depends ultimately on how strongly umklapp processes couple the corresponding
bands. This allows us to show, in particular, that the Stoner peak in SPEELS
spectra does not necessarily indicate the value of the exchange splitting
energy. Thus, the common assumption that this peak allows us to estimate the
magnetic moment through its correlation with exchange splitting should be
reconsidered, both in bulk and surface studies. Furthermore, we are able to
show that the above mechanism is one of the main causes for the typical
broadness of experimental spectra. Finally, our model predicts that optical
spin waves should be excited in SPEELS experiments.Comment: 11 pages, 7 eps figures, REVTeX fil
Dynamics of an Antiferromagnet at Low Temperatures: Spin-Wave Damping and Hydrodynamics
Moral essays wherein some of Mr. Locks and Monsir. Malbranch's opinions are briefly examin'd : together with an answer to some chapters in the oracles of reason concerning deism / by Ja. Lowde ...
The reasonableness of the Christian religion a sermon preached at the visitation held at Stoakesly in Cleveland, in Yorkshire : being the first visitation of the Reverend Mr. Long, B.D. and Arch-Deacon of Cleveland / by Ja. Lowde ...
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