126 research outputs found
Surface Influence on Flux Penetration into HTS Bulks
The influence of surface treatment on AC loss in melt-processed quasi-single
crystal HTS was investigated with resonance oscillations technique. We have
found that amplitude dependencies of AC loss on magnetic field amplitude become
rather complicated after surface polishing. The experimental data show well
distinguished dynamic crossover from absence of barrier at low rates of field
variation to its appearence at higher rates. An explaination of such a dynamic
surface barrier appearance based on consideration of along surface vortex
propagation was suggested.Comment: 2 pages with 1 EPS figure, to be presented at LT2
Simple technique for superconducting joints quality estimation in bulk melt-processed high temperature superconductors
We propose an empirical approach to estimate the quality of superconducting
joints (welds) between blocks of bulk high temperature superconductors (HTS).
As a measuring value, we introduce a joint's quality factor and show its
natural correlation with joint's critical current density. Being simple and
non-destructive, this approach is considered to be quite important to solve the
problem of utilization of HTS in large scale applications. The approach has
been applied to characterize the joint's quality of melt-processed Y-123 joined
by Tm-123 solder.Comment: 3 pages with 2 figures (revtex
Determination of critical current density in melt-processed HTS bulks from levitation force measurements
A simple approach to describe the levitation force measurements on
melt-processed HTS bulks was developed. A couple of methods to determine the
critical current density were introduced. The averaged -plane
values for the field parallel to this plane were determined. The first and
second levitation force hysteresis loops calculated with these values
coincide remarkably well with the experimental data.Comment: 10 pages (tex), 2 figures (in jpeg
Peculiarities of phonon spectra and lattice heat capacity in Ir and Rh
A simple pseudopotential model is proposed, which allows the phonon spectra
and temperature dependence of the lattice heat capacity of Ir and Rh be
described with a high enough accuracy. A careful comparison of the calculated
and experimental values of the lattice heat capacity is carried out, with the
procedure of the identification of the phonon contribution to the heat capacity
and determination of the characteristics (momenta) of the phonon density of
states from the experimental values of the total heat capacity of metal at a
constant pressure being described in detail. The results of the theoretical
calculations explain, in particular, such peculiar feature of Ir and Rh,
unusual for cubic metals, as a sharp (more than by a factor of 1.5) decrease in
the effective Debye temperature with increasing termperature. The temperature
dependence of the mean square amplitude of atomic displacements in Ir and Rh
has been calculated. Basing on the band calculations the manifestation of the
Kohn singularities in the phonon spectra of Ir are discussed.Comment: 15 pages, LaTeX2e, 12 figures in postscrip
Reduced Bloch mode expansion for periodic media band structure calculations
Reduced Bloch mode expansion is presented for fast periodic media band
structure calculations. The expansion employs a natural basis composed of a
selected reduced set of Bloch eigenfunctions. The reduced basis is selected
within the irreducible Brillouin zone at high symmetry points determined by the
medium's crystal structure and group theory (and possibly at additional related
points). At each of the reciprocal lattice selection points, a number of Bloch
eigenfunctions are selected up to the frequency range of interest for the band
structure calculations. Since it is common to initially discretize the periodic
unit cell and solution field using some choice of basis, reduced Bloch mode
expansion is practically a secondary expansion that uses a selected set of
Bloch eigenvectors. Such expansion therefore keeps, and builds on, any
favorable attributes a primary expansion approach might exhibit. Being in line
with the well known concept of modal analysis, the proposed approach maintains
accuracy while reducing the computation time by up to two orders of magnitudes
or more depending on the size and extent of the calculations. Results are
presented for phononic, photonic and electronic band structures.Comment: 15 pages of text, 8 figures, submitted for journal publication, minor
edits and correction of typo
An embedding scheme for the Dirac equation
An embedding scheme is developed for the Dirac Hamiltonian H. Dividing space
into regions I and II separated by surface S, an expression is derived for the
expectation value of H which makes explicit reference to a trial function
defined in I alone, with all details of region II replaced by an effective
potential acting on S and which is related to the Green function of region II.
Stationary solutions provide approximations to the eigenstates of H within I.
The Green function for the embedded Hamiltonian is equal to the Green function
for the entire system in region I. Application of the method is illustrated for
the problem of a hydrogen atom in a spherical cavity and an Au(001)/Ag/Au(001)
sandwich structure using basis sets that satisfy kinetic balance.Comment: 16 pages, 5 figure
Born Effective Charges of Barium Titanate: band by band decomposition and sensitivity to structural features
The Born effective charge tensors of Barium Titanate have been calculated for
each of its 4 phases. Large effective charges of Ti and O, also predicted by
shell model calculations and made plausible by a simplified model, reflect the
partial covalent character of the chemical bond. A band by band decomposition
confirms that orbital hybridization is not restricted to Ti and O atoms but
also involves Ba which appears more covalent than generally assumed. Our
calculations reveal a strong dependence of the effective charges on the atomic
positions contrasting with a relative insensitivity on isotropic volume
changes.Comment: 13 page
The physics of dynamical atomic charges: the case of ABO3 compounds
Based on recent first-principles computations in perovskite compounds,
especially BaTiO3, we examine the significance of the Born effective charge
concept and contrast it with other atomic charge definitions, either static
(Mulliken, Bader...) or dynamical (Callen, Szigeti...). It is shown that static
and dynamical charges are not driven by the same underlying parameters. A
unified treatment of dynamical charges in periodic solids and large clusters is
proposed. The origin of the difference between static and dynamical charges is
discussed in terms of local polarizability and delocalized transfers of charge:
local models succeed in reproducing anomalous effective charges thanks to large
atomic polarizabilities but, in ABO3 compounds, ab initio calculations favor
the physical picture based upon transfer of charges. Various results concerning
barium and strontium titanates are presented. The origin of anomalous Born
effective charges is discussed thanks to a band-by-band decomposition which
allows to identify the displacement of the Wannier center of separated bands
induced by an atomic displacement. The sensitivity of the Born effective
charges to microscopic and macroscopic strains is examined. Finally, we
estimate the spontaneous polarization in the four phases of barium titanate.Comment: 25 pages, 6 Figures, 10 Tables, LaTe
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