1,465 research outputs found
Alkali oxide-tantalum oxide and alkali oxide-niobium oxide ionic conductors
A search was made for new cationic conducting phases in alkali-tantalate and niobate systems. The phase equilibrium diagrams were constructed for the six binary systems Nb2O5-LiNbO3, Nb2O5-NaNbO3, Nb2O5-KNbO3, Ta2O5-NaTaO3, Ta2O5-LiTaO3, and Ta2O5-KTaO3. Various other binary and ternary systems were also examined. Pellets of nineteen phases were evaluated (by the sponsoring agency) by dielectric loss measurements. Attempts were made to grow large crystals of eight different phases. The system Ta2O5-KTaO3 contains at least three phases which showed peaks in dielectric loss vs. temperature. All three contain structures related to the tungsten bronzes with alkali ions in non-stoichiometric crystallographic positions
Alkali oxide-tantalum, niobium and antimony oxide ionic conductors
The phase equilibrium relations of four systems were investigated in detail. These consisted of sodium and potassium antimonates with antimony oxide and tantalum and niobium oxide with rubidium oxide as far as the ratio 4Rb2O:llB2O5 (B=Nb, Ta). The ternary system NaSbO3-Sb2O4-NaF was investigated extensively to determine the actual composition of the body centered cubic sodium antimonate. Various other binary and ternary oxide systems involving alkali oxides were examined in lesser detail. The phases synthesized were screened by ion exchange methods to determine mobility of the mobility of the alkali ion within the niobium, tantalum or antimony oxide (fluoride) structural framework. Five structure types warranted further investigation; these structure types are (1) hexagonal tungsten bronze (HTB), (2) pyrochlore, (3) the hybrid HTB-pyrochlore hexagonal ordered phases, (4) body centered cubic antimonates and (5) 2K2O:3Nb2O5. Although all of these phases exhibit good ion exchange properties only the pyrochlore was prepared with Na(+) ions as an equilibrium phase and as a low porosity ceramic. Sb(+3) in the channel interferes with ionic conductivity in this case, although relatively good ionic conductivity was found for the metastable Na(+) ion exchanged analogs of RbTa2O5F and KTaWO6 pyrochlore phases
The Perfect Quark-Gluon Vertex Function
We evaluate a perfect quark-gluon vertex function for QCD in coordinate space
and truncate it to a short range. We present preliminary results for the
charmonium spectrum using this quasi-perfect action.Comment: 3 pages LaTex, 4 figures, poster presented at LATTICE9
Lagrangian planetary equations in Schwarzschild space--time
We have developed a method to study the effects of a perturbation to the
motion of a test point--like object in a Schwarzschild spacetime. Such a method
is the extension of the Lagrangian planetary equations of classical celestial
mechanics into the framework of the full theory of general relativity. The
method provides a natural approach to account for relativistic effects in the
unperturbed problem in an exact way.Comment: 7 pages; revtex; accepted for publication in Class. Quantum Gra
Progress on Perfect Lattice Actions for QCD
We describe a number of aspects in our attempt to construct an approximately
perfect lattice action for QCD. Free quarks are made optimally local on the
whole renormalized trajectory and their couplings are then truncated by
imposing 3-periodicity. The spectra of these short ranged fermions are
excellent approximations to continuum spectra. The same is true for free
gluons. We evaluate the corresponding perfect quark-gluon vertex function,
identifying in particular the ``perfect clover term''. First simulations for
heavy quarks show that the mass is strongly renormalized, but again the
renormalized theory agrees very well with continuum physics. Furthermore we
describe the multigrid formulation for the non-perturbative perfect action and
we present the concept of an exactly (quantum) perfect topological charge on
the lattice.Comment: 14 pages, 17 figures, Talk presented at LATTICE96(improvement
Perfect Lattice Topology: The Quantum Rotor as a Test Case
Lattice actions and topological charges that are classically and quantum
mechanically perfect (i.e. free of lattice artifacts) are constructed
analytically for the quantum rotor. It is demonstrated that the Manton action
is classically perfect while the Villain action is quantum perfect. The
geometric construction for the topological charge is only perfect at the
classical level. The quantum perfect lattice topology associates a topological
charge distribution, not just a single charge, with each lattice field
configuration. For the quantum rotor with the classically perfect action and
topological charge, the remaining cut-off effects are exponentially suppressed.Comment: 12 pages, including two figures. ordinary LaTeX, requires fps.sty;
Submitted to Phys. Lett.
The Dynamics of a Meandering River
We present a statistical model of a meandering river on an alluvial plane
which is motivated by the physical non-linear dynamics of the river channel
migration and by describing heterogeneity of the terrain by noise. We study the
dynamics analytically and numerically. The motion of the river channel is
unstable and we show that by inclusion of the formation of ox-bow lakes, the
system may be stabilised. We then calculate the steady state and show that it
is in agreement with simulations and measurements of field data.Comment: Revtex, 12 pages, 2 postscript figure
Effective Field Theories
Effective field theories encode the predictions of a quantum field theory at
low energy. The effective theory has a fairly low ultraviolet cutoff. As a
result, loop corrections are small, at least if the effective action contains a
term which is quadratic in the fields, and physical predictions can be read
straight from the effective Lagrangean.
Methods will be discussed how to compute an effective low energy action from
a given fundamental action, either analytically or numerically, or by a
combination of both methods. Basically,the idea is to integrate out the high
frequency components of fields. This requires the choice of a "blockspin",i.e.
the specification of a low frequency field as a function of the fundamental
fields. These blockspins will be the fields of the effective field theory. The
blockspin need not be a field of the same type as one of the fundamental
fields, and it may be composite. Special features of blockspins in nonabelian
gauge theories will be discussed in some detail.
In analytical work and in multigrid updating schemes one needs interpolation
kernels \A from coarse to fine grid in addition to the averaging kernels
which determines the blockspin. A neural net strategy for finding optimal
kernels is presented.
Numerical methods are applicable to obtain actions of effective theories on
lattices of finite volume. The constraint effective potential) is of particular
interest. In a Higgs model it yields the free energy, considered as a function
of a gauge covariant magnetization. Its shape determines the phase structure of
the theory. Its loop expansion with and without gauge fields can be used to
determine finite size corrections to numerical data.Comment: 45 pages, 9 figs., preprint DESY 92-070 (figs. 3-9 added in ps
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