13,036 research outputs found
Spectral flow of monopole insertion in topological insulators
Inserting a magnetic flux into a two-dimensional one-particle Hamiltonian
leads to a spectral flow through a given gap which is equal to the Chern number
of the associated Fermi projection. This paper establishes a generalization to
higher even dimension by inserting non-abelian monopoles of the Wu-Yang type.
The associated spectral flow is then equal to a higher Chern number. For the
study of odd spacial dimensions, a new so-called `chirality flow' is introduced
which, for the insertion of a monopole, is then linked to higher winding
numbers. This latter fact follows from a new index theorem for the spectral
flow between two unitaries which are conjugates of each other by a self-adjoint
unitary.Comment: title changed; final corrections before publication; to appear in
Commun. Math. Phy
Comet P/Tempel: Some highlights and conclusions from the 1988 apparition
From the brightness development and a sequence of imaging observations of the coma activity onset of comet P/Tempel 2 in 1988, it is concluded that there might have happened eruptive events of strong dust and gas outbursts during May and June 1988. A comparison of dust coma modeling calculations with CCD observations of the coma widely confirms Sekanina's nucleus model for the comet
High performance structures
Materials selection, structural geometry, proof testing and statistical screening, prestressing, and system energy as tools for designing optimum trusses and other high performance structure
Studies in prestressed and segmented brittle structures
Application of nonlinear bending theory to prestressed and segmented brittle structure
Multiphoton Bloch-Siegert shifts and level-splittings in spin-one systems
We consider a spin-boson model in which a spin 1 system is coupled to an
oscillator. A unitary transformation is applied which allows a separation of
terms responsible for the Bloch-Siegert shift, and terms responsible for the
level splittings at anticrossings associated with Bloch-Siegert resonances.
When the oscillator is highly excited, the system can maintain resonance for
sequential multiphoton transitions. At lower levels of excitation, resonance
cannot be maintained because energy exchange with the oscillator changes the
level shift. An estimate for the critical excitation level of the oscillator is
developed.Comment: 14 pages, 3 figure
Laboratory studies on cometary crust formation: The importance of sintering
It is demonstrated by experiments and theoretical considerations that sintering processes, so far used to describe the densification of metal and ceramic powders, are relevant for icy materials and therefore probably also for comets. A theoretical model is presented which describes the evolution of so called sinter necks, the contact zone between ice particles. With this model the strength increase of a porous, loosley packed icy body is calculated in which the sinter necks grow by evaporation and condensation of water vapor at a constant temperature. Experiments with ice powders validate the model qualitatively. An increase in strength up to a factor of four is observed during isothermal sintering. In order to check the relevance of the experimental results and the basic theoretical ideas with respect to real comets, more exact theories and improved experiments taking into account additional mass transport mechanisms are needed
Pattern Selection in the Complex Ginzburg-Landau Equation with Multi-Resonant Forcing
We study the excitation of spatial patterns by resonant, multi-frequency
forcing in systems undergoing a Hopf bifurcation to spatially homogeneous
oscillations. Using weakly nonlinear analysis we show that for small amplitudes
only stripe or hexagon patterns are linearly stable, whereas square patterns
and patterns involving more than three modes are unstable. In the case of
hexagon patterns up- and down-hexagons can be simultaneously stable. The
third-order, weakly nonlinear analysis predicts stable square patterns and
super-hexagons for larger amplitudes. Direct simulations show, however, that in
this regime the third-order weakly nonlinear analysis is insufficient, and
these patterns are, in fact unstable
On Keller Theorem for Anisotropic Media
The Keller theorem in the problem of effective conductivity in anisotropic
two-dimensional (2D) many-component composites makes it possible to establish a
simple inequality for the isotropic part of
the 2-nd rank symmetric tensor of effective
conductivity.Comment: 1 page, 1 figur
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