72,174 research outputs found
A Non-Crossing Approximation for the Study of Intersite Correlations
We develop a Non-Crossing Approximation (NCA) for the effective cluster
problem of the recently developed Dynamical Cluster Approximation (DCA). The
DCA technique includes short-ranged correlations by mapping the lattice problem
onto a self-consistently embedded periodic cluster of size . It is a fully
causal and systematic approximation to the full lattice problem, with
corrections in two dimensions. The NCA we develop is a
systematic approximation with corrections . The method will
be discussed in detail and results for the one-particle properties of the
Hubbard model are shown. Near half filling, the spectra display pronounced
features including a pseudogap and non-Fermi-liquid behavior due to
short-ranged antiferromagnetic correlations.Comment: 12 pages, 11 figures, EPJB styl
Optical waveguide sensors
An overview of the field of optical waveguide sensors is presented. Some emphasis is laid on the development of a single scheme under which the diversity of sensor principles can be arranged. First three types of sensors are distinguished: intrinsic, extrinsic and active. Next, two steps are distinguished in the sensing process: the physical alteration of the waveguide (via geometrical and/or materials effects) and the effect thereof on the guided light (changing phase and/or amplitude of the light modes). The diversity and common features of the field of optical waveguide sensors are demonstrated with the help of many examples.\ud
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Tracing the development of dust around evolved stars: The case of 47 Tuc
We observed mid-infrared (7.5-22 mum) spectra of AGB stars in the globular
cluster 47 Tuc with the Spitzer telescope and find significant dust features of
various types. Comparison of the characteristics of the dust spectra with the
location of the stars in a logP-K-diagram shows that dust mineralogy and
position on the AGB are related. A 13 mum feature is seen in spectra of low
luminosity AGB stars. More luminous AGB stars show a broad feature at 11.5 mum.
The spectra of the most luminous stars are dominated by the amorphous silicate
bending vibration centered at 9.7 mum. For 47 Tuc AGB stars, we conclude that
early on the AGB dust consisting primarily of Mg-, Al- and Fe oxides is formed.
With further AGB evolution amorphous silicates become the dominant species.Comment: 2 figures, accepted for publication in ApJ Letter
The Dynamics of Silica Melts under High Pressure: Mode-Coupling Theory Results
The high-pressure dynamics of a computer-modeled silica melt is studied in
the framework of the mode-coupling theory of the glass transition (MCT) using
static-structure input from molecular-dynamics (MD) computer simulation. The
theory reproduces the experimentally known viscosity minimum (diffusivity
maximum) as a function of density or pressure and explains it in terms of a
corresponding minimum in its critical temperature. This minimum arises from a
gradual change in the equilibrium static structure which shifts from being
dominated by tetrahedral ordering to showing the cageing known from
high-density liquids. The theory is in qualitative agreement with computer
simulation results.Comment: Presented at ESF EW Glassy Liquids under Pressure, to be published in
Journal of Physic
Light scattering from mesoscopic objects in diffusive media
The diffuse intensity propagating in turbid media is sensitive to the
presence of any kind of object embedded in the medium, e.g. obstacles or
defects. The long-ranged effects of isolated objects can be described by a
stationary diffusion equation, the effect of any single object being
parametrized in terms of a multipole expansion. An absorbing object is chiefly
characterized by a negative charge, while the leading effect of a non-absorbing
object is due to its dipole moment. The associated intrinsic characteristics of
the object (capacitance or effective radius , polarizability
) can be evaluated within the diffusion approximation for large enough
objects. The situation of mesoscopic objects, with a size comparable to the
mean free path, requires a more careful treatment, for which the appropriate
framework is radiative transfer theory. This formalism is worked out in detail
for spheres and cylinders of the following kinds: totally absorbing (black),
transparent, and totally reflecting.Comment: 31 pages, 2 tables, 7 figures. To appear in Eur. J. Phys.
Thermodynamic description of a dynamical glassy transition
For the dynamical glassy transition in the -spin mean field spin glass
model a thermodynamic description is given. The often considered marginal
states are not the relevant ones for this purpose. This leads to consider a
cooling experiment on exponential timescales, where lower states are accessed.
The very slow configurational modes are at quasi-equilibrium at an effective
temperature. A system independent law is derived that expresses their
contribution to the specific heat. -scaling in the aging regime of
two-time quantities is explained.Comment: 5 pages revte
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