224,492 research outputs found
Thermoelectric properties of AgGaTe and related chalcopyrite structure materials
We present an analysis of the potential thermoelectric performance of p-type
AgGaTe, which has already shown a of 0.8 with partial optimization,
and observe that the same band structure features, such as a mixture of light
and heavy bands and isotropic transport, that lead to this good performance are
present in certain other ternary chalcopyrite structure semiconductors. We find
that optimal performance of AgGaTe will be found for hole concentrations
between 4 and 2 cm at 900 K, and 2
and 10 cm at 700 K, and that certain other
chalcopyrite semiconductors might show good thermoelectric performance at
similar doping ranges and temperatures if not for higher lattice thermal
conductivity
Galaxy-Galaxy Flexion: Weak Lensing to Second Order
In this paper, we develop a new gravitational lensing inversion technique.
While traditional approaches assume that the lensing field varies little across
a galaxy image, we note that this variation in the field can give rise to a
``Flexion'' or bending of a galaxy image, which may then be used to detect a
lensing signal with increased signal to noise. Since the significance of the
Flexion signal increases on small scales, this is ideally suited to
galaxy-galaxy lensing. We develop an inversion technique based on the
``Shapelets'' formalism of Refregier (2003). We then demonstrate the proof of
this concept by measuring a Flexion signal in the Deep Lens Survey. Assuming an
intrinsically isothermal distribution, we find from the Flexion signal alone a
velocity width of v_c=221\pm 12 km/s for lens galaxies of r < 21.5, subject to
uncertainties in the intrinsic Flexion distribution.Comment: 11 pages, Latex, 4 figures. Accepted by ApJ, changes include revision
of errors from previous draf
Selfdual Einstein metrics and conformal submersions
Weyl derivatives, Weyl-Lie derivatives and conformal submersions are defined,
then used to generalize the Jones-Tod correspondence between selfdual
4-manifolds with symmetry and Einstein-Weyl 3-manifolds with an abelian
monopole. In this generalization, the conformal symmetry is replaced by a
particular kind of conformal submersion with one dimensional fibres. Special
cases are studied in which the conformal submersion is holomorphic, affine, or
projective. All scalar-flat Kahler metrics with such a holomorphic conformal
submersion, and all four dimensional hypercomplex structures with a compatible
Einstein metric, are obtained from solutions of the resulting ``affine monopole
equations''. The ``projective monopole equations'' encompass Hitchin's
twistorial construction of selfdual Einstein metrics from three dimensional
Einstein-Weyl spaces, and lead to an explicit formula for carrying out this
construction directly. Examples include new selfdual Einstein metrics depending
explicitly on an arbitrary holomorphic function of one variable or an arbitrary
axially symmetric harmonic function. The former generically have no continuous
symmetries.Comment: 34 page
Integrable Background Geometries
This work has its origins in an attempt to describe systematically the
integrable geometries and gauge theories in dimensions one to four related to
twistor theory. In each such dimension, there is a nondegenerate integrable
geometric structure, governed by a nonlinear integrable differential equation,
and each solution of this equation determines a background geometry on which,
for any Lie group , an integrable gauge theory is defined. In four
dimensions, the geometry is selfdual conformal geometry and the gauge theory is
selfdual Yang-Mills theory, while the lower-dimensional structures are
nondegenerate (i.e., non-null) reductions of this. Any solution of the gauge
theory on a -dimensional geometry, such that the gauge group acts
transitively on an -manifold, determines a -dimensional
geometry () fibering over the -dimensional geometry with
as a structure group. In the case of an -dimensional group acting
on itself by the regular representation, all -dimensional geometries
with symmetry group are locally obtained in this way. This framework
unifies and extends known results about dimensional reductions of selfdual
conformal geometry and the selfdual Yang-Mills equation, and provides a rich
supply of constructive methods. In one dimension, generalized Nahm equations
provide a uniform description of four pole isomonodromic deformation problems,
and may be related to the Toda and dKP equations via a
hodograph transformation. In two dimensions, the Hitchin
equation is shown to be equivalent to the hyperCR Einstein-Weyl equation, while
the Hitchin equation leads to a Euclidean analogue of
Plebanski's heavenly equations.Comment: for Progress in Twistor Theory, SIGM
Selfdual 4-Manifolds, Projective Surfaces, and the Dunajski-West Construction
I present a construction of real or complex selfdual conformal 4-manifolds
(of signature (2,2) in the real case) from a natural gauge field equation on a
real or complex projective surface, the gauge group being the group of
diffeomorphisms of a real or complex 2-manifold. The 4-manifolds obtained are
characterized by the existence of a foliation by selfdual null surfaces of a
special kind. The classification by Dunajski and West of selfdual conformal
4-manifolds with a null conformal vector field is the special case in which the
gauge group reduces to the group of diffeomorphisms commuting with a vector
field, and I analyse the presence of compatible scalar-flat K\"ahler,
hypercomplex and hyperk\"ahler structures from a gauge-theoretic point of view.
In an appendix, I discuss the twistor theory of projective surfaces, which is
used in the body of the paper, but is also of independent interest.Comment: for Progress in Twistor Theory, SIGM
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