609 research outputs found
Free Quantum Fields on the Poincare' Group
A class of free quantum fields defined on the Poincare' group, is described
by means of their two-point vacuum expectation values. They are not equivalent
to fields defined on the Minkowski spacetime and they are "elementary" in the
sense that they describe particles that transform according to irreducible
unitary representations of the symmetry group, given by the product of the
Poincare' group and of the group SL(2, C) considered as an internal symmetry
group. Some of these fields describe particles with positive mass and arbitrary
spin and particles with zero mass and arbitrary helicity or with an infinite
helicity spectrum. In each case the allowed SL(2, C) internal quantum numbers
are specified. The properties of local commutativity and the limit in which one
recovers the usual field theories in Minkowski spacetime are discussed. By
means of a superposition of elementary fields, one obtains an example of a
field that present a broken symmetry with respect to the group Sp(4, R), that
survives in the short-distance limit. Finally, the interaction with an
accelerated external source is studied and and it is shown that, in some
theories, the average number of particles emitted per unit of proper time
diverges when the acceleration exceeds a finite critical value.Comment: 49 pages, plain tex with vanilla.st
The Case Against Commodity Agreements
Depth sensing nano-indentation investigations have been performed to determine the radial dependence of the hardness through the cross section of a Fe-based bulk glassy rod. We have found the hardness of the material decreases along radius from the center to the outermost surface. This phenomenon is attributed to the ‘cooling rate induced surface softening’. Furthermore, a significant change (~15 %) in elastic modulus is noticed along the radius as well. QC 20120906Hero-
Oxidation states and magnetism of Fe nanoparticles prepared by a laser evaporation technique
Nanoparticles of iron and iron oxide have been prepared in a thermal diffusion cloud chamber using pulsed laser evaporation. SEM/TEM studies of these particles reveal a size distribution with a mean diameter of about 60 Ã…. This is consistent with the mean particle size estimated from the magnetic data. The oxidation levels of these nanoparticles prepared at different partial oxygen pressures were investigated using FTIR. All the samples are found to exhibit superparamagnetism with blocking temperatures ranging from 50 K to above room temperature. Magnetic anisotropy constants are calculated from the frequency dependence of the blocking temperatures are found to be one quarter of magnitude higher than is known for the bulk
Spherical functions on the de Sitter group
Matrix elements and spherical functions of irreducible representations of the
de Sitter group are studied on the various homogeneous spaces of this group. It
is shown that a universal covering of the de Sitter group gives rise to
quaternion Euler angles. An explicit form of Casimir and Laplace-Beltrami
operators on the homogeneous spaces is given. Different expressions of the
matrix elements and spherical functions are given in terms of multiple
hypergeometric functions both for finite-dimensional and unitary
representations of the principal series of the de Sitter group.Comment: 40 page
Cosmic multi-muon events observed in the underground CERN-LEP tunnel with the ALEPH experiment
Multimuon events have been recorded with the ALEPH-detector, located 140 m underground, in parallel with ee data taking. Benefitting from the high spatial and momentum resolution of the ALEPH tracking chambers narrowly spaced muons in high multiplicity bundles could be analysed. The bulk of the data can be successfully described by standard production phenomena. The multiplicity distribution favors, though not with very high significance, a chemical composition which changes from light to heavier elements with increasing energy around the ``knee". The five highest multiplicity events, with up to 150 muons within an area of 8 m, occur with a frequency which is almost an order of magnitude above the simulation. To establish a possible effect, more of these events should be recorded with a larger area detector
Graded Contractions of Affine Kac-Moody Algebras
The method of graded contractions, based on the preservation of the
automorphisms of finite order, is applied to the affine Kac-Moody algebras and
their representations, to yield a new class of infinite dimensional Lie
algebras and representations. After the introduction of the horizontal and
vertical gradings, and the algorithm to find the horizontal toroidal gradings,
I discuss some general properties of the graded contractions, and compare them
with the In\"on\"u-Wigner contractions. The example of is discussed
in detail.Comment: 23 pages, Ams-Te
Electromagnetic properties of non-Dirac particles with rest spin 1/2
We resolve a number of questions related to an analytic description of
electromagnetic form factors of non-Dirac particles with the rest spin 1/2. We
find the general structure of a matrix antisymmetric tensor operator. We obtain
two recurrence relations for matrix elements of finite transformations of the
proper Lorentz group and explicit formulas for a certain set of such elements.
Within the theory of fields with double symmetry, we discuss writing the
components of wave vectors of particles in the form of infinite continued
fractions. We show that for (GeV/c), where is
the transferred momentum squared, electromagnetic form factors that decrease as
increases and are close to those experimentally observed in the proton
can be obtained without explicitly introducing an internal particle structure.Comment: 18 pages, 2 figure
Ferromagnetic Domain Distribution in Thin Films During Magnetization Reversal
We have shown that polarized neutron reflectometry can determine in a
model-free way not only the mean magnetization of a ferromagnetic thin film at
any point of a hysteresis cycle, but also the mean square dispersion of the
magnetization vectors of its lateral domains. This technique is applied to
elucidate the mechanism of the magnetization reversal of an exchange-biased
Co/CoO bilayer. The reversal process above the blocking temperature is governed
by uniaxial domain switching, while below the blocking temperature the reversal
of magnetization for the trained sample takes place with substantial domain
rotation
Correlated local distortions of the TlO layers in TlBaCuO: An x-ray absorption study
We have used the XAFS (x-ray-absorption fine structure) technique to
investigate the local structure about the Cu, Ba, and Tl atoms in orthorhombic
Tl-2201 with a superconducting transition temperature T=60 K. Our results
clearly show that the O(1), O(2), Cu, and Ba atoms are at their ideal sites as
given by the diffraction measurements, while the Tl and O(3) atoms are more
disordered than suggested by the average crystal structure. The Tl-Tl distance
at 3.5 \AA{ } between the TlO layers does not change, but the Tl-Tl distance at
3.9 \AA{ } within the TlO layer is not observed and the Tl-Ba and Ba-Tl peaks
are very broad. The shorter Tl-O(3) distance in the TlO layer is about 2.33
\AA, significantly shorter than the distance calculated with both the Tl and
O(3) atoms at their ideal sites ( 0 or ). A model based
on these results shows that the Tl atom is displaced along the
directions from its ideal site by about 0.11 \AA; the displacements of
neighboring Tl atoms are correlated. The O(3) atom is shifted from the $4e$
site by about 0.53 \AA{ } roughly along the directions. A comparison of
the Tl L-edge XAFS spectra from three samples, with T=60 K, 76 K,
and 89 K, shows that the O environment around the Tl atom is sensitive to T
while the Tl local displacement is insensitive to T and the structural
symmetry. These conclusions are compared with other experimental results and
the implications for charge transfer and superconductivity are discussed. This
paper has been submitted to Phys. Rev. B.Comment: 20 pages plus 14 ps figures, REVTEX 3.
Merging teaching and research: how to use your teaching to increase your research outputs
We all need research outputs for:
We all need research outputs for:
Promotion,
job applications,
peer esteem,
attracting good students,
evidence for attracting funding
This presentation is for architectural staff and gives some examples of how to make the most of teaching so that it can be converted into research output
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