3,354 research outputs found
Highlights of Symmetry Groups
The concepts of symmetry and symmetry groups are at the heart of several
developments in modern theoretical and mathematical physics. The present paper
is devoted to a number of selected topics within this framework: Euclidean and
rotation groups; the properties of fullerenes in physical chemistry; Galilei,
Lorentz and Poincare groups; conformal transformations and the Laplace
equation; quantum groups and Sklyanin algebras. For example, graphite can be
vaporized by laser irradiation, producing a remarkably stable cluster
consisting of 60 carbon atoms. The corresponding theoretical model considers a
truncated icosahedron, i.e. a polygon with 60 vertices and 32 faces, 12 of
which are pentagonal and 20 hexagonal. The Carbon 60 molecule obtained when a
carbon atom is placed at each vertex of this structure has all valences
satisfied by two single bonds and one double bond. In other words, a structure
in which a pentagon is completely surrounded by hexagons is stable. Thus, a
cage in which all 12 pentagons are completely surrounded by hexagons has
optimum stability. On a more formal side, the exactly solvable models of
quantum and statistical physics can be studied with the help of the quantum
inverse problem method. The problem of enumerating the discrete quantum systems
which can be solved by the quantum inverse problem method reduces to the
problem of enumerating the operator-valued functions that satisfy an equation
involving a fixed solution of the quantum Yang--Baxter equation. Two basic
equations exist which provide a systematic procedure for obtaining completely
integrable lattice approximations to various continuous completely integrable
systems. This analysis leads in turn to the discovery of Sklyanin algebras.Comment: Plain Tex with one figur
Twistors and Spin 3/2 Potentials in Quantum Gravity
Local boundary conditions involving field strengths and the normal to the
boundary, originally studied in anti-de Sitter space-time, have been recently
considered in one-loop quantum cosmology. This paper derives the conditions
under which spin-lowering and spin-raising operators preserve these local
boundary conditions on a 3-sphere for fields of spin 0,1/2,1,3/2 and 2.
Moreover, the two-component spinor analysis of the four potentials of the
totally symmetric and independent field strengths for spin 3/2 is applied to
the case of a 3-sphere boundary. It is shown that such boundary conditions can
only be imposed in a flat Euclidean background, for which the gauge freedom in
the choice of the potentials remains. Alternative boundary conditions for
supergravity involving the spinor-valued 1-forms for gravitinos and the normal
to the boundary are also studied.Comment: 20 pages, plain-tex, recently appearing in: Twistor Theory, edited by
Stephen Huggett (Marcel Dekker, New York, 1994). The authors apologize for
the delay in circulating the paper, which was due to technical problems now
fixe
Classical brackets for dissipative systems
We show how to write a set of brackets for the Langevin equation, describing
the dissipative motion of a classical particle, subject to external random
forces. The method does not rely on an action principle, and is based solely on
the phenomenological description of the dissipative dynamics as given by the
Langevin equation. The general expression for the brackets satisfied by the
coordinates, as well as by the external random forces, at different times, is
determined, and it turns out that they all satisfy the Jacobi identity. Upon
quantization, these classical brackets are found to coincide with the
commutation rules for the quantum Langevin equation, that have been obtained in
the past, by appealing to microscopic conservative quantum models for the
friction mechanism.Comment: Latex file, 8 pages, prepared for the Conference Spacetime and
Fundamental Interactions: Quantum Aspects, Vietri sul Mare, Italy, 26-31 May
200
Detection of trace elements in the bivalve Ruditapes decussatus from Sardinian coastal lagoons: effects on food safety and pathological findings in target organs
Shellfish can bioaccumulate toxic metals due to their ability to concentrate inorganic contaminants. Since biomonitoring by using bivalve molluscs is currently considered one of the most effective approaches for assessing the degree of pollution of brackish
environments, the aims of this PhD thesis were: a) to detect the content of trace elements in the Grooved carpet shell Ruditapes decussatus collected in Sardinian coastal lagoons (Italy) and their effects on food safety; b) to evaluate the pathological findings in its target
organs. The concentration of 16 trace elements (Al, Ag, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, Sn, Tl and Zn) was quantified. The legal limits set by European Regulations for Cd, Hg and Pb were never exceeded. Unexpectedly high values for Al and Fe were found. A total of six histopathological alterations were analysed in the digestive gland, gills and kidney following a weighted condition indices approach. Gills show the highest prevalence of lesions than digestive gland, followed by kidney. The clam R. decussatus confirmed the capacity of bivalves as suitable bioindicators of trace elements pollution.
The determination of trace elements combined with histopathological analysis, should be a powerful tool in environmental monitoring plans. It is useful for understanding directly the health status of the marine organisms and indirectly the impact which different
anthropogenic activities have on shellfish harvested in coastal environments
Variations of Casimir energy from a superconducting transition
We consider a five-layer Casimir cavity, including a thin superconducting
film. We show that when the cavity is cooled below the critical temperature for
the onset of superconductivity, the sharp variation (in the microwave region)
of the reflection coefficient of the film produces a variation in the value of
the Casimir energy. Even though the relative variation in the Casimir energy is
very small, its magnitude can be comparable to the condensation energy of the
superconducting film, and thus causes a significant increase in the value of
the critical magnetic field, required to destroy the superconductivity of the
film. The proposed scheme might also help clarifying the current controversy
about the magnitude of the contribution to Casimir free energy from the TE zero
mode, as we find that alternative treatments of this mode strongly affect the
shift of critical field.Comment: RevTex 4, 15 pages, 8 figures. Revised version with improved
presentation, a brief description of the ALADIN experiment, and a few
references adde
Push on a Casimir apparatus in a weak gravitational field
The influence of the gravity acceleration on the regularized energy-momentum
tensor of the quantized electromagnetic field between two plane parallel
conducting plates is derived. We use Fermi coordinates and work to first order
in the constant acceleration parameter. A new simple formula for the trace
anomaly is found to first order in the constant acceleration, and a more
systematic derivation is therefore obtained of the theoretical prediction
according to which the Casimir device in a weak gravitational field will
experience a tiny push in the upwards direction.Comment: 14 pages, Plain Tex. Talk given at the 17th SIGRAV Conference on
General Relativity and Gravitational Physics, Torino, September 200
Boundary Terms for Massless Fermionic Fields
Local supersymmetry leads to boundary conditions for fermionic fields in
one-loop quantum cosmology involving the Euclidean normal to the boundary and a
pair of independent spinor fields. This paper studies the corresponding
classical properties, i.e. the classical boundary-value problem and boundary
terms in the variational problem. Interestingly, a link is found with the
classical boundary-value problem when spectral boundary conditions are imposed
on a 3-sphere in the massless case. Moreover, the boundary term in the action
functional is derived.Comment: 8 pages, plain-tex, recently appearing in Foundations of Physics
Letters, volume 7, pages 303-308, year 199
Ultrabiomicroscopy anterior segment evaluation of ocular contusive trauma caused by pressurized bottled drink caps. a case report
We report the case of a patient presented to the emergency department because of a contu-sive trauma from a pressurized bottled drink cap. During the visit, the patient indicated that he had been hit in his left eye by a cork while he was opening a sparkling wine bottle. He underwent a total ophthalmology examination. He had an important reduction of visual acuity, corneal swelling, Descemet’s folds, and hyphema. Therefore, we decided to perform ultrabio-microscopy (UBM) of the anterior segment to study the endothelial damage and Descemet’s membrane. UBM images confirmed the direct biomicroscopy, highlighting the damaged loca-tion
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