496 research outputs found
Modified Landau levels, damped harmonic oscillator and two-dimensional pseudo-bosons
In a series of recent papers one of us has analyzed in some details a class
of elementary excitations called {\em pseudo-bosons}. They arise from a special
deformation of the canonical commutation relation [a,a^\dagger]=\1, which is
replaced by [a,b]=\1, with not necessarily equal to . Here,
after a two-dimensional extension of the general framework, we apply the theory
to a generalized version of the two-dimensional Hamiltonian describing Landau
levels. Moreover, for this system, we discuss coherent states and we deduce a
resolution of the identity. We also consider a different class of examples
arising from a classical system, i.e. a damped harmonic oscillator.Comment: in press in Journal of Mathematical Physic
A discrete nonetheless remarkable brick in de Sitter: the "massless minimally coupled field"
Over the last ten years interest in the physics of de Sitter spacetime has
been growing very fast. Besides the supposed existence of a "de sitterian
period" in inflation theories, the observational evidence of an acceleration of
the universe expansion (interpreted as a positive cosmological constant or a
"dark energy" or some form of "quintessence") has triggered a lot of attention
in the physics community. A specific de sitterian field called "massless
minimally coupled field" (mmc) plays a fundamental role in inflation models and
in the construction of the de sitterian gravitational field. A covariant
quantization of the mmc field, `a la Krein-Gupta-Bleuler was proposed in [1].
In this talk, we will review this construction and explain the relevance of
such a field in the construction of a massless spin 2 field in de Sitter
space-time.Comment: Proceedings of the XXVII Colloquium on Group Theoretical Methods in
Physics, Yerevan, August 200
Complex Effects of Ecosystem Engineer Loss on Benthic Ecosystem Response to Detrital Macroalgae
Ecosystem engineers change abiotic conditions, community assembly and ecosystem functioning. Consequently, their loss may modify thresholds of ecosystem response to disturbance and undermine ecosystem stability. This study investigates how loss of the bioturbating lugworm Arenicola marina modifies the response to macroalgal detrital enrichment of sediment biogeochemical properties, microphytobenthos and macrofauna assemblages. A field manipulative experiment was done on an intertidal sandflat (Oosterschelde estuary, The Netherlands). Lugworms were deliberately excluded from 1x m sediment plots and different amounts of detrital Ulva (0, 200 or 600 g Wet Weight) were added twice. Sediment biogeochemistry changes were evaluated through benthic respiration, sediment organic carbon content and porewater inorganic carbon as well as detrital macroalgae remaining in the sediment one month after enrichment. Microalgal biomass and macrofauna composition were measured at the same time. Macroalgal carbon mineralization and transfer to the benthic consumers were also investigated during decomposition at low enrichment level (200 g WW). The interaction between lugworm exclusion and detrital enrichment did not modify sediment organic carbon or benthic respiration. Weak but significant changes were instead found for porewater inorganic carbon and microalgal biomass. Lugworm exclusion caused an increase of porewater carbon and a decrease of microalgal biomass, while detrital enrichment drove these values back to values typical of lugworm-dominated sediments. Lugworm exclusion also decreased the amount of macroalgae remaining into the sediment and accelerated detrital carbon mineralization and CO2 release to the water column. Eventually, the interaction between lugworm exclusion and detrital enrichment affected macrofauna abundance and diversity, which collapsed at high level of enrichment only when the lugworms were present. This study reveals that in nature the role of this ecosystem engineer may be variable and sometimes have no or even negative effects on stability, conversely to what it should be expected based on current research knowledge
Mathematical aspects of intertwining operators: the role of Riesz bases
In this paper we continue our analysis of intertwining relations for both
self-adjoint and not self-adjoint operators. In particular, in this last
situation, we discuss the connection with pseudo-hermitian quantum mechanics
and the role of Riesz bases.Comment: Journal of Physics A, in pres
Generating functions for generalized binomial distributions
In a recent article a generalization of the binomial distribution associated
with a sequence of positive numbers was examined. The analysis of the
nonnegativeness of the formal expressions was a key-point to allow to give them
a statistical interpretation in terms of probabilities. In this article we
present an approach based on generating functions that solves the previous
difficulties: the constraints of nonnegativeness are automatically fulfilled, a
complete characterization in terms of generating functions is given and a large
number of analytical examples becomes available.Comment: PDFLaTex, 27 pages, 5 figure
Dissipation in ferrofluids: Mesoscopic versus hydrodynamic theory
Part of the field dependent dissipation in ferrofluids occurs due to the
rotational motion of the ferromagnetic grains relative to the viscous flow of
the carrier fluid. The classical theoretical description due to Shliomis uses a
mesoscopic treatment of the particle motion to derive a relaxation equation for
the non-equilibrium part of the magnetization. Complementary, the hydrodynamic
approach of Liu involves only macroscopic quantities and results in dissipative
Maxwell equations for the magnetic fields in the ferrofluid. Different stress
tensors and constitutive equations lead to deviating theoretical predictions in
those situations, where the magnetic relaxation processes cannot be considered
instantaneous on the hydrodynamic time scale. We quantify these differences for
two situations of experimental relevance namely a resting fluid in an
oscillating oblique field and the damping of parametrically excited surface
waves. The possibilities of an experimental differentiation between the two
theoretical approaches is discussed.Comment: 14 pages, 2 figures, to appear in PR
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