3,978 research outputs found
Two-time Green's functions and spectral density method in nonextensive quantum statistical mechanics
We extend the formalism of the thermodynamic two-time Green's functions to
nonextensive quantum statistical mechanics. Working in the optimal Lagrangian
multipliers representation, the -spectral properties and the methods for a
direct calculation of the two-time % -Green's functions and the related
-spectral density ( measures the nonextensivity degree) for two generic
operators are presented in strict analogy with the extensive ()
counterpart. Some emphasis is devoted to the nonextensive version of the less
known spectral density method whose effectiveness in exploring equilibrium and
transport properties of a wide variety of systems has been well established in
conventional classical and quantum many-body physics. To check how both the
equations of motion and the spectral density methods work to study the
-induced nonextensivity effects in nontrivial many-body problems, we focus
on the equilibrium properties of a second-quantized model for a high-density
Bose gas with strong attraction between particles for which exact results exist
in extensive conditions. Remarkably, the contributions to several thermodynamic
quantities of the -induced nonextensivity close to the extensive regime are
explicitly calculated in the low-temperature regime by overcoming the
calculation of the grand-partition function.Comment: 48 pages, no figure
Spectral density method in quantum nonextensive thermostatistics and magnetic systems with long-range interactions
Motived by the necessity of explicit and reliable calculations, as a valid
contribution to clarify the effectiveness and, possibly, the limits of the
Tsallis thermostatistics, we formulate the Two-Time Green Functions Method in
nonextensive quantum statistical mechanics within the optimal Lagrange
multiplier framework, focusing on the basic ingredients of the related Spectral
Density Method. Besides, to show how the SDM works we have performed, to the
lowest order of approximation, explicit calculations of the low-temperature
properties for a quantum -dimensional spin-1/2 Heisenberg ferromagnet with
long-range interactions decaying as ( is the distance between
spins in the lattice)Comment: Contribution to Next-SigmaPhi conference in Kolymbari, Crete, Greece,
August 13-18, 2005, 9 page
The Classical Spectral Density Method at Work: The Heisenberg Ferromagnet
In this article we review a less known unperturbative and powerful many-body
method in the framework of classical statistical mechanics and then we show how
it works by means of explicit calculations for a nontrivial classical model.
The formalism of two-time Green functions in classical statistical mechanics is
presented in a form parallel to the well known quantum counterpart, focusing on
the spectral properties which involve the important concept of spectral
density. Furthermore, the general ingredients of the classical spectral density
method (CSDM) are presented with insights for systematic nonperturbative
approximations to study conveniently the macroscopic properties of a wide
variety of classical many-body systems also involving phase transitions. The
method is implemented by means of key ideas for exploring the spectrum of
elementary excitations and the damping effects within a unified formalism.
Then, the effectiveness of the CSDM is tested with explicit calculations for
the classical -dimensional spin- Heisenberg ferromagnetic model with
long-range exchange interactions decaying as () with distance
between spins and in the presence of an external magnetic field. The analysis
of the thermodynamic and critical properties, performed by means of the CSDM to
the lowest order of approximation, shows clearly that nontrivial results can be
obtained in a relatively simple manner already to this lower stage. The basic
spectral density equations for the next higher order level are also presented
and the damping of elementary spin excitations in the low temperature regime is
studied. The results appear in reasonable agreement with available exact ones
and Monte Carlo simulations and this supports the CSDM as a promising method of
investigation in classical many-body theory.Comment: Latex, 58 pages, 12 figure
Missing hard states and regular outbursts: the puzzling case of the black hole candidate 4U 1630-472
4U 1630-472 is a recurrent X-ray transient classified as a black-hole
candidate from its spectral and timing properties. One of the peculiarities of
this source is the presence of regular outbursts with a recurrence period
between 600 and 730 d that has been observed since the discovery of the source
in 1969. We report on a comparative study on the spectral and timing behaviour
of three consecutive outbursts occurred in 2006, 2008 and 2010. We analysed all
the data collected by the INTErnational Gamma-Ray Astrophysics Laboratory
(INTEGRAL) and the Rossi X-ray timing Explorer (RXTE) during these three years
of activity. We show that, in spite of having a similar spectral and timing
behaviour in the energy range between 3 and 30 keV, these three outbursts show
pronounced differences above 30 keV. In fact, the 2010 outburst extends at high
energies without any detectable cut-off until 150-200 keV, while the two
previous outbursts that occurred in 2006 and 2008 are not detected at all above
30 keV. Thus, in spite of a very similar accretion disk evolution, these three
outbursts exhibit totally different characteristics of the Compton electron
corona, showing a softening in their evolution rarely observed before in a low
mass X-ray binary hosting a black hole. We argue the possibility that the
unknown perturbation that causes the outbursts to be equally spaced in time
could be at the origin of this particular behaviour. Finally we describe
several possible scenarios that could explain the regularity of the outbursts,
identifying the most plausible, such as a third body orbiting around the binary
system.Comment: April 2015: accepted for publication in MNRAS. May 2015: in pres
Assessment of the radiological impact of a decommissioning nuclear power plant in Italy
The assessment of the radiological impact of a decommissioning Nuclear Power
Plant is presented here through the results of an environmental monitoring
survey carried out in the area surrounding the Garigliano Power Plant. The
levels of radioactivity in soil, water, air and other environmental matrices
are shown, in which {\alpha}, {\beta} and {\gamma} activity and {\gamma}
equivalent dose rate are measured. Radioactivity levels of the samples from the
Garigliano area are analyzed and then compared to those from a control zone
situated more than 100 km away. Moreover, a comparison is made with a previous
survey held in 2001. The analyses and comparisons show no significant
alteration in the radiological characteristics of the area surroundings the
plant, with an overall radioactivity depending mainly from the global fallout
and natural sources.Comment: 13 pages, 6 figures, 2 table
Assessment of the radiological impact of a decommissioning nuclear power plant in Italy
The assessment of the radiological impact of a decommissioning Nuclear Power
Plant is presented here through the results of an environmental monitoring
survey carried out in the area surrounding the Garigliano Power Plant. The
levels of radioactivity in soil, water, air and other environmental matrices
are shown, in which {\alpha}, {\beta} and {\gamma} activity and {\gamma}
equivalent dose rate are measured. Radioactivity levels of the samples from the
Garigliano area are analyzed and then compared to those from a control zone
situated more than 100 km away. Moreover, a comparison is made with a previous
survey held in 2001. The analyses and comparisons show no significant
alteration in the radiological characteristics of the area surroundings the
plant, with an overall radioactivity depending mainly from the global fallout
and natural sources
Comprehending 3D and 4D ontology-driven conceptual models: An empirical study
This paper presents an empirical study that investigates the extent to which the pragmatic quality of ontology-driven models is influenced by the choice of a particular ontology, given a certain understanding of that ontology. To this end, we analyzed previous research efforts and distilled three hypotheses based on different metaphysical characteristics. An experiment based on two foundational ontologies (UFO and BORO) involving 158 participants was then carried out, followed by a protocol analysis to gain further insights into the results of experiment. We then extracted five derivations from the results of the empirical study in order to summarize our findings. Overall, the results confirm that the choice of a foundational ontology can lead to significant differences in the interpretation and comprehension of the conceptual models produced. Moreover, the effect of applying a certain foundational ontology can cause considerable variations in the effort required to comprehend these models
Study of 236U/238U ratio at CIRCE using a 16-strip silicon detector with a TOF system
Accelerator Mass Spectrometry (AMS) is presently the most sensitive technique for the measurement of long-lived actinides, e.g. 236 U and x Pu isotopes. A new actinide AMS system, based on a 3-MV pelletron tandem accelerator, is operated at the Center for Isotopic Research on Cultural and Environmental Heritage (CIRCE) in Caserta, Italy. In this paper we report on the procedure adopted to increase the 236 U abundance sensitivity as low as possible. The energy and position determinations of the 236 U ions, using a 16-strip silicon detector have been obtained. A 236 U/ 238 U isotopic ratio background level of about 2.9×10 −11 was obtained, summing over all the strips, using a Time of Flight-Energy (TOF-E) system with a 16-strip silicon detector (4.9×10 −12 just with one strip)
a smart nanofibrous material for adsorbing and detecting elemental mercury in air
Abstract. The combination of the affinity of gold for mercury and nanosized frameworks has allowed for the design and fabrication of novel kinds of sensors with promising sensing features for environmental applications. Specifically, conductive sensors based on composite nanofibrous electrospun layers of titania easily decorated with gold nanoparticles were developed to obtain nanostructured hybrid materials capable of entrapping and revealing gaseous elemental mercury (GEM) traces from the environment. The electrical properties of the resulting chemosensors were measured. A few minutes of air sampling were sufficient to detect the concentration of mercury in the air, ranging between 20 and 100 ppb, without using traps or gas carriers (LOD: 1.5 ppb). Longer measurements allowed the sensor to detect lower concentrations of GEM. The resulting chemosensors are expected to be low cost and very stable (due to the peculiar structure), requiring low power, low maintenance, and simple equipment
- …