1,401 research outputs found
The Ising phase in the J1-J2 Heisenberg Model
The two dimensional Heisenberg antiferromagnet on the square lattice with
nearest (J1) and next-nearest (J2) neighbor couplings is investigated in the
strong frustration regime (J2/J1>1/2). A new effective field theory describing
the long wavelength physics of the model is derived from the quantum
hamiltonian. The structure of the resulting non linear sigma model allows to
recover the known spin wave results in the collinear regime, supports the
presence of an Ising phase transition at finite temperature and suggests the
possible occurrence of a non-magnetic ground state breaking rotational
symmetry. By means of Lanczos diagonalizations we investigate the spin system
at T=0, focusing our attention on the region where the collinear order
parameter is strongly suppressed by quantum fluctuations and a transition to a
non-magnetic state occurs. Correlation functions display a remarkable size
independence and allow to identify the transition between the magnetic and
non-magnetic region of the phase diagram. The numerical results support the
presence of a non-magnetic phase with orientational ordering.Comment: 6 pages, 4 figures, to be published in PR
Recent developments of the Hierarchical Reference Theory of Fluids and its relation to the Renormalization Group
The Hierarchical Reference Theory (HRT) of fluids is a general framework for
the description of phase transitions in microscopic models of classical and
quantum statistical physics. The foundations of HRT are briefly reviewed in a
self-consistent formulation which includes both the original sharp cut-off
procedure and the smooth cut-off implementation, which has been recently
investigated. The critical properties of HRT are summarized, together with the
behavior of the theory at first order phase transitions. However, the emphasis
of this presentation is on the close relationship between HRT and non
perturbative renormalization group methods, as well as on recent
generalizations of HRT to microscopic models of interest in soft matter and
quantum many body physics.Comment: 17 pages, 5 figures. Review paper to appear in Molecular Physic
Dynamical arrest and replica symmetry breaking in attractive colloids
Within the Replica Symmetry Breaking (RSB) framework developed by M.Mezard
and G.Parisi we investigate the occurrence of structural glass transitions in a
model of fluid characterized by hard sphere repulsion together with short range
attraction. This model is appropriate for the description of a class of
colloidal suspensions. The transition line in the density-temperature plane
displays a reentrant behavior, in agreement with Mode Coupling Theory (MCT), a
dynamical approach based on the Mori-Zwanzig formalism. Quantitative
differences are however found, together with the absence of the predicted
glass-glass transition at high density. We also perform a systematic study of
the pure hard sphere fluid in order to ascertain the accuracy of the adopted
method and the convergence of the numerical procedure.Comment: 7 pages, 6 figure
Phase transitions in simple and not so simple binary fluids
Compared to pure fluids, binary mixtures display a very diverse phase
behavior, which depends sensitively on the parameters of the microscopic
potential. Here we investigate the phase diagrams of simple model mixtures by
use of a microscopic implementation of the renormalization group technique.
First, we consider a symmetric mixture with attractive interactions, possibly
relevant for describing fluids of molecules with internal degrees of freedom.
Despite the simplicity of the model, slightly tuning the strength of the
interactions between unlike species drastically changes the topology of the
phase boundary, forcing or inhibiting demixing, and brings about several
interesting features such as double critical points, tricritical points, and
coexistence domains enclosing `islands' of homogeneous, mixed fluid.
Homogeneous phase separation in mixtures can be driven also by purely repulsive
interactions. As an example, we consider a model of soft particles which has
been adopted to describe binary polymer solutions. This is shown to display
demixing (fluid-fluid) transition at sufficiently high density. The nature and
the physical properties of the corresponding phase transition are investigated.Comment: 6 pages + 3 figures, presented at the 5th EPS Liquid Matter
Conference, Konstanz, 14-18 September 200
Smooth cutoff formulation of hierarchical reference theory for a scalar phi4 field theory
The phi4 scalar field theory in three dimensions, prototype for the study of
phase transitions, is investigated by means of the hierarchical reference
theory (HRT) in its smooth cutoff formulation. The critical behavior is
described by scaling laws and critical exponents which compare favorably with
the known values of the Ising universality class. The inverse susceptibility
vanishes identically inside the coexistence curve, providing a first principle
implementation of the Maxwell construction, and shows the expected
discontinuity across the phase boundary, at variance with the usual sharp
cutoff implementation of HRT. The correct description of first and second order
phase transitions within a microscopic, nonperturbative approach is thus
achieved in the smooth cutoff HRT.Comment: 8 pages, 4 figure
Heavily obscured AGN in the local Universe
We present here a new powerful diagnostic plot to select heavily obscured AGN
in the local universe by combining infrared (Spitzer, IRAS) and X-ray (XMM)
information. On the basis of this plot, we selected a sample of X-ray obscured
sources in the 2XMM catalogue and found seven newly discovered Compton-thick
AGN candidates.Comment: 2 pages, 2 figures, To appear in refereed Proceedings of "X-ray
Astronomy 2009: Present Status, Multi-Wavelength Approach and Future
Perspectives", Bologna, Italy, September 7-11, 2009, AIP, eds. A. Comastri,
M. Cappi, and L. Angelin
Exact Renormalization Group : A New Method for Blocking the Action
We consider the exact renormalization group for a non-canonical scalar field
theory in which the field is coupled to the external source in a special non
linear way. The Wilsonian action and the average effective action are then
simply related by a Legendre transformation up to a trivial quadratic form. An
exact mapping between canonical and non-canonical theories is obtained as well
as the relations between their flows. An application to the theory of liquids
is sketched
The Swift-BAT survey reveals the orbital period of three high-mass X-ray binaries
A growing number of previously hidden Galactic X-ray sources are now detected
with recent surveys performed by the Integral and Swift satellites. Most of
these new sources eluded past surveys due to their large local X-ray extinction
and consequent low soft X-ray flux. The Swift-BAT performs daily monitoring of
the sky in an energy band (15-150 keV) which is only marginally affected by
X-ray extinction, thus allowing for the search of long periodicities in the
light curve and identification of the nature of the X-ray sources. We performed
a period search using the folding technique in the Swift-BAT light curves of
three Integral sources: IGR J05007-7047, IGR J13186-6257 and IGR J17354-3255.
Their periodograms show significant peaks at 30.770.01 d, 19.9940.01
d and 8.4480.002 d, respectively. We estimate the significance of these
features from the chi squared distribution of all the trials, finding a
probability less than 1.5 that the detections occurred due to
chance. We complement our analysis with the study of their broadband X-ray
emission. We identify the periodicities with the orbital periods of the
sources. The periods are typical for the wind accretors X-ray binaries and we
support this identification showing that also their energy spectra are
compatible with an X-ray spectral emission characteristic of high-mass X-ray
binaries. The spectrum of IGR J05007-704 that resides in the Large Magellanic
Cloud, does not show any intrinsic local absorption, whereas the spectra of the
Galactic sources IGR J17354-3255 and IGR J13186-6257 may be affected by a local
absorber. The folded light curve for IGR J13186-6257 suggests a possible Be
companion star.Comment: 10 pages, 14 figures. Accepted for publication in A&
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