176 research outputs found
Il restauro della chiesa di Ognissanti: il barocco a Mantova. Storia e arte. Note sul restauro
Storia della chiesa di Ognissanti in Mantova alla luce dei documenti e dei nuovi restaur
Topology Induced Spatial Bose-Einstein Condensation for Bosons on Star-Shaped Optical Networks
New coherent states may be induced by pertinently engineering the topology of
a network. As an example, we consider the properties of non-interacting bosons
on a star network, which may be realized with a dilute atomic gas in a
star-shaped deep optical lattice. The ground state is localized around the star
center and it is macroscopically occupied below the Bose-Einstein condensation
temperature T_c. We show that T_c depends only on the number of the star arms
and on the Josephson energy of the bosonic Josephson junctions and that the
non-condensate fraction is simply given by the reduced temperature T/T_c.Comment: 20 Pages, 5 Figure
Bose-Einstein Condensation on inhomogeneous complex networks
The thermodynamic properties of non interacting bosons on a complex network
can be strongly affected by topological inhomogeneities. The latter give rise
to anomalies in the density of states that can induce Bose-Einstein
condensation in low dimensional systems also in absence of external confining
potentials. The anomalies consist in energy regions composed of an infinite
number of states with vanishing weight in the thermodynamic limit. We present a
rigorous result providing the general conditions for the occurrence of
Bose-Einstein condensation on complex networks in presence of anomalous
spectral regions in the density of states. We present results on spectral
properties for a wide class of graphs where the theorem applies. We study in
detail an explicit geometrical realization, the comb lattice, which embodies
all the relevant features of this effect and which can be experimentally
implemented as an array of Josephson Junctions.Comment: 11 pages, 9 figure
Quantum States of Topologically Massive Electrodynamics and Gravity
The free quantum states of topologically massive electrodynamics and gravity
in 2+1 dimensions, are explicitly found. It is shown that in both theories the
states are described by infrared-regular polarization tensors containing a
regularization phase which depends on the spin. This is done by explicitly
realizing the quantum algebra on a functional Hilbert space and by finding the
Wightman function to define the scalar product on such a Hilbert space. The
physical properties of the states are analyzed defining creation and
annihilation operators.
For both theories, a canonical and covariant quantization procedure is
developed. The higher order derivatives in the gravitational lagrangian are
treated by means of a preliminary Dirac procedure.
The closure of the Poincar\'e algebra is guaranteed by the
infrared-finiteness of the states which is related to the spin of the
excitations through the regularization phase. Such a phase may have interesting
physical consequences.Comment: 21 page, latex, no figure
Chiral Dynamics and Fermion Mass Generation in Three Dimensional Gauge Theory
We examine the possibility of fermion mass generation in 2+1- dimensional
gauge theory from the current algebra point of view.In our approach the
critical behavior is governed by the fluctuations of pions which are the
Goldstone bosons for chiral symmetry breaking. Our analysis supports the
existence of an upper critical number of Fermion flavors and exhibits the
explicit form of the gap equation as well as the form of the critical exponent
for the inverse correlation lenght of the order parameterComment: Latex,10 pages,DFUPG 70/9
Dynamical delocalization of Majorana edge states by sweeping across a quantum critical point
We study the adiabatic dynamics of Majorana fermions across a quantum phase
transition. We show that the Kibble-Zurek scaling, which describes the density
of bulk defects produced during the critical point crossing, is not valid for
edge Majorana fermions. Therefore, the dynamics governing an edge state quench
is nonuniversal and depends on the topological features of the system. Besides,
we show that the localization of Majorana fermions is a necessary ingredient to
guaranty robustness against defect production.Comment: Submitted to the Special Issue on "Dynamics and Thermalization in
Isolated Quantum Many-Body Systems" in New Journal of Physics. Editors:M.
Cazalilla, M. Rigol. New references and some typos correcte
Confinement-Deconfinement Transition in 3-Dimensional QED
We argue that, at finite temperature, parity invariant non-compact
electrodynamics with massive electrons in 2+1 dimensions can exist in both
confined and deconfined phases. We show that an order parameter for the
confinement-deconfinement phase transition is the Polyakov loop operator whose
average measures the free energy of a test charge that is not an integral
multiple of the electron charge. The effective field theory for the Polyakov
loop operator is a 2-dimensional Euclidean scalar field theory with a global
discrete symmetry , the additive group of the integers. We argue that the
realization of this symmetry governs confinement and that the
confinement-deconfinement phase transition is of
Berezinskii-Kosterlitz-Thouless type. We compute the effective action to
one-loop order and argue that when the electron mass is much greater than
the temperature and dimensional coupling , the effective field theory
is the Sine-Gordon model. In this limit, we estimate the critical temperature,
.Comment: 11 pages, latex, no figure
Oblique Confinement and Phase Transitions in Chern-Simons Gauge Theories
We investigate non-perturbative features of a planar Chern-Simons gauge
theory modeling the long distance physics of quantum Hall systems, including a
finite gap M for excitations. By formulating the model on a lattice, we
identify the relevant topological configurations and their interactions. For M
bigger than a critical value, the model exhibits an oblique confinement phase,
which we identify with Lauglin's incompressible quantum fluid. For M smaller
than the critical value, we obtain a phase transition to a Coulomb phase or a
confinement phase, depending on the value of the electromagnetic coupling.Comment: 8 pages, harvmac, DFUPG 91/94 and MPI-PhT/94-9
Chiral Symmetry Breaking on the Lattice: a Study of the Strongly Coupled Lattice Schwinger Model
We revisit the strong coupling limit of the Schwinger model on the lattice
using staggered fermions and the hamiltonian approach to lattice gauge
theories. Although staggered fermions have no continuous chiral symmetry, they
posses a discrete axial invari ance which forbids fermion mass and which must
be broken in order for the lattice Schwinger model to exhibit the features of
the spectrum of the continuum theory. We show that this discrete symmetry is
indeed broken spontaneously in the strong coupling li mit. Expanding around a
gauge invariant ground state and carefully considering the normal ordering of
the charge operator, we derive an improved strong coupling expansion and
compute the masses of the low lying bosonic excitations as well as the chiral
co ndensate of the model. We find very good agreement between our lattice
calculations and known continuum values for these quantities already in the
fourth order of strong coupling perturbation theory. We also find the exact
ground state of the antiferromag netic Ising spin chain with long range Coulomb
interaction, which determines the nature of the ground state in the strong
coupling limit.Comment: 24 pages, Latex, no figure
Politics of nanotechnologies in food and agriculture
The chapter discusses the reasons for the delay in the regulatory intervention
concerning nanotechnologies used in the agriculture and food sectors. The
main finding is that unregulated introduction of nanoinnovation into the food system
is due to the current neoliberal food policy and to the power struggles that
characterize the economic, social and political dynamics within the global supply
chain. Therefore, it is necessary to put the âquestion concerning technologyâ at the
center of the regulatory debate in order to implement a regulatory system able to
face nanorisks. Which means looking at the way in which technology controls
power relationships within society. Attention should be shifted from efficiency to
power issues, and new technologies should be assessed from a political rather than
an economic or ethical perspective
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