40 research outputs found
The index theorem in QCD with a finite cut-off
The fixed point Dirac operator on the lattice has exact chiral zero modes on
topologically non-trivial gauge field configurations independently whether
these configurations are smooth, or coarse. The relation ,
where is the number of left (right)-handed zero modes and
is the fixed point topological charge holds not only in the continuum
limit, but also at finite cut-off values. The fixed point action, which is
determined by classical equations, is local, has no doublers and complies with
the no-go theorems by being chirally non-symmetric. The index theorem is
reproduced exactly, nevertheless. In addition, the fixed point Dirac operator
has no small real eigenvalues except those at zero, i.e. there are no
'exceptional configurations'.Comment: 9 pages, 1 figure. Minor clarifying changes are made and new
references adde
Does a relativistic metric generalization of Newtonian gravity exist in 2+1 dimensions?
It is shown that, contrary to previous claims, a scalar tensor theory of
Brans-Dicke type provides a relativistic generalization of Newtonian gravity in
2+1 dimensions. The theory is metric and test particles follow the space-time
geodesics. The static isotropic solution is studied in vacuum and in regions
filled with an incompressible perfect fluid. It is shown that the solutions can
be consistently matched at the matter vacuum interface, and that the Newtonian
behavior is recovered in the weak field regime.Comment: 6 pages, no figures, Revtex4. Some discussions on the physical nature
of the interior solution and on the omega->infinity limit and some references
added. Version to appear in Phys. Rev.
Statistical mechanics and phase diagrams of rotating self-gravitating fermions
We compute statistical equilibrium states of rotating self-gravitating
systems enclosed within a box by maximizing the Fermi-Dirac entropy at fixed
mass, energy and angular momentum. We increase the rotation up to the Keplerian
limit and describe the flattening of the configuration until mass shedding
occurs. At the maximum rotation, the system develops a cusp at the equator. We
draw the equilibrium phase diagram of the rotating self-gravitating Fermi gas
and discuss the structure of the caloric curve as a function of degeneracy
parameter and angular velocity. We argue that systems described by the
Fermi-Dirac distribution in phase space do not bifurcate to non-axisymmetric
structures, in continuity with the case of polytropes with index n>0.808 (the
Fermi gas at T=0 corresponds to n=3/2). This contrasts with the study of
Votyakov et al. (2002) who consider a Fermi-Dirac distribution in configuration
space and find ``double star'' structures (their model at T=0 corresponds to
n=0). We also discuss the influence of rotation on the onset of the
gravothermal catastrophe for globular clusters. On general grounds, we complete
previous investigations concerning the nature of phase transitions in
self-gravitating systems. We emphasize the inequivalence of statistical
ensembles regarding the formation of binaries (or low-mass condensates) in the
microcanonical ensemble and Dirac peaks (or massive condensates) in the
canonical ensemble. We also describe an hysteretic cycle between the gaseous
phase and the condensed phase that are connected by a ``collapse'' or an
``explosion''. This notion of hysteresis in self-gravitating systems is new.Comment: submitted to A&
Gravitational instability of slowly rotating isothermal spheres
We discuss the statistical mechanics of rotating self-gravitating systems by
allowing properly for the conservation of angular momentum. We study
analytically the case of slowly rotating isothermal spheres by expanding the
solutions of the Boltzmann-Poisson equation in a series of Legendre
polynomials, adapting the procedure introduced by Chandrasekhar (1933) for
distorted polytropes. We show how the classical spiral of Lynden-Bell & Wood
(1967) in the temperature-energy plane is deformed by rotation. We find that
gravitational instability occurs sooner in the microcanonical ensemble and
later in the canonical ensemble. According to standard turning point arguments,
the onset of the collapse coincides with the minimum energy or minimum
temperature state in the series of equilibria. Interestingly, it happens to be
close to the point of maximum flattening. We determine analytically the
generalization of the singular isothermal solution to the case of a slowly
rotating configuration. We also consider slowly rotating configurations of the
self-gravitating Fermi gas at non zero temperature.Comment: Submitted to A&
Microcanonical mean-field thermodynamics of self-gravitating and rotating systems
We derive the global phase diagram of a self-gravitating -body system
enclosed in a finite three-dimensional spherical volume as a function of
total energy and angular momentum, employing a microcanonical mean-field
approach. At low angular momenta (i.e. for slowly rotating systems) the known
collapse from a gas cloud to a single dense cluster is recovered. At high
angular momenta, instead, rotational symmetry can be spontaneously broken and
rotationally asymmetric structures (double clusters) appear.Comment: 4 pages, 4 figures; to appear in Phys. Rev. Let
Properties of the Fixed Point Lattice Dirac Operator in the Schwinger Model
We present a numerical study of the properties of the Fixed Point lattice
Dirac operator in the Schwinger model. We verify the theoretical bounds on the
spectrum, the existence of exact zero modes with definite chirality, and the
Index Theorem. We show by explicit computation that it is possible to find an
accurate approximation to the Fixed Point Dirac operator containing only very
local couplings.Comment: 38 pages, LaTeX, 3 figures, uses style [epsfig], a few comments and
relevant references adde
The interplay between double exchange, super-exchange, and Lifshitz localization in doped manganites
Considering the disorder caused in manganites by the substitution of Mn by Fe
or Ga, we accomplish a systematic study of doped manganites begun in previous
papers. To this end, a disordered model is formulated and solved using the
Variational Mean Field technique. The subtle interplay between double exchange,
super-exchange, and disorder causes similar effects on the dependence of T_C on
the percentage of Mn substitution in the cases considered. Yet, in
LaCaMnGaO our results suggest a quantum
critical point (QCP) for , associated to the localization of
the electronic states of the conduction band. In the case of
LaCaMnFeO (with ) no such QCP is expected.Comment: 6 pages + 3 postscript figures. Largely extended discussio
Effect of angular momentum on equilibrium properties of a self-gravitating system
The microcanonical properties of a two dimensional system of N classical
particles interacting via a smoothed Newtonian potential as a function of the
total energy E and the total angular momentum L are discussed. In order to
estimate suitable observables a numerical method based on an importance
sampling algorithm is presented. The entropy surface shows a negative specific
heat region at fixed L for all L. Observables probing the average mass
distribution are used to understand the link between thermostatistical
properties and the spatial distribution of particles. In order to define a
phase in non-extensive system we introduce a more general observable than the
one proposed by Gross and Votyakov [Eur. Phys. J. B:15, 115 (2000)]: the sign
of the largest eigenvalue of the entropy surface curvature. At large E the
gravitational system is in a homogeneous gas phase. At low E there are several
collapse phases; at L=0 there is a single cluster phase and for L>0 there are
several phases with 2 clusters. All these pure phases are separated by first
order phase transition regions. The signal of critical behaviour emerges at
different points of the parameter space (E,L). We also discuss the ensemble
introduced in a recent pre-print by Klinko & Miller; this ensemble is the
canonical analogue of the one at constant energy and constant angular momentum.
We show that a huge loss of informations appears if we treat the system as a
function of intensive parameters: besides the known non-equivalence at first
order phase transitions, there exit in the microcanonical ensemble some values
of the temperature and the angular velocity for which the corresponding
canonical ensemble does not exist, i.e. the partition sum diverges.Comment: 17 pages, 11 figures, submitted to Phys. Rev.
Diquark condensation at strong coupling
The possibility of diquark condensation at sufficiently large baryon chemical
potential and zero temperature is analyzed in QCD at strong coupling. In
agreement with other strong coupling analysis, it is found that a first order
phase transition separates a low density phase with chiral symmetry
spontaneously broken from a high density phase where chiral symmetry is
restored. In none of the phases diquark condensation takes place as an
equilibrium state, but, for any value of the chemical potential, there is a
metastable state characterized by a non-vanishing diquark condensate. The
energy difference between this metastable state and the equilibrium state
decreases with the chemical potential and is minimum in the high density phase.
The results indicate that there is attraction in the quark-quark sector also at
strong coupling, and that the attraction is more effective at high baryon
density, but for infinite coupling it is not enough to produce diquark
condensation. It is argued that the absence of diquark condensation is not a
peculiarity of the strong coupling limit, but persists at sufficiently large
finite couplings.Comment: 10 pages, 2 figures. An important discussion concerning the extension
of the results to finite couplings adde
Variational Mean Field approach to the Double Exchange Model
It has been recently shown that the double exchange Hamiltonian, with weak
antiferromagnetic interactions, has a richer variety of first and second order
transitions than previously anticipated, and that such transitions are
consistent with the magnetic properties of manganites. Here we present a
thorough discussion of the variational Mean Field approach that leads to the
these results. We also show that the effect of the Berry phase turns out to be
crucial to produce first order Paramagnetic-Ferromagnetic transitions near half
filling with transition temperatures compatible with the experimental
situation. The computation relies on two crucial facts: the use of a Mean Field
ansatz that retains the complexity of a system of electrons with off-diagonal
disorder, not fully taken into account by the Mean Field techniques, and the
small but significant antiferromagnetic superexchange interaction between the
localized spins.Comment: 13 pages, 11 postscript figures, revte