71 research outputs found
On the chiral and deconfinement phase transitions in parity-conserving QED_3 at finite temperature
We present some results about the interplay between the chiral and
deconfinement phase transitions in parity-conserving QED3 (with N flavours of
massless 4 component fermions) at finite temperature. Following Grignani et al
(Phys. Rev. D53, 7157 (1996), Nucl. Phys. B473, 143 (1996)), confinement is
discussed in terms of an effective Sine-Gordon theory for the timelike
component of the gauge field A_0. But whereas in the references above the
fermion mass m is a Lagrangian parameter, we consider the m=0 case and ask
whether an effective S-G theory can again be derived with m replaced by the
dynamically generated mass Sigma which appears below T_{ch}, the critical
temperature for the chiral phase transition. The fermion and gauge sectors are
strongly interdependent, but as a first approximation we decouple them by
taking Sigma to be a constant, depending only on the constant part of the gauge
field. We argue that the existence of a low-temperature confining phase may be
associated with the generation of Sigma; and that, analogously, the vanishing
of Sigma for T > T_{ch} drives the system to its deconfining phase. The effect
of the gauge field dynamics on mass generation is also indicated. (38kb)Comment: 1 reference adde
The Pioneer anomaly: the measure of a topological phase defect of light in cosmology
It is shown that a wave vector representing a light pulse in an adiabatically
evolving expanding space should develop, after a round trip (back and forth to
the emitter) a geometric phase for helicity states at a given fixed position
coordinate of this expanding space.In a section of the Hopf fibration of the
Poincare sphere that identifies a projection to the physically allowed states,
the evolution defines a parallel transported state that can be joined
continuously with the initial state by means of the associated
Berry-Pancharatnam connection. The connection allows to compute an anomaly in
the frequency for the vector modes in terms of the scale factor of the
space-time background being identical to the reported Pioneer Anomaly.Comment: 10 pages, some minor notation changes have been made. Some additional
remarks were writte
Longitudinal and transverse fermion-boson vertex in QED at finite temperature in the HTL approximation
We evaluate the fermion-photon vertex in QED at the one loop level in Hard
Thermal Loop approximation and write it in covariant form. The complete vertex
can be expanded in terms of 32 basis vectors. As is well known, the
fermion-photon vertex and the fermion propagator are related through a
Ward-Takahashi Identity (WTI). This relation splits the vertex into two parts:
longitudinal (Gamma_L) and transverse (Gamma_T). Gamma_L is fixed by the WTI.
The description of the longitudinal part consumes 8 of the basis vectors. The
remaining piece Gamma_T is then written in terms of 24 spin amplitudes.
Extending the work of Ball and Chiu and Kizilersu et. al., we propose a set of
basis vectors T^mu_i(P_1,P_2) at finite temperature such that each of these is
transverse to the photon four-momentum and also satisfies T^mu_i(P,P)=0, in
accordance with the Ward Identity, with their corresponding coefficients being
free of kinematic singularities. This basis reduces to the form proposed by
Kizilersu et. al. at zero temperature. We also evaluate explicitly the
coefficient of each of these vectors at the above-mentioned level of
approximation.Comment: 13 pages, uses RevTe
A UV completion of scalar electrodynamics
In previous works, we constructed UV-finite and unitary scalar field theories
with an infinite spectrum of propagating modes for arbitrary polynomial
interactions. In this paper, we introduce infinitely many massive vector fields
into a U(1) gauge theory to construct a theory with UV-finiteness and
unitarity.Comment: 25 page
Inverse Landau-Khalatnikov Transformation and Infrared Critical Exponents of (2+1)-dimensional Quantum Electrodynamics
By applying an inverse Landau-Khalatnikov transformation, connecting
(resummed) Schwinger-Dyson treatments in non-local and Landau gauges of
, we derive the infrared behaviour of the wave-function renormalization
in the Landau gauge, and the associated critical exponents in the normal phase
of the theory (no mass generation). The result agrees with the one conjectured
in earlier treatments. The analysis involves an approximation, namely an
expansion of the non-local gauge in powers of momenta in the infrared. This
approximation is tested by reproducing the critical number of flavours
necessary for dynamical mass generation in the chiral-symmetry-broken phase of
.Comment: 13 pages LATEX, 1 Figure (included automatically
Derivative expansion and gauge independence of the false vacuum decay rate in various gauges
In theories with radiative symmetry breaking, the calculation of the false
vacuum decay rate requires the inclusion of higher-order terms in the
derivative expansion of the effective action. I show here that, in the case of
covariant gauges, the presence of infrared singularities forbids the consistent
calculation by keeping the lowest-order terms. The situation is remedied,
however, in the case of gauges. Using the Nielsen identities I show
that the final result is gauge independent for generic values of the gauge
parameter that are not anomalously small.Comment: Some comments and references adde
Non-linear Dynamics in QED_3 and Non-trivial Infrared Structure
In this work we consider a coupled system of Schwinger-Dyson equations for
self-energy and vertex functions in QED_3. Using the concept of a
semi-amputated vertex function, we manage to decouple the vertex equation and
transform it in the infrared into a non-linear differential equation of
Emden-Fowler type. Its solution suggests the following picture: in the absence
of infrared cut-offs there is only a trivial infrared fixed-point structure in
the theory. However, the presence of masses, for either fermions or photons,
changes the situation drastically, leading to a mass-dependent non-trivial
infrared fixed point. In this picture a dynamical mass for the fermions is
found to be generated consistently. The non-linearity of the equations gives
rise to highly non-trivial constraints among the mass and effective (`running')
gauge coupling, which impose lower and upper bounds on the latter for dynamical
mass generation to occur. Possible implications of this to the theory of
high-temperature superconductivity are briefly discussed.Comment: 29 pages LATEX, 7 eps figures incorporated, uses axodraw style.
Discussion on the massless case (section 2) modified; no effect on
conclusions, typos correcte
The low-energy phase-only action in a superconductor: a comparison with the XY model
The derivation of the effective theory for the phase degrees of freedom in a
superconductor is still, to some extent, an open issue. It is commonly assumed
that the classical XY model and its quantum generalizations can be exploited as
effective phase-only models. In the quantum regime, however, this assumption
leads to spurious results, such as the violation of the Galilean invariance in
the continuum model. Starting from a general microscopic model, in this paper
we explicitly derive the effective low-energy theory for the phase, up to
fourth-order terms. This expansion allows us to properly take into account
dynamic effects beyond the Gaussian level, both in the continuum and in the
lattice model. After evaluating the one-loop correction to the superfluid
density we critically discuss the qualitative and quantitative differences
between the results obtained within the quantum XY model and within the correct
low-energy theory, both in the case of s-wave and d-wave symmetry of the
superconducting order parameter. Specifically, we find dynamic anharmonic
vertices, which are absent in the quantum XY model, and are crucial to restore
Galilean invariance in the continuum model. As far as the more realistic
lattice model is concerned, in the weak-to-intermediate-coupling regime we find
that the phase-fluctuation effects are quantitatively reduced with respect to
the XY model. On the other hand, in the strong-coupling regime we show that the
correspondence between the microscopically derived action and the quantum XY
model is recovered, except for the low-density regime.Comment: 29 pages, 11 figures. Slightly revised presentation, accepted for
publication in Phys. Rev.
Phase Structure of QED3 at Finite Temperature
Dynamical symmetry breaking in three-dimensional QED with N fermion flavours
is considered at finite temperature, in the large approximation. Using an
approximate treatment of the Schwinger-Dyson equation for the fermion
self-energy, we find that chiral symmetry is restored above a certain critical
temperature which depends itself on . We find that the ratio of the
zero-momentum zero-temperature fermion mass to the critical temperature has a
large value compared with four-fermion theories, as had been suggested in a
previous work with a momentum-independent self-energy. Evidence of a
temperature- dependent critical is shown to appear in this approximation.
The phase diagram for spontaneous mass generation in the theory is presented in
space.Comment: 9 page
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