134 research outputs found

### PHASE TRANSITION OF N-COMPONENT SUPERCONDUCTORS

We investigate the phase transition in the three-dimensional abelian Higgs
model for N complex scalar fields, using the gauge-invariant average action
\Gamma_{k}. The dependence of \Gamma_{k} on the effective infra-red cut-off k
is described by a non-perturbative flow equation. The transition turns out to
be first- or second-order, depending on the ratio between scalar and gauge
coupling. We look at the fixed points of the theory for various N and compute
the critical exponents of the model. Comparison with results from the
\epsilon-expansion shows a rather poor convergence for \epsilon=1 even for
large N. This is in contrast to the surprisingly good results of the
\epsilon-expansion for pure scalar theories. Our results suggest the existence
of a parameter range with a second-order transition for all N, including the
case of the superconductor phase transition for N=1.Comment: 30p. with 9 uuencoded .eps-figures appended, LaTe

### The Thermal Renormalization Group for Fermions, Universality, and the Chiral Phase-Transition

We formulate the thermal renormalization group, an implementation of the
Wilsonian RG in the real-time (CTP) formulation of finite temperature field
theory, for fermionic fields. Using a model with scalar and fermionic degrees
of freedom which should describe the two-flavor chiral phase-transition, we
discuss the mechanism behind fermion decoupling and universality at second
order transitions. It turns out that an effective mass-like term in the fermion
propagator which is due to thermal fluctuations and does not break chiral
symmetry is necessary for fermion decoupling to work. This situation is in
contrast to the high-temperature limit, where the dominance of scalar over
fermionic degrees of freedom is due to the different behavior of the
distribution functions. The mass-like contribution is the leading thermal
effect in the fermionic sector and is missed if a derivative expansion of the
fermionic propagator is performed. We also discuss results on the
phase-transition of the model considered where we find good agreement with
results from other methods.Comment: References added, minor typos correcte

### Heisenberg frustrated magnets: a nonperturbative approach

Frustrated magnets are a notorious example where the usual perturbative
methods are in conflict. Using a nonperturbative Wilson-like approach, we get a
coherent picture of the physics of Heisenberg frustrated magnets everywhere
between $d=2$ and $d=4$. We recover all known perturbative results in a single
framework and find the transition to be weakly first order in $d=3$. We compute
effective exponents in good agreement with numerical and experimental data.Comment: 5 pages, Revtex, technical details available at
http://www.lpthe.jussieu.fr/~tissie

### Renormalization group flows for gauge theories in axial gauges

Gauge theories in axial gauges are studied using Exact Renormalisation Group flows. We introduce a background field in the infrared regulator, but not in the gauge fixing, in contrast to the usual background field gauge. It is shown how heat-kernel methods can be used to obtain approximate solutions to the flow and the corresponding Ward identities. Expansion schemes are discussed, which are not applicable in covariant gauges. As an application, we derive the one-loop effective action for covariantly constant field strength, and the one-loop beta-function for arbitrary regulator

### Effective average action in statistical physics and quantum field theory

An exact renormalization group equation describes the dependence of the free
energy on an infrared cutoff for the quantum or thermal fluctuations. It
interpolates between the microphysical laws and the complex macroscopic
phenomena. We present a simple unified description of critical phenomena for
O(N)-symmetric scalar models in two, three or four dimensions, including
essential scaling for the Kosterlitz-Thouless transition.Comment: 34 pages,5 figures,LaTe

### Critical Behavior of the Meissner Transition in the Lattice London Superconductor

We carry out Monte Carlo simulations of the three dimensional (3D) lattice
London superconductor in zero applied magnetic field, making a detailed finite
size scaling analysis of the Meissner transition. We find that the magnetic
penetration length \lambda, and the correlation length \xi, scale as \lambda ~
\xi ~ |t|^{-\nu}, with \nu = 0.66 \pm 0.03, consistent with ordinary 3D XY
universality, \nu_XY ~ 2/3. Our results confirm the anomalous scaling dimension
of magnetic field correlations at T_c.Comment: 4 pages, 5 ps figure

### A non perturbative approach of the principal chiral model between two and four dimensions

We investigate the principal chiral model between two and four dimensions by
means of a non perturbative Wilson-like renormalization group equation. We are
thus able to follow the evolution of the effective coupling constants within
this whole range of dimensions without having recourse to any kind of small
parameter expansion. This allows us to identify its three dimensional critical
physics and to solve the long-standing discrepancy between the different
perturbative approaches that characterizes the class of models to which the
principal chiral model belongs.Comment: 5 pages, 1 figure, Revte

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