140 research outputs found
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
Abelian versus Non-Abelian Higgs Model in Three Dimensions
We study the phase structure of the abelian Higgs model in three dimensions
based on perturbation theory and a set of gauge independent gap equations for
Higgs boson and vector boson masses. Contrary to the non-abelian Higgs model,
the vector boson mass vanishes in the symmetric phase. In the Higgs phase the
gap equations yield masses consistent with perturbation theory. The phase
transition is first-order for small values of the scalar self-coupling
, where the employed loop expansion is applicable.Comment: 8 pages Latex, 4 figures in separate uu-fil
Magnetic Screening in the High Temperature Phase of the Standard Model
Non-perturbative effects in the high-temperature phase of the electroweak
theory are characterized by a magnetic screening length. Its size influences
the range of validity of perturbation theory, and it also determines the
critical Higgs boson mass where the first-order phase transition changes to a
crossover. We propose a gauge-invariant definition of the magnetic screening
length and discuss its role in several gauge-dependent and gauge-invariant
correlation functions.Comment: 11 pages Latex, 3 figures, uses epsf.st
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 and . We recover all known perturbative results in a single
framework and find the transition to be weakly first order in . 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
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
Critical Phenomena with Linked Cluster Expansions in a Finite Volume
Linked cluster expansions are generalized from an infinite to a finite
volume. They are performed to 20th order in the expansion parameter to approach
the critical region from the symmetric phase. A new criterion is proposed to
distinguish 1st from 2nd order transitions within a finite size scaling
analysis. The criterion applies also to other methods for investigating the
phase structure such as Monte Carlo simulations. Our computational tools are
illustrated at the example of scalar O(N) models with four and six-point
couplings for and in three dimensions. It is shown how to localize
the tricritical line in these models. We indicate some further applications of
our methods to the electroweak transition as well as to models for
superconductivity.Comment: 36 pages, latex2e, 7 eps figures included, uuencoded, gzipped and
tarred tex file hdth9607.te
Masses and Phase Structure in the Ginzburg-Landau Model
We study numerically the phase structure of the Ginzburg-Landau model, with
particular emphasis on mass measurements. There is no local gauge invariant
order parameter, but we find that there is a phase transition characterized by
a vanishing photon mass. For type I superconductors the transition is of 1st
order. For type II 1st order is excluded by susceptibility analysis, but the
photon correlation length suggests 2nd order critical behaviour with \nu ~ 1/2.
The scalar mass, in contrast, does not show clear critical behaviour in the
type II regime for V \to \infty, contrary to the conventional picture.Comment: 16 pages, 6 figures. More data gathered, allowing more definite
conclusion
Spatial distribution of photoelectrons participating in formation of x-ray absorption spectra
Interpretation of x-ray absorption near-edge structure (XANES) experiments is
often done via analyzing the role of particular atoms in the formation of
specific peaks in the calculated spectrum. Typically, this is achieved by
calculating the spectrum for a series of trial structures where various atoms
are moved and/or removed. A more quantitative approach is presented here, based
on comparing the probabilities that a XANES photoelectron of a given energy can
be found near particular atoms. Such a photoelectron probability density can be
consistently defined as a sum over squares of wave functions which describe
participating photoelectron diffraction processes, weighted by their normalized
cross sections. A fine structure in the energy dependence of these
probabilities can be extracted and compared to XANES spectrum. As an
illustration of this novel technique, we analyze the photoelectron probability
density at the Ti K pre-edge of TiS2 and at the Ti K-edge of rutile TiO2.Comment: Journal abstract available on-line at
http://link.aps.org/abstract/PRB/v65/e20511
QED_3 theory of underdoped high temperature superconductors II: the quantum critical point
We study the effect of gapless quasiparticles in a d-wave superconductor on
the T=0 end point of the Kosterlitz-Thouless transition line in underdoped
high-temperature superconductors. Starting from a lattice model that has
gapless fermions coupled to 3D XY phase fluctuations of the superconducting
order parameter, we propose a continuum field theory to describe the quantum
phase transition between the d-wave superconductor and the spin-density-wave
insulator. Without fermions the theory reduces to the standard Higgs scalar
electrodynamics (HSE), which is known to have the critical point in the
inverted XY universality class. Extending the renormalization group calculation
for the HSE to include the coupling to fermions, we find that the qualitative
effect of fermions is to increase the portion of the space of coupling
constants where the transition is discontinuous. The critical exponents at the
stable fixed point vary continuously with the number of fermion fields , and
we estimate the correlation length exponent (nu = 0.65) and the vortex field
anomalous dimension(eta_Phi=-0.48) at the quantum critical point for the
physical case N=2. The stable critical point in the theory disappears for the
number of Dirac fermions N > N_c, with N_c ~ 3.4 in our approximation. We
discuss the relationship between the superconducting and the chiral (SDW)
transitions, and point to some interesting parallels between our theory and the
Thirring model.Comment: 13 pages including figures in tex
Phase Structure and Phase Transition of the SU(2) Higgs Model in Three Dimensions
We derive a set of gauge independent gap equations for Higgs boson and vector
boson masses for the SU(2) Higgs model in three dimensions. The solutions can
be associated with the Higgs phase and the symmetric phase, respectively. In
the Higgs phase the calculated masses are in agreement with results from
perturbation theory. In the symmetric phase a non-perturbative vector boson
mass is generated by the non-abelian gauge interactions, whose value is rather
independent of the scalar self-coupling . For small values of
the phase transition is first-order. Its strength decreases with
increasing , and at a critical value the first-order
transition changes to a crossover. Based on a perturbative matching the
three-dimensional theory is related to the four-dimensional theory at high
temperatures. The critical Higgs mass , corresponding to the critical
coupling , is estimated to be below 100 GeV. The ``symmetric phase''
of the theory can be interpreted as a Higgs phase whose parameters are
determined non-perturbatively. The obtained Higgs boson and vector boson masses
are compared with recent results from lattice Monte Carlo simulations.Comment: 22 pages Latex, 9 figures as uuencoded EPS files. Figure 9 replaced,
no changes in the tex
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