1,653 research outputs found
Renormalized thermodynamics from the 2PI effective action
High-temperature resummed perturbation theory is plagued by poor convergence
properties. The problem appears for theories with bosonic field content such as
QCD, QED or scalar theories. We calculate the pressure as well as other
thermodynamic quantities at high temperature for a scalar one-component field
theory, solving a three-loop 2PI effective action numerically without further
approximations. We present a detailed comparison with the two-loop
approximation. One observes a strongly improved convergence behavior as
compared to perturbative approaches. The renormalization employed in this work
extends previous prescriptions, and is sufficient to determine all counterterms
required for the theory in the symmetric as well as the spontaneously broken
phase.Comment: 20 pages, 7 figures; PRD version, references added, very minor
change
Critical phenomena from the two-particle irreducible 1/N expansion
The 1/N expansion of the two-particle irreducible (2PI) effective action is
employed to compute universal properties at the second-order phase transition
of an O(N)-symmetric N-vector model directly in three dimensions. At
next-to-leading order the approach cures the spurious small-N divergence of the
standard (1PI) 1/N expansion for a computation of the critical anomalous
dimension eta(N), and leads to improved estimates already for moderate values
of N.Comment: 18 pages, 3 figure
Dynamic universality class of Model C from the functional renormalization group
We establish new scaling properties for the universality class of Model C,
which describes relaxational critical dynamics of a nonconserved order
parameter coupled to a conserved scalar density. We find an anomalous diffusion
phase, which satisfies weak dynamic scaling while the conserved density
diffuses only asymptotically. The properties of the phase diagram for the
dynamic critical behavior include a significantly extended weak scaling region,
together with a strong and a decoupled scaling regime. These calculations are
done directly in 2 < d < 4 space dimensions within the framework of the
nonperturbative functional renormalization group. The scaling exponents
characterizing the different phases are determined along with subleading
indices featuring the stability properties.Comment: 5 pages, 3 figures; PRB version, minor change
Critical Phenomena in Continuous Dimension
We present a calculation of critical phenomena directly in continuous
dimension d employing an exact renormalization group equation for the effective
average action. For an Ising-type scalar field theory we calculate the critical
exponents nu(d) and eta(d) both from a lowest--order and a complete
first--order derivative expansion of the effective average action. In
particular, this can be used to study critical behavior as a function of
dimensionality at fixed temperature.Comment: 5 pages, 1 figure, PLB version, references adde
Turning a First Order Quantum Phase Transition Continuous by Fluctuations: General Flow Equations and Application to d-Wave Pomeranchuk Instability
We derive renormalization group equations which allow us to treat order
parameter fluctuations near quantum phase transitions in cases where an
expansion in powers of the order parameter is not possible. As a prototypical
application, we analyze the nematic transition driven by a d-wave Pomeranchuk
instability in a two-dimensional electron system. We find that order parameter
fluctuations suppress the first order character of the nematic transition
obtained at low temperatures in mean-field theory, so that a continuous
transition leading to quantum criticality can emerge
Linking the Quark Meson Model with QCD at High Temperature
We model the transition of a system of quarks and gluons at high energies to
a system of quarks and mesons at low energies in a consistent renormalization
group approach. Flow equations interpolate between the physics of the
high-temperature degrees of freedom and the low-temperature dynamics at a scale
of 1 GeV. We also discuss the dependence of the equation of state on baryon
density and compare our results with recent lattice gauge simulations.Comment: 11 pages, 4 figures additional discussion of the second order phase
transitio
Incommensurate antiferromagnetic fluctuations in the two-dimensional Hubbard model
Commensurate and incommensurate antiferromagnetic fluctuations in the
two-dimensional repulsive t-t'-Hubbard model are investigated using functional
renormalization group equations. For a sufficient deviation from half filling
we establish the existence of local incommensurate order below a pseudocritical
temperature T_{pc}. Fluctuations not accounted for in the mean field
approximation are important--they lower T_{pc} by a factor \approx2.5.Comment: 7 pages, 8 figures, some changes due to referees' comments,
equivalent to published versio
Transport coefficients from the 2PI effective action
We show that the lowest nontrivial truncation of the two-particle irreducible
(2PI) effective action correctly determines transport coefficients in a weak
coupling or 1/N expansion at leading (logarithmic) order in several
relativistic field theories. In particular, we consider a single real scalar
field with cubic and quartic interactions in the loop expansion, the O(N) model
in the 2PI-1/N expansion, and QED with a single and many fermion fields.
Therefore, these truncations will provide a correct description, to leading
(logarithmic) order, of the long time behavior of these systems, i.e. the
approach to equilibrium. This supports the promising results obtained for the
dynamics of quantum fields out of equilibrium using 2PI effective action
techniques.Comment: 5 pages, explanation in introduction expanded, summary added; to
appear in PR
Comparison of Boltzmann Equations with Quantum Dynamics for Scalar Fields
Boltzmann equations are often used to study the thermal evolution of particle
reaction networks. Prominent examples are the computation of the baryon
asymmetry of the universe and the evolution of the quark-gluon plasma after
relativistic heavy ion collisions. However, Boltzmann equations are only a
classical approximation of the quantum thermalization process which is
described by the so-called Kadanoff-Baym equations. This raises the question
how reliable Boltzmann equations are as approximations to the full
Kadanoff-Baym equations. Therefore, we present in this paper a detailed
comparison between the Kadanoff-Baym and Boltzmann equations in the framework
of a scalar Phi^4 quantum field theory in 3+1 space-time dimensions. The
obtained numerical solutions reveal significant discrepancies in the results
predicted by both types of equations. Apart from quantitative discrepancies, on
a qualitative level the universality respected by the Kadanoff-Baym equations
is severely restricted in the case of Boltzmann equations. Furthermore, the
Kadanoff-Baym equations strongly separate the time scales between kinetic and
chemical equilibration. This separation of time scales is absent for the
Boltzmann equation.Comment: text and figures revised, references added, results unchanged, 21
pages, 10 figures, published in Phys. Rev. D73 (2006) 12500
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