3,472 research outputs found
Broken phase effective potential in the two-loop Phi-derivable approximation and nature of the phase transition in a scalar theory
We study the phase transition of a real scalar phi^4 theory in the two-loop
Phi-derivable approximation using the imaginary time formalism, extending our
previous (analytical) discussion of the Hartree approximation. We combine Fast
Fourier Transform algorithms and accelerated Matsubara sums in order to achieve
a high accuracy. Our results confirm and complete earlier ones obtained in the
real time formalism [1] but which were less accurate due to the integration in
Minkowski space and the discretization of the spectral density function. We
also provide a complete and explicit discussion of the renormalization of the
two-loop Phi-derivable approximation at finite temperature, both in the
symmetric and in the broken phase, which was already used in the real-time
approach, but never published. Our main result is that the two-loop
Phi-derivable approximation suffices to cure the problem of the Hartree
approximation regarding the order of the transition: the transition is of the
second order type, as expected on general grounds. The corresponding critical
exponents are, however, of the mean-field type. Using a "RG-improved" version
of the approximation, motivated by our renormalization procedure, we find that
the exponents are modified. In particular, the exponent delta, which relates
the field expectation value phi to an external field h, changes from 3 to 5,
getting then closer to its expected value 4.789, obtained from accurate
numerical estimates [2].Comment: 54 pages, 16 figure
Evaluasi Buku Teks Bahasa Inggris “Stairway: A Fun and Easy English Book” Grade VI Elementary School
Choosing a suitable coursebook is not an easy task since coursebook is a key component in most language program. It serves as language input and language practice for learners. It provides the basis for content lesson, the skills taught and the variety of language practice for learner, it also serves as a supplement for teacher’s instruction. The variety of coursebook are available to use by teacher for their classroom. However, teacher need to know the strength and the weakness of the coursebook so that they can choose the best coursebook for their classroom. This study aimed to evaluate coursebook entitled Stairway: A Fun and Easy English Book for sixth grader of Elementary school. The evaluation using the list of criteria of coursebook from Cunningsworth. This study is qualitative descriptive since this study aimed to give the details information about a phenomenon in this case is the coursebook. The result of this study showed that the coursebook entitled Stairway: A Fun and Easy English Book is suitable to use with some adaptation. The result obtained from the suitability from the book with the criteria given. From 44 items of criteria, the book has suitability on 28 criteria or 63% suitability. Accordingly, the book can be used with some adaptation to minimize the weakness in the book. Keywords: ELT Coursebook, Evaluation, Suitability
Gravitational waves from self-ordering scalar fields
Gravitational waves were copiously produced in the early Universe whenever
the processes taking place were sufficiently violent. The spectra of several of
these gravitational wave backgrounds on subhorizon scales have been extensively
studied in the literature. In this paper we analyze the shape and amplitude of
the gravitational wave spectrum on scales which are superhorizon at the time of
production. Such gravitational waves are expected from the self ordering of
randomly oriented scalar fields which can be present during a thermal phase
transition or during preheating after hybrid inflation. We find that, if the
gravitational wave source acts only during a small fraction of the Hubble time,
the gravitational wave spectrum at frequencies lower than the expansion rate at
the time of production behaves as with an
amplitude much too small to be observable by gravitational wave observatories
like LIGO, LISA or BBO. On the other hand, if the source is active for a much
longer time, until a given mode which is initially superhorizon (), enters the horizon, for , we find that the gravitational
wave energy density is frequency independent, i.e. scale invariant. Moreover,
its amplitude for a GUT scale scenario turns out to be within the range and
sensitivity of BBO and marginally detectable by LIGO and LISA. This new
gravitational wave background can compete with the one generated during
inflation, and distinguishing both may require extra information.Comment: 21 pages, 2 figures, added discussion about numerical integration and
a new figure to illustrate the scale-invariance of the GW power spectrum,
conclusions unchange
Variational approximations for correlation functions in quantum field theories
Applying the time-dependent variational principle of Balian and V\'en\'eroni,
we derive variational approximations for multi-time correlation functions in
field theory. We assume first that the initial state is given and
characterized by a density operator equal to a Gaussian density matrix. Then,
we study the more realistic situation where only a few expectation values are
given at the initial time and we perform an optimization with respect to the
initial state. We calculate explicitly the two-time correlation functions with
two and four field operators at equilibrium in the symmetric phase.Comment: 60 pages, Latex, to be published in Annals of Physic
Jet quenching and broadening: the transport coefficient in an anisotropic plasma
The jet quenching parameter is analyzed for a quark jet propagating
in an anisotropic plasma. The momentum anisotropy is calculated at high
temperature of the underlying quark-gluon plasma. is explicitly
estimated in leading-logarithmic approximation by the broadening of the
massless quark interacting via gluon exchange. A plasma instability is present.
Strong indications are found that is increasing with increasing
anisotropy. Possible implications for the saturation scale in
collisions are pointed out.Comment: 1
A many-body perturbation theory approach to the electron-phonon interaction with density-functional theory as a starting point
The electron-phonon interaction plays a crucial role in many fields of
physics and chemistry. Nevertheless, its actual calculation by means of modern
many-body perturbation theory is weakened by the use of model Hamiltonians that
are based on parameters difficult to extract from the experiments. Such
shortcoming can be bypassed by using density-functional theory to evaluate the
electron-phonon scattering amplitudes, phonon frequencies and electronic bare
energies. In this work, we discuss how a consistent many-body diagrammatic
expansion can be constructed on top of density-functional theory. In that
context, the role played by screening and self-consistency when all the
components of the electron-nucleus and nucleus-nucleus interactions are taken
into account is paramount. A way to avoid over-screening is notably presented.
Finally, we derive cancellations rules as well as internal consistency
constraints in order to draw a clear, sound and practical scheme to merge
many-body perturbation and density-functional theory.Comment: 25 pages, 13 figure
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