1,136 research outputs found
Quantum energies with worldline numerics
We present new results for Casimir forces between rigid bodies which impose
Dirichlet boundary conditions on a fluctuating scalar field. As a universal
computational tool, we employ worldline numerics which builds on a combination
of the string-inspired worldline approach with Monte-Carlo techniques.
Worldline numerics is not only particularly powerful for inhomogeneous
background configurations such as involved Casimir geometries, it also provides
for an intuitive picture of quantum-fluctuation-induced phenomena. Results for
the Casimir geometries of a sphere above a plate and a new perpendicular-plates
configuration are presented.Comment: 8 pages, 2 figures, Submitted to the Proceedings of the Seventh
Workshop QFEXT'05 (Barcelona, September 5-9, 2005), Refs updated, version to
appear in JPhys
Chiral phase structure of QCD with many flavors
We investigate QCD with a large number of massless flavors with the aid of
renormalization group flow equations. We determine the critical number of
flavors separating the phases with and without chiral symmetry breaking in
SU(Nc) gauge theory with many fermion flavors. Our analysis includes all
possible fermionic interaction channels in the pointlike four-fermion limit.
Constraints from gauge invariance are resolved explicitly and regulator-scheme
dependencies are studied. Our findings confirm the existence of an Nf window
where the system is asymptotically free in the ultraviolet, but remains
massless and chirally invariant on all scales, approaching a conformal fixed
point in the infrared. Our prediction for the critical number of flavors of the
zero-temperature chiral phase transition in SU(3) is Nf^{cr}=10.0\pm
0.29(fermion)[+1.55;-0.63](gluon), with the errors arising from
approximations in the fermionic and gluonic sectors, respectively.Comment: 7 pages, 3 figures, updated discussion of the uncertainties in the
gauge secto
Optical probes of the quantum vacuum: The photon polarization tensor in external fields
The photon polarization tensor is the central building block of an effective
theory description of photon propagation in the quantum vacuum. It accounts for
the vacuum fluctuations of the underlying theory, and in the presence of
external electromagnetic fields, gives rise to such striking phenomena as
vacuum birefringence and dichroism. Standard approximations of the polarization
tensor are often restricted to on-the-light-cone dynamics in homogeneous
electromagnetic fields, and are limited to certain momentum regimes only. We
devise two different strategies to go beyond these limitations: First, we aim
at obtaining novel analytical insights into the photon polarization tensor for
homogeneous fields, while retaining its full momentum dependence. Second, we
employ wordline numerical methods to surpass the constant-field limit.Comment: 13 pages, 4 figures; typo in Eq. (5) corrected (matches journal
version
RG flow of the Polyakov-loop potential: First status report
We study SU(2) Yang-Mills theory at finite temperature in the framework of
the functional renormalization group. We concentrate on the effective potential
for the Polyakov loop which serves as an order parameter for confinement. In
this first status report, we focus on the behaviour of the effective
Polyakov-loop potential at high temperatures. In addition to the standard
perturbative result, our findings provide information about the ``RG improved''
backreactions of Polyakov-loop fluctuations on the potential. We demonstrate
that these fluctuations establish the convexity of the effective potential.Comment: 10 pages, 2 figure
Renormalization Group Study of Magnetic Catalysis in the 3d Gross-Neveu Model
Magnetic catalysis describes the enhancement of symmetry breaking quantum
fluctuations in chirally symmetric quantum field theories by the coupling of
fermionic degrees of freedom to a magnetic background configuration. We use the
functional renormalization group to investigate this phenomenon for interacting
Dirac fermions propagating in (2+1)-dimensional spacetime, described by the
Gross-Neveu model. We identify pointlike operators up to quartic fermionic
terms that can be generated in the renormalization group flow by the presence
of an external magnetic field. We employ the beta function for the fermionic
coupling to quantitatively analyze the field dependence of the induced spectral
gap. Within our pointlike truncation, the renormalization group flow provides a
simple picture for magnetic catalysis.Comment: 14 pages, 6 figures, typos correcte
Monte-Carlo calculation of the lateral Casimir forces between rectangular gratings within the formalism of lattice quantum field theory
We propose a new Monte-Carlo method for calculation of the Casimir forces.
Our method is based on the formalism of noncompact lattice quantum
electrodynamics. This approach has been tested in the simplest case of two
ideal conducting planes. After this the method has been applied to the
calculation of the lateral Casimir forces between two ideal conducting
rectangular gratings. We compare our calculations with the results of PFA and
"Optimal" PFA methods.Comment: 12 pages, 6 figures, accepted in Int. J. Mod. Phys.
Worldline algorithms for Casimir configurations
We present improved worldline numerical algorithms for high-precision
calculations of Casimir interaction energies induced by scalar-field
fluctuations with Dirichlet boundary conditions for various Casimir geometries.
Significant reduction of numerical cost is gained by exploiting the symmetries
of the worldline ensemble in combination with those of the configurations. This
facilitates high-precision calculations on standard PCs or small clusters. We
illustrate our strategies using the experimentally most relevant sphere-plate
and cylinder-plate configuration. We compute Casimir curvature effects for a
wide parameter range, revealing the tight validity bounds of the commonly used
proximity force approximation (PFA). We conclude that data analysis of future
experiments aiming at a precision of 0.1% must no longer be based on the PFA.
Revisiting the parallel-plate configuration, we find a mapping between the
D-dimensional Casimir energy and properties of a random-chain polymer ensemble.Comment: 23 pages, 9 figure
Analytical results for the confinement mechanism in QCD_3
We present analytical methods for investigating the interaction of two heavy
quarks in QCD_3 using the effective action approach. Our findings result in
explicit expressions for the static potentials in QCD_3 for long and short
distances. With regard to confinement, our conclusion reflects many features
found in the more realistic world of QCD_4.Comment: 24 pages, uses REVTe
Pressure-induced isostructural phase transition of metal-doped silicon clathrates
We propose an atomistic model for the pressure-induced isostructural phase
transition of metal-doped silicon clathrates, Ba8Si46 and K8Si46, that has been
observed at 14 GPa and 23 GPa, respectively. The model explains successfully
the equation of state, transition pressure, change of Raman spectra and
dependence on the doped cations as well as the effects of substituting Si(6c)
atoms with noble metals.Comment: 5 pages, two coumn, 5 figures. See http://www.iitaka.org/down.html
for more informatio
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