Ventilation of double facades

This paper deals with the development and thetesting of a simulation algorithm for the temperaturebehaviour and the flow characteristics of doublefaçades. It has been developed in order to obtain atool which enables the energy consultant to makequick design decisions without being required to usefairly complicated CFD tools.In order to determine the degree of accuracy of thealgorithm, a double façade has been monitored undercontrolled conditions and the results have beencompared against the predicted values for severaldesign situations. The resulting inaccuracy in somecases can be traced back to how the flow resistanceof various geometries are modelled. This paper deals with the development and thetesting of a simulation algorithm for the temperaturebehaviour and the flow characteristics of doublefaçades. It has been developed in order to obtain atool which enables the energy consultant to makequick design decisions without being required to usefairly complicated CFD tools.In order to determine the degree of accuracy of thealgorithm, a double façade has been monitored undercontrolled conditions and the results have beencompared against the predicted values for severaldesign situations. The resulting inaccuracy in somecases can be traced back to how the flow resistanceof various geometries are modelled

Fractional electric charge and quark confinement

Owing to their fractional electric charges, quarks are blind to transformations that combine a color center phase with an appropriate electromagnetic one. Such transformations are part of a global $Z_6$-like center symmetry of the Standard Model that is lost when quantum chromodynamics (QCD) is treated as an isolated theory. This symmetry and the corresponding topological defects may be relevant to non-perturbative phenomena such as quark confinement, much like center symmetry and ordinary center vortices are in pure SU($N$) gauge theories. Here we report on our investigations of an analogous symmetry in a 2-color model with dynamical Wilson quarks carrying half-integer electric charge.Comment: Conference proceedings for the XXIX International Symposium on Lattice Field Theory, 201

Universal Aspects of Deconfinement in 2+1 Dimensions

The 2+1 dimensional pure SU(N) gauge theories with N <= 4 are candidates for applying the powerful tools of scaling and universality to their deconfinement transitions at finite temperature. The corresponding 2 dimensional q-state Potts models with q <= 4 have 2nd order transitions, and we can exploit many exact results to obtain accurate critical couplings, transition temperatures, critical exponents, and the leading behavior of the continuum string tension near the phase transition on one side, together with its dual on the other. Thereby, the self-duality of the 2d spin models is reflected in a duality between spacelike vortices and confining electric fluxes. We also discuss the relevance of center symmetry and the corresponding vortices for confinement in full QCD when the electromagnetic interactions of fractionally charged quarks are included.Comment: 3 pages, 4 figures (pdflatex), for the proceedings of `Quark Confinement and the Hadron Spectrum IX', August 30 - September 3, Madrid, Spai

Infrared behaviour and fixed points in Landau gauge QCD

We investigate the infrared behaviour of gluon and ghost propagators in Landau gauge QCD by means of an exact renormalisation group equation. We explain how, in general, the infrared momentum structure of Green functions can be extracted within this approach. An optimisation procedure is devised to remove residual regulator dependences. In Landau gauge QCD this framework is used to determine the infrared leading terms of the propagators. The results support the Kugo-Ojima confinement scenario. Possible extensions are discussed.Comment: 4 pages, 1 figur

Fe-implanted ZnO: Magnetic precipitates versus dilution

Nowadays ferromagnetism is often found in potential diluted magnetic semiconductor systems. However, many authors argue that the observed ferromagnetism stems from ferromagnetic precipitates or spinodal decomposition rather than from carrier mediated magnetic impurities, as required for a diluted magnetic semiconductor. In the present paper we answer this question for Fe-implanted ZnO single crystals comprehensively. Different implantation fluences and temperatures and post-implantation annealing temperatures have been chosen in order to evaluate the structural and magnetic properties over a wide range of parameters. Three different regimes with respect to the Fe concentration and the process temperature are found: 1) Disperse Fe$^{2+}$ and Fe$^{3+}$ at low Fe concentrations and low processing temperatures, 2) FeZn$_2$O$_4$ at very high processing temperatures and 3) an intermediate regime with a co-existence of metallic Fe (Fe$^0$) and ionic Fe (Fe$^{2+}$ and Fe$^{3+}$). Ferromagnetism is only observed in the latter two cases, where inverted ZnFe$_2$O$_4$ and $\alpha$-Fe nanocrystals are the origin of the observed ferromagnetic behavior, respectively. The ionic Fe in the last case could contribute to a carrier mediated coupling. However, their separation is too large to couple ferromagnetically due to the lack of p-type carrier. For comparison investigations of Fe-implanted epitaxial ZnO thin films are presented.Comment: 14 pages, 17 figure

Scaling behavior and positivity violation of the gluon propagator in full QCD

The Landau-gauge gluon propagator is studied using the coarse and fine dynamical MILC configurations. The effects of dynamical quarks are clearly visible and lead to a reduction of the nonperturbative infrared enhancement relative to the quenched case. Lattice spacing effects are studied and found to be small. The gluon spectral function is shown to clearly violate positivity in both quenched and full QCD.Comment: 7 pages, 9 figures. References and 1 figure added, minor text modifications, version to be published in PR

Spectral density in resonance region and analytic confinement

We study the role of finite widths of resonances in a nonlocal version of the Wick-Cutkosky model. The spectrum of bound states is known analytically in this model and forms linear Regge tragectories. We compute the widths of resonances, calculate the spectral density in an extension of the Breit-Wigner {\it ansatz} and discuss a mechanism for the damping of unphysical exponential growth of observables at high energy due to finite widths of resonances.Comment: 13 pages, RevTeX, 6 figures. Revised version with typographical corrections and additional comments in conclusion

Infrared exponents and the strong-coupling limit in lattice Landau gauge

We study the gluon and ghost propagators of lattice Landau gauge in the strong-coupling limit beta=0 in pure SU(2) lattice gauge theory to find evidence of the conformal infrared behavior of these propagators as predicted by a variety of functional continuum methods for asymptotically small momenta $q^2 \ll \Lambda_\mathrm{QCD}^2$. In the strong-coupling limit, this same behavior is obtained for the larger values of a^2q^2 (in units of the lattice spacing a), where it is otherwise swamped by the gauge field dynamics. Deviations for a^2q^2 < 1 are well parameterized by a transverse gluon mass $\propto 1/a$. Perhaps unexpectedly, these deviations are thus no finite-volume effect but persist in the infinite-volume limit. They furthermore depend on the definition of gauge fields on the lattice, while the asymptotic conformal behavior does not. We also comment on a misinterpretation of our results by Cucchieri and Mendes in Phys. Rev. D81 (2010) 016005.Comment: 17 pages, 12 figures. Revised version (mainly sections I and II); references and comments on subsequent work on the subject added