Divergent IR gluon propagator from Ward-Slavnov-Taylor identities?

We exploit the Ward-Slavnov-Taylor identity relating the 3-gluons to the ghost-gluon vertices to conclude either that the ghost dressing function is finite and non vanishing at zero momentum while the gluon propagator diverges (although it may do so weakly enough not to be in contradiction with current lattice data) or that the 3-gluons vertex is non-regular when one momentum goes to zero. We stress that those results should be kept in mind when one studies the Infrared properties of the ghost and gluon propagators, for example by means of Dyson-Schwinger equations.Comment: 6 pages, bibte

Instanton dominance over αs\alpha_s at low momenta from lattice QCD simulations at Nf=0N_f=0, Nf=2+1N_f=2+1 and Nf=2+1+1N_f=2+1+1

We report on an instanton-based analysis of the gluon Green functions in the Landau gauge for low momenta; in particular we use lattice results for $\alpha_s$ in the symmetric momentum subtraction scheme (${\rm MOM}$) for large-volume lattice simulations. We have exploited quenched gauge field configurations, $N_f=0$, with both Wilson and tree-level Symanzik improved actions, and unquenched ones with $N_f=2+1$ and $N_f=2+1+1$ dynamical flavors (domain wall and twisted-mass fermions, respectively). We show that the dominance of instanton correlations on the low-momenta gluon Green functions can be applied to the determination of phenomenological parameters of the instanton liquid and, eventually, to a determination of the lattice spacing. We furthermore apply the Gradient Flow to remove short-distance fluctuations. The Gradient Flow gets rid of the QCD scale, $\Lambda_{\rm QCD}$, and reveals that the instanton prediction extents to large momenta. For those gauge field configurations free of quantum fluctuations, the direct study of topological charge density shows the appearance of large-scale lumps that can be identified as instantons, giving access to a direct study of the instanton density and size distribution that is compatible with those extracted from the analysis of the Green functions.Comment: Proceedings of the 35th International Symposium on Lattice Field Theory, Granada, Spai

Non-perturbative Power Corrections to Ghost and Gluon Propagators

We study the dominant non-perturbative power corrections to the ghost and gluon propagators in Landau gauge pure Yang-Mills theory using OPE and lattice simulations. The leading order Wilson coefficients are proven to be the same for both propagators. The ratio of the ghost and gluon propagators is thus free from this dominant power correction. Indeed, a purely perturbative fit of this ratio gives smaller value ($\simeq 270$MeV) of \Lambda_{\ms} than the one obtained from the propagators separately($\simeq 320$MeV). This argues in favour of significant non-perturbative $\sim 1/q^2$ power corrections in the ghost and gluon propagators. We check the self-consistency of the method.Comment: 14 pages, 4 figures; replaced with revised version, to appear in JHE

The strong coupling constant at small momentum as an instanton detector

We present a study of $\alpha_{MOM}(p)$ at small p computed from the lattice. It shows a dramatic $\propto p^4$ law which can be understood within an instanton liquid model. In this framework the prefactor gives a direct measure of the instanton density in thermalised configurations. A preliminary result for this density is 5.27(4) fm^{-4}.Comment: 12 pages, 4 figure

Preliminary Calculation of αs\alpha_s from Green Functions with Dynamical Quarks

We present preliminary results on the computation of the QCD running coupling constant in the $\widetilde{MOM}$ scheme and Landau gauge with two flavours of dynamical Wilson quarks. Gluon momenta range up to about 7 GeV ($\beta =$ 5.6, 5.8 and 6.0) with a constant dynamical-quark mass. This range already allows to exhibit some evidence for a sizable $1/\mu^2$ correction to the asymptotic behaviour, as in the quenched approximation, although a fit without power corrections is still possible with a reasonable $\chi^2$. Following the conclusions of our quenched study, we take into account $1/\mu^2$ correction to the asymptotic behaviour. We find $\Lambda_{\rm \bar{MS}}^{N_f=2} = 264(27) {\rm MeV} \times [{a^{-1}(5.6,0.1560)}/{2.19 {\rm GeV}}]$, which leads to $\alpha_s(M_Z) = 0.113(3)(4)$. The latter result has to be taken as a preliminary indication rather than a real prediction in view of the systematic errors still to be controlled. Still, being two sigmas below the experimental result makes it very encouraging.Comment: 14 pages, 3 figs., 2 tabs., revte

Testing OPE for ghosts, gluons and αs\alpha_s

We present here our results on extracting Wilson coefficients from different quantities such as ghost and gluon propagators which are calculated by means of Lattice QCD. The results confirm the validity of our method for the calculation of the strong coupling constant as well as allow to estimate the range of momenta where OPE is applicable.Comment: arXiv admin note: substantial text overlap with arXiv:1301.759

Vers un laser germanium dopé N et contraint en tension

Dans ce travail de thèse, nous avons étudié différentes approches qui devraient permettre d obtenir l effet laser dans le germanium. Nous avons pu montrer expérimentalement l influence du dopage et de la déformation sur la structure de bande du germanium, et l adéquation avec les modèles concluants à l existence de gain. Nous avons exploré les possibilités offertes par l hétéro-épitaxie sur III-V pour obtenir une déformation en tension du germanium. Nous avons évalué la déformation résultante par des mesures croisées de rayons X, de diffusion Raman et de photoluminescence, et étudié l évolution de la qualité des couches épitaxiées en fonction de la déformation et de l épaisseur. Une nouvelle méthode de déformation du germanium, s appuyant sur le dépôt par plasma de couches contraintes de nitrure, a été introduite et étudiée. L effet laser a été recherché par la conception de guides ridges et microdisques déformés par ces dépôts. Plusieurs voies d application de la déformation dans ces cavités ont été explorées à travers des simulations par éléments finis et la conception de structures de test. Cette optimisation préalable nous a permis d observer sur les microdisques une déformation biaxiale de 1.11%. En uniaxial, nous avons pu appliquer au germanium une déformation de 1.07% et montrer expérimentalement l importance de la direction de la déformation dans l augmentation de la luminescence. Nous avons pu observer et mesurer un gain optique net de 80 cm dans des structures déformées uniaxialement à 0.8%.In this PhD work, we studied different approaches that should lead to a germanium laser. We have experimentally shown the influence of strain and doping on the germanium band structure, and the adequacy of the existing models. We explored the possibilities offered by heteroepitaxy on III-V compounds to apply stress. We investigated the resulting strain by cross-checking X-rays, Raman spectroscopy and photoluminescence measurements, and analysed the quality of the grown layers depending on strain and thickness. A new method to apply strain to the germanium, by means of plasma deposited stressed nitride layers, was introduced and studied. Lasing has been pursued by conceiving ridges and microdisks strained by this method. An optimization of the geometry was performed through finite element modeling and the production of test structures. This optimization allowed to achieve a maximum biaxial strain of 1.1%. For uniaxial strains, we observed a maximum of 1.07% and showed experimentally the importance of the crystalline orientation in the enhancement of the emission. We demonstrated a modal gain value of 80 cm in ridges uniaxially strained at 0.8%.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF