The role of center vortices in Gribov's confinement scenario

The connection of Gribov's confinement scenario in Coulomb gauge with the center vortex picture of confinement is investigated. For this purpose we assume a vacuum wave functional which models the infrared properties of the theory and in particular shows strict confinement, i.e. an area law of the Wilson loop. We isolate the center vortex content of this wave functional by standard lattice methods and investigate their contributions to various static propagators of the Hamilton approach to Yang-Mills theory in Coulomb gauge. We find that the infrared properties of these quantities, in particular the infrared divergence of the ghost form factor, are dominated by center vortices.Comment: 18 pages, 5 figure

Broad boron sheets and boron nanotubes: An ab initio study of structural, electronic, and mechanical properties

Based on a numerical ab initio study, we discuss a structure model for a broad boron sheet, which is the analog of a single graphite sheet, and the precursor of boron nanotubes. The sheet has linear chains of sp hybridized sigma bonds lying only along its armchair direction, a high stiffness, and anisotropic bonds properties. The puckering of the sheet is explained as a mechanism to stabilize the sp sigma bonds. The anisotropic bond properties of the boron sheet lead to a two-dimensional reference lattice structure, which is rectangular rather than triangular. As a consequence the chiral angles of related boron nanotubes range from 0 to 90 degrees. Given the electronic properties of the boron sheets, we demonstrate that all of the related boron nanotubes are metallic, irrespective of their radius and chiral angle, and we also postulate the existence of helical currents in ideal chiral nanotubes. Furthermore, we show that the strain energy of boron nanotubes will depend on their radii, as well as on their chiral angles. This is a rather unique property among nanotubular systems, and it could be the basis of a different type of structure control within nanotechnology.Comment: 16 pages, 17 figures, 2 tables, Versions: v1=preview, v2=first final, v3=minor corrections, v4=document slightly reworke

A general solution in the Newtonian limit of f(R)- gravity

We show that any analytic $f(R)$-gravity model, in the metric approach, presents a weak field limit where the standard Newtonian potential is corrected by a Yukawa-like term. This general result has never been pointed out but often derived for some particular theories. This means that only $f(R)=R$ allows to recover the standard Newton potential while this is not the case for other relativistic theories of gravity. Some considerations on the physical consequences of such a general solution are addressed.Comment: 5 page

Gravitational lensing in fourth order gravity

Gravitational lensing is investigated in the weak field limit of fourth order gravity in which the Lagrangian of the gravitational field is modified by replacing the Ricci scalar curvature R with an analytical expression $f(R)$. Considering the case of a pointlike lens, we study the behaviour of the deflection angle in the case of power law Lagrangians, i.e. with f(R) = f_0 R^n. In order to investigate possible detectable signatures, the position of the Einstein ring and the solutions of the lens equation are evaluated considering the change with respect to the standard case. Effects on the amplification of the images and the Paczynski curve in microlensing experiments are also estimated.Comment: 10 pages, 3 figures, accepted for publication on Physical Review

Boron Fullerenes: A First-Principles Study

A family of unusually stable boron cages was identified and examined using first-principles local density functional method. The structure of the fullerenes is similar to that of the B12 icosahedron and consists of six crossing double-rings. The energetically most stable fullerene is made up of 180 boron atoms. A connection between the fullerene family and its precursors, boron sheets, is made. We show that the most stable boron sheets are not necessarily precursors of very stable boron cages. Our finding is a step forward in the understanding of the structure of the recently produced boron nanotubes.Comment: 10 pages, 4 figures, 1 tabl

Timing and magma evolution of the Chelopech volcanic complex (Bulgaria)

The Chelopech volcanic complex is located in the Central Srednogorie magmatic zone and hosts one of the largest Cu–Au deposits in Europe. Field observations and sedimentary relationships allow to distinguish three units of the volcanic complex: (I) dome-like bodies, (II) lava to agglomerate flows, and (III) the Vozdol lava breccias and volcanites. The volcanic rocks are porphyritic with plagioclase and amphibole phenocrysts, quartz and biotite are rare. The lava flows contain fully crystallised, fine-grained enclaves of more basic composition. Their mineral chemistry indicates mingling and mixing between two parental magmas. The geochemical evolution of the Chelopech volcanic complex developed from intermediate to basic lavas, but the evolution of the magmatism was more complex including magmatic differentiation, assimilation, mingling and mixing. The trace element distribution is typical for an active continental margin. The magmatic activity commenced at the northern border of the Chelopech region with the intrusion of dome-like bodies at 92.2 ± 0.3 Ma (U–Pb single zircon ID-TIMS dating). The products of the second and the third units are geochronologically indistinguishable within the error uncertainties, and representative samples yield a crystallisation age of 91.3 ± 0.3 Ma. REE abundances reveal a striking positive Ce-anomaly in zircons of unit 2 and zircon core parts of unit 3, which relates to a higher oxidation state of the parental magma. Sr and Nd isotopic compositions suggest a mixed mantle and crustal source of the Turonian magma. Initial Sr ratios range between 0.70470 and 0.70554, and 90(Nd) varies between –2.27 and –3.55. 90(Hf) values of concordant zircons corroborate this data and range between +2.90 to +5.02 in the andesite of the first unit and from +1.06 to +1.38 in the volcanites of the second and third unit

String Nature of Confinement in (Non-)Abelian Gauge Theories

Recent progress achieved in the solution of the problem of confinement in various (non-)Abelian gauge theories by virtue of a derivation of their string representation is reviewed. The theories under study include QCD within the so-called Method of Field Correlators, QCD-inspired Abelian-projected theories, and compact QED in three and four space-time dimensions. Various nonperturbative properties of the vacua of the above mentioned theories are discussed. The relevance of the Method of Field Correlators to the study of confinement in Abelian models, allowing for an analytical description of this phenomenon, is illustrated by an evaluation of field correlators in these models.Comment: 100 pages, LaTeX2e, no figures, 1 table, based on the Ph.D. thesises at the Humboldt University of Berlin (1999) (available under http://dochost.rz.hu-berlin.de) and the Institute of Theoretical and Experimental Physics, Moscow (2000), new results are included, extended with respect to the journal versio