Probing the ground state in gauge theories

We consider two very different models of the flux tube linking two heavy quarks: a string linking the matter fields and a Coulombic description of two separately gauge invariant charges. We compare how close they are to the unknown true ground state in compact U(1) and the SU(2) Higgs model. Simulations in compact U(1) show that the string description is better in the confined phase but the Coulombic description is best in the deconfined phase; the last result is shown to agree with analytical calculations. Surprisingly in the non-abelian theory the Coulombic description is better in both the Higgs and confined phases. This indicates a significant difference in the width of the flux tubes in the two theories.Comment: 13 pages, 10 .eps figures. V2: conclusions extende

220 fs Er-Yb:glass laser mode-locked by a broadband low-loss Si/Ge saturable absorber

We demonstrate femtosecond performance of an ultra-broadband high-index-contrast saturable Bragg reflector consisting of a silicon/silicon-dioxide/germanium structure that is fully compatible with CMOS processing. This device offers a reflectivity bandwidth of over 700 nm and sub-picosecond recovery time of the saturable loss. It is used to achieve mode-locking of an Er-Yb:glass laser centered at 1540 nm, generating 220 fs pulses, with the broadest output spectrum to date

The infrastructural power of the military: The geoeconomic role of the US Army Corps of Engineers in the Arabian Peninsula

In analysing the role of the US in the global expansion of capitalist relations, most critical accounts see the US military’s invasion and conquest of various states as paving the way for the arrival of US businesses and capitalist relations. However, beyond this somewhat simplified image, and even in peacetime, the US military has been a major geoeconomic actor that has wielded its infrastructural power via its US Army Corps of Engineers’ overseas activities. The transformation of global economies in the 20th century has depended on the capitalisation of the newly independent states and the consolidation of liberal capitalist relations in the subsequent decades. The US Army Corps of Engineers has not only extended lucrative contracts to private firms (based not only in the US and host country, but also in geopolitically allied states), but also, and perhaps most important, has itself established a grammar of capitalist relations. It has done so by forging both physical infrastructures (roads, ports, utilities and telecommunications infrastructures) and virtual capitalist infrastructures through its practices of contracting, purchasing, design, accounting, regulatory processes and specific regimes of labour and private property ownership

Mathematical surprises and Dirac's formalism in quantum mechanics

By a series of simple examples, we illustrate how the lack of mathematical concern can readily lead to surprising mathematical contradictions in wave mechanics. The basic mathematical notions allowing for a precise formulation of the theory are then summarized and it is shown how they lead to an elucidation and deeper understanding of the aforementioned problems. After stressing the equivalence between wave mechanics and the other formulations of quantum mechanics, i.e. matrix mechanics and Dirac's abstract Hilbert space formulation, we devote the second part of our paper to the latter approach: we discuss the problems and shortcomings of this formalism as well as those of the bra and ket notation introduced by Dirac in this context. In conclusion, we indicate how all of these problems can be solved or at least avoided.Comment: Largely extended and reorganized version, with new title and abstract and with 2 figures added (published version), 54 page

Probing the reaction mechanism of IspH protein by x-ray structure analysis

Isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) represent the two central intermediates in the biosynthesis of isoprenoids. The recently discovered deoxyxylulose 5-phosphate pathway generates a mixture of IPP and DMAPP in its final step by reductive dehydroxylation of 1-hydroxy-2-methyl-2-butenyl 4-diphosphate. This conversion is catalyzed by IspH protein comprising a central iron-sulfur cluster as electron transfer cofactor in the active site. The five crystal structures of IspH in complex with substrate, converted substrate, products and PPi reported in this article provide unique insights into the mechanism of this enzyme. While IspH protein crystallizes with substrate bound to a [4Fe-4S] cluster, crystals of IspH in complex with IPP, DMAPP or inorganic pyrophosphate feature [3Fe-4S] clusters. The IspH:substrate complex reveals a hairpin conformation of the ligand with the C(1) hydroxyl group coordinated to the unique site in a [4Fe-4S] cluster of aconitase type. The resulting alkoxide complex is coupled to a hydrogen-bonding network, which serves as proton reservoir via a Thr167 proton relay. Prolonged x-ray irradiation leads to cleavage of the C(1)-O bond (initiated by reducing photo electrons). The data suggest a reaction mechanism involving a combination of Lewis-acid activation and proton coupled electron transfer. The resulting allyl radical intermediate can acquire a second electron via the iron-sulfur cluster. The reaction may be terminated by the transfer of a proton from the β-phosphate of the substrate to C(1) (affording DMAPP) or C(3) (affording IPP)

Biosynthesis of terpenes: Studies on 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate reductase

Earlier in vivo studies showed the involvement of IspH protein in the conversion of 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate into isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). We have demonstrated now that cell extract of an Escherichia coli strain engineered for hyperexpression of the ispH (lytB) gene catalyzes the in vitro conversion of 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate into IPP and DMAPP. The reaction requires NADH, FAD, divalent cations (preferably Co(2+)), and probably one or more as-yet-unidentified proteins. The low intrinsic catalytic activities of wild-type E. coli cell extract and isolated chromoplasts of red pepper (Capsicum annuum) are enhanced by the addition of purified recombinant IspH protein