The Moduli Space and Phase Structure of Heterotic Strings in Two Dimensions

We explore the moduli space of heterotic strings in two dimensions. In doing so, we introduce new lines of compactified theories with Spin(24) gauge symmetry and discuss compactifications with Wilson lines. The phase structure of d=2 heterotic string theory is examined by classifying the hypersurfaces in moduli space which support massless quanta or discrete states. Finally, we compute the torus amplitude over much of the moduli space.Comment: 23 pages, 1 figure, harvmac.tex; v2: minor changes to introduction, version to be published in Phys. Rev.

Two Flaws In Business Cycle Accounting

Using 'business cycle accounting' (BCA), Chari, Kehoe and McGrattan (2006) (CKM) conclude that models of financial frictions which create a wedge in the intertemporal Euler equation are not promising avenues for modeling business cycle dynamics. There are two reasons that this conclusion is not warranted. First, small changes in the implementation of BCA overturn CKM's conclusions. Second, one way that shocks to the intertemporal wedge impact on the economy is by their spillover effects onto other wedges. This potentially important mechanism for the transmission of intertemporal wedge shocks is not identified under BCA. CKM potentially understate the importance of these shocks by adopting the extreme position that spillover effects are zero.

Two flaws in business cycle dating

Using “business cycle accounting,” Chari, Kehoe, and McGrattan (2006) conclude that models of financial frictions which create a wedge in the intertemporal Euler equation are not promising avenues for modeling business cycle dynamics. There are two reasons that this conclusion is not warranted. First, small changes in the implementation of business cycle accounting overturn Chari, Kehoe, and McGrattan’s conclusions. Second, one way that shocks to the intertemporal wedge affect the economy is by their spillover effects onto other wedges. This potentially important mechanism for the transmission of intertemporal-wedge shocks is not identified under business cycle accounting. Chari, Kehoe, and McGrattan potentially understate the importance of these shocks by adopting the extreme position that spillover effects are zero.Business cycles

Synthesis of terephthalic acid via Diels-Alder reactions with ethylene and oxidized variants of 5-hydroxymethylfurfural

Terephthalic acid (PTA), a monomer in the synthesis of polyethylene terephthalate (PET), is obtained by the oxidation of petroleum-derived p-xylene. There is significant interest in the synthesis of renewable, biomass-derived PTA. Here, routes to PTA starting from oxidized products of 5-hydroxymethylfurfural (HMF) that can be produced from biomass are reported. These routes involve Diels-Alder reactions with ethylene and avoid the hydrogenation of HMF to 2,5-dimethylfuran. Oxidized derivatives of HMF are reacted with ethylene over solid Lewis acid catalysts that do not contain strong Brønsted acids to synthesize intermediates of PTA and its equally important diester, dimethyl terephthalate (DMT). The partially oxidized HMF, 5-(hydroxymethyl)furoic acid (HMFA), is reacted with high pressure ethylene over a pure-silica molecular sieve containing framework tin (Sn-Beta) to produce the Diels-Alder dehydration product, 4-(hydroxymethyl)benzoic acid (HMBA), with 31% selectivity at 61% HMFA conversion after 6 h at 190 °C. If HMFA is protected with methanol to form methyl 5-(methoxymethyl)furan-2-carboxylate (MMFC), MMFC can react with ethylene in the presence of Sn-Beta for 2 h to produce methyl 4-(methoxymethyl)benzenecarboxylate (MMBC) with 46% selectivity at 28% MMFC conversion or in the presence of a pure-silica molecular sieve containing framework zirconium (Zr-Beta) for 6 h to produce MMBC with 81% selectivity at 26% MMFC conversion. HMBA and MMBC can then be oxidized to produce PTA and DMT, respectively. When Lewis acid containing mesoporous silica (MCM-41) and amorphous silica, or Brønsted acid containing zeolites (Al-Beta), are used as catalysts, a significant decrease in selectivity/yield of the Diels-Alder dehydration product is observed

Striking the Right Balance When Users are Good at IT Too!: Partnering for Enterprise System Success

Enterprise-level information system (IS) implementations are risky endeavors that require the active engagement of diverse parties from within the organization to meet the technical and functional requirements of the implementation, and mitigate possible resistance to the implemented system. Past research on IS implementation has traditionally pointed to the IT department as the sole source of technical competence, and confined the potential contribution of the user base to functional expertise. Furthermore, this line of research consistently identifies the IT department as the leaders of such projects, further confining the user base to sideline consulting roles during these initiatives. However, today’s business professionals increasingly possess IT competence and capability of contributing to the technical side of IS implementations. This study focuses on the balance of IT competence within the multi-functional enterprise-level IS implementation team, brought on by a more technically competent user base, and examines its impact on project success

Recent Trends in California Hispanic Fertility Rates — a Comparative Analysis

In this article we did a comparative analysis of recent trends and differentials in Hispanic fertility rates in the State of California. Hispanics, largely composed of individuals of Mexican origins, were the largest racial/ethnic minority population in the State of California from 1999 to 2001, composing roughly 30 percent of the population, contributed approximately 50 percent of the births during this period. Hispanics, compared to other racial/ethnic groups, have high teenage fertility rates, which persist throughout the reproductive years. Hispanic total fertility rates were well above replacement level fertility and higher than those of other racial/ethnic populations. Two prime policy issues related to human reproduction emerged from an analysis of the data—education and medical care. Although teenage fertility rates declined for all racial/ethnic groups during this period, they still remain high, especially for Hispanics and also for Blacks and American Indians, highlighting the need for education on human reproduction for teenagers. With high teenage fertility rates and a diverse racial/ethnic population, the need for more pervasive prenatal care, especially in the first trimester, also emerged as a significant policy issue. / En este artículo hicimos un análisis comparativo de tendencias recientes y diferenciales en las tasas de fertilidad de hispanos en el estado de California. Hispanos, los cuales están compuestos mayormente por individuos de origen Mexicano, fueron el grupo étnico/racial mas grande de la población minoritaria en el estado de California desde el 1999 a el 2001, de manera que componen más o menos 30 porciento de la población y contribuyen a aproximadamente 50 porciento de los nacimientos en este periodo. Hispanos, comparados con otros grupos étnicos/raciales, tienen altas tasas de fertilidad adolescente, las cuales persisten a través de sus años reproductivos. Las tasas totales de fertilidad para Hispanos estuvieron bastante sobre el nivel de reemplazo de fertilidad y más altas que aquellas de otras poblaciones étnicas/raciales. Dos asuntos de política relacionados a la reproducción humana salieron del análisis de la data- educación y cuidado médico. Aunque las tasas de fertilidad para adolescentes disminuyó para todos los grupos étnicos/raciales en este periodo, todavía permanecen muy altas, especialmente para los hispanos pero también para afro-americanos e Indios Americanos, señalando la necesidad de educar a nuestros adolescentes acerca de la reproducción humana. Con las altas tasas de fertilidad de adolescentes y una población étnica/racial tan diversa, existe la necesidad para cuidados prenatales más permeables, especialmente en el primer trimestre de embarazo, lo cual también surgió ser un tema de política muy significante

Structural Classification of Metal Complexes with Three-Coordinate Centres

Attempts to describe the geometry about three-coordinate silver(I) complexes have proven difficult because interatomic angles generally vary wildly and there is no adequate or readily available classification system found in the literature. A search of the Cambridge Structural Database shows that complexes formed between any metal centre and three non-metal donors (18001 examples) usually adopt geometries that are quite different than ideal ‘textbook’ extremes of either trigonal planar (∼4% with α = β = γ = 120 ± 2°), T-shaped (∼0.05% with α = 180 ± 2°, β = γ = 90 ± 2°), or trigonal pyramidal (∼0.3% with α = β = γ = 110 ± 2°). Moreover, there are multiple variations of “Y-type” and “other” shapes that require elaboration. Thus, to assist in future structural descriptions, we developed a classification system that spans all known and yet-to-be-discovered three-coordinate geometries. A spreadsheet has also been constructed that utilizes the “shape-space” approach to extract the structural description from a user input of three angles about a tri-coordinate centre and the number of atoms in a plane. The structures of two silver(I) complexes of new N-donor ligands p-NH2C6H4C6H4CH(pz = pyrazol-1-yl)2, L1, and 2-ferrocenyl-4,5-di(2-pyridyl)imidazole, L2, illustrate the utility of this classification system

Fast, low-loss all-optical phase modulation in warm rubidium vapour

High-speed switching with low loss would be a versatile tool for photonic quantum technologies, with applications in state generation, multiplexing, and the implementation of quantum gates. Phase modulation is one method of achieving this switching, but existing optical phase modulators either achieve high bandwidth or low loss, but not both. We demonstrate fast ($100\,\mathrm{MHz}$) bandwidth), low-loss ($74(2)\,\%$) transmission) phase shifting ($\Delta\phi = (0.90(5))\pi$) in a signal field, induced by a control field, and mediated by the two-photon $5S_{1/2} \rightarrow{} 5P_{3/2} \rightarrow{} 5D_{5/2}$ transition in rubidium-87 vapour. We discuss routes to enhance both performance and scalability for application to a range of quantum and classical technologies.Comment: 10 pages, 6 figure