Axigluons cannot explain the observed top quark forward-backward asymmetry

We study an SU(3)^2 axigluon model introduced by Frampton, Shu, and Wang to explain the recent Fermilab Tevatron observation of a significant positive enhancement in the top quark forward-backward asymmetry relative to standard model predictions. First, we demonstrate that data on neutral B_d-meson mixing excludes the region of model parameter space where the top asymmetry is predicted to be the largest. Keeping the gauge couplings below the critical value that would lead to fermion condensation imposes further limits at large axigluon mass, while precision electroweak constraints on the model are relatively mild. Furthermore, by considering an extension to an SU(3)^3 color group, we demonstrate that embedding the model in an extra-dimensional framework can only dilute the axigluon effect on the forward-backward asymmetry. We conclude that axigluon models are unlikely to be the source of the observed top quark asymmetry.Comment: 12 pages, 7 eps figures included. Minor changes to conform with published versio

Effects of strain on orbital ordering and magnetism at perovskite oxide interfaces: LaMnO3/SrMnO3

We study how strain affects orbital ordering and magnetism at the interface between SrMnO3 and LaMnO3 from density-functional calculations and interpret the basic results in terms of a three-site Mn-O-Mn model. Magnetic interaction between the Mn atoms is governed by a competition between the antiferromagnetic superexchange of the Mn t2g core spins and the ferromagnetic double exchange of the itinerant eg electrons. While the core electrons are relatively unaffected by the strain, the orbital character of the itinerant electron is strongly affected, which in turn causes a large change in the strength of the ferromagnetic double exchange. The epitaxial strain produces the tetragonal distortion of the MnO6 octahedron, splitting the Mn eg states into x2−y2 and 3z2−1 states, with the former being lower in energy, if the strain is tensile in the plane and opposite if the strain is compressive. For the case of the tensile strain, the resulting higher occupancy of the x2−y2 orbital enhances the in-plane ferromagnetic double exchange owing to the larger electron hopping in the plane, causing at the same time a reduction in the out-of-plane double exchange. This reduction is large enough to be overcome by antiferromagnetic superexchange, which wins to produce a net antiferromagnetic interaction between the out-of-plane Mn atoms. For the case of the in-plane compressive strain, the reverse happens, viz., that the higher occupancy of the 3z2−1 orbital results in the out-of-plane ferromagnetic interaction, while the in-plane magnetic interaction remains antiferromagnetic. Concrete density-functional results are presented for the (LaMnO3)1/(SrMnO3)1 and (LaMnO3)1/ (SrMnO3)3 superlattices for various strain conditions.This work was supported by the U.S. Department of Energy under Grant No. DE-FG02-00ER45818

Electronic and magnetic structure of the (LaMnO3)2n/(SrMnO3)n superlattices

We study the magnetic structure of the (LaMnO3)2n/(SrMnO3)n superlattices from density-functional calculations. In agreement with the experiments, we find that the magnetism changes with the layer thickness n. The reason for the different magnetic structures is shown to be the varying potential barrier across the interface, which controls the leakage of the Mn-eg electrons from the LaMnO3 side to the SrMnO3 side. This in turn affects the interfacial magnetism via the carrier-mediated Zener double exchange. For the n=1 superlattice, the Mn-eg electrons are more or less spread over the entire lattice so that the magnetic behavior is similar to the equivalent alloy compound La2/3Sr1/3MnO3. For larger n, the eg electron transfer occurs mostly between the two layers adjacent to the interface, thus leaving the magnetism unchanged and bulklike away from the interface region.This work was supported by the U.S. Department of Energy under Grant No. DE-FG02-00ER45818. We thank J. W. Freeland for stimulating this work and for valuable discussions

Electronic Phases and Phase Separation in the Hubbard-Holstein Model of a Polar Interface

http://arxiv.org/abs/1012.0889From a mean-field solution of the Hubbard-Holstein model, we show that a rich variety of different electronic phases can result at the interface between two polar materials such as LaAlO$_3$/SrTiO$_3$. Depending on the strengths of the various competing interactions, viz., the electronic kinetic energy, electron-phonon interaction, Coulomb energy, and electronic screening strength, the electrons could (i) either be strongly confined to the interface forming a 2D metallic or an insulating phase, (ii) spread deeper into the bulk making a 3D phase, or (iii) become localized at individual sites forming a Jahn-Teller polaronic phase. In the polaronic phase, the Coulomb interaction could lead to unpaired electrons resulting in magnetic Kondo centers. Under appropriate conditions, electronic phase separation may also occur resulting in the coexistence of metallic and insulating regions at the interface.This work was supported by the U. S. Department of Energy through Grant No. DE-FG02-00ER45818

Polar catastrophe, electron leakage, and magnetic ordering at the LaMnO3/SrMnO3 interface

Electronic reconstruction at the polar interface LaMnO3/SrMnO3 (LMO/SMO) (100) resulting from the polar catastrophe is studied from a model Hamiltonian that includes the double and super exchange interactions, the Madelung potential, and the Jahn-Teller coupling terms relevant for the manganites. We show that the polar catastrophe, originating from the alternately charged LMO layers and neutral SMO layers, is quenched by the accumulation of an extra half electron per cell in the interface region as in the case of the LaAlO3/SrTiO3 interface. In addition, the Mn eg electrons leak out from the LMO side to the SMO side, the extent of the leakage being controlled by the interfacial potential barrier and the substrate induced epitaxial strain. The leaked electrons mediate a Zener double exchange, making the layers adjacent to the interface ferromagnetic, while the two bulk materials away from the interface retain their original type A or G antiferromagnetic structures. A half-metallic conduction band results at the interface, sandwiched by the two insulating bulks. We have also studied how the electron leakage and consequently the magnetic ordering are affected by the substrate induced epitaxial strain. Comparisons are made with the results of the density-functional calculations for the (LMO)6/(SMO)4 superlattice.This work was supported by the U. S. Department of Energy through Grant No. DE-FG02-00ER45818

Magnetic and Orbital Order in LaMnO3 under Uniaxial Strain: A Model Study

The effect of uniaxial strain on electronic structure and magnetism in LaMnO3 is studied from a model Hamiltonian that illustrates the competition between the Jahn-Teller, super exchange, and double exchange interactions. We retain in our model the three main octahedral distortions (Q1,Q2, and Q3), which couple to the Mn (eg) electrons. Our results show the ground state to be a type A antiferromagnetic (AFM) insulating state for the unstrained case, consistent with experiments. With tensile strain (stretching along the c axis), the ground state changes into a ferromagnetic and eventually into a type G0 AFM structure, while with compressive strain, we find the type A switching into a type G structure. The orbital ordering, which displays the well known checkerboard x2−1/y2−1 structure for the unstrained case, retains more or less the same character for compressive strain, while changing into the z2 − 1 character for tensile strains. While Q1 and Q3 are fixed by the strain components "xx and "zz in our model, the magnitude of the in-plane distortion mode (Q2), which varies to minimize the total energy, slowly diminishes with tensile strain, completely disappearing as the FM state is entered. Within our model, the FM state is metallic, while the three AFM states are insulating.This work was supported by the U. S. Department of Energy through Grant No. DE-FG02-00ER45818

Evaluation of the Blood-Glucose Reducing Effects of Aqueous Extracts of the Selected Umbelliferous Fruits Used in Culinary Practices

Purpose: Oxidative stress in diabetes coexists with a reduction in the antioxidant status, which can increase the deleterious effects of free radicals. Many plant extracts and plant products have been shown to have significant antioxidant activity. The role of antioxidant plants in diabetes has not been studied. Food additives like spices, which have free radical scavenging activity, may also be useful in maintaining normal glucose levels in diabetic patients. The objective of the study is to evaluate the aqueous extracts of selected umbelliferous fruits for hypoglycaemic and antidiabetic activities in normal and diabetic rats respectively. Methods: All the aqueous extracts of (Caraway) CA, (Coriander) CO, (Cumin) CU, (Dill) DI and (Fennel) FE were administered at dose levels of 300 mg/kg body weight orally to different groups each containing 5 animals. A control group was also maintained simultaneously and received distilled water orally.Blood samples were collected from retro-orbital plexus of each rat before and at 2, 4, 6 and 8 h after extract administration and were analyzed for glucose content by using glucose oxidase peroxidase (GOD-POD) method using a visible spectrophotometer at 505 nm. Results: In normal rats only caraway extract produced significant hypoglycaemic action. The rest of the extracts produced hypoglycaemia, which was not significant different from that of control animals. In diabetic rats also the caraway was found to be the best among the extracts. Conclusion: The study indicates that since the umbelliferous fruits are used in the preparation of foods, they may be useful in the control of postprandial rise of blood glucose particularly in diabetic condition. Additionally, their daily use may help in reducing complications associated with chronic diabetes. Keywords: Diabetes, Oxidative stress, Glucose Oxidase – Peroxidase (GOD-POD), Antioxidants. > Tropical Journal of Pharmaceutical Research Vol. 5 (2) 2006: pp. 613-61

Export performance of Indian fisheries in the context of globalisation

Internationally traded fisheries products are characterised by a high degree of heterogenity, reflecting the wide range of species and of processing techniques. Indian sea food industry, by and large still remains as a supplier of raw materials to the pre processors in foreign countries and 90 per cent goes in bulk packs, which is the prime reason for the drastic reduction in the unit value realisation (Rao and Prakash, 1999). India 's share in the overall trade of the world is 1.5 to 2 per cent

Scalable parallel communications

Coarse-grain parallelism in networking (that is, the use of multiple protocol processors running replicated software sending over several physical channels) can be used to provide gigabit communications for a single application. Since parallel network performance is highly dependent on real issues such as hardware properties (e.g., memory speeds and cache hit rates), operating system overhead (e.g., interrupt handling), and protocol performance (e.g., effect of timeouts), we have performed detailed simulations studies of both a bus-based multiprocessor workstation node (based on the Sun Galaxy MP multiprocessor) and a distributed-memory parallel computer node (based on the Touchstone DELTA) to evaluate the behavior of coarse-grain parallelism. Our results indicate: (1) coarse-grain parallelism can deliver multiple 100 Mbps with currently available hardware platforms and existing networking protocols (such as Transmission Control Protocol/Internet Protocol (TCP/IP) and parallel Fiber Distributed Data Interface (FDDI) rings); (2) scale-up is near linear in n, the number of protocol processors, and channels (for small n and up to a few hundred Mbps); and (3) since these results are based on existing hardware without specialized devices (except perhaps for some simple modifications of the FDDI boards), this is a low cost solution to providing multiple 100 Mbps on current machines. In addition, from both the performance analysis and the properties of these architectures, we conclude: (1) multiple processors providing identical services and the use of space division multiplexing for the physical channels can provide better reliability than monolithic approaches (it also provides graceful degradation and low-cost load balancing); (2) coarse-grain parallelism supports running several transport protocols in parallel to provide different types of service (for example, one TCP handles small messages for many users, other TCP's running in parallel provide high bandwidth service to a single application); and (3) coarse grain parallelism will be able to incorporate many future improvements from related work (e.g., reduced data movement, fast TCP, fine-grain parallelism) also with near linear speed-ups

One-Loop Corrections to the S and T Parameters in a Three Site Higgsless Model

In this paper we compute the the one-loop chiral logarithmic corrections to the S and T parameters in a highly deconstructed Higgsless model with only three sites. In addition to the electroweak gauge bosons, this model contains a single extra triplet of vector states (which we denote \rho^{\pm} and \rho^0), rather than an infinite tower of "KK" modes. We compute the corrections to S and T in 'tHooft-Feynman gauge, including the ghost, unphysical Goldstone-boson, and appropriate "pinch" contributions required to obtain gauge-invariant results for the one-loop self-energy functions. We demonstrate that the chiral-logarithmic corrections naturally separate into two parts, a model-independent part arising from scaling below the \rho mass, which has the same form as the large Higgs-mass dependence of the S or T parameter in the standard model, and a second model-dependent contribution arising from scaling between the \rho mass and the cutoff of the model. The form of the universal part of the one-loop result allows us to correctly interpret the phenomenologically derived limits on the S and T parameters (which depend on a "reference" Higgs-boson mass) in this three-site Higgsless model. Higgsless models may be viewed as dual to models of dynamical symmetry breaking akin to "walking technicolor", and in these terms our calculation is the first to compute the subleading 1/N corrections to the S and T parameters. We also discuss the reduction of the model to the ``two-site'' model, which is the usual electroweak chiral lagrangian, noting the ``non-decoupling'' contributions present in the limit as M_\rho goes to infinity.Comment: 58 pages; uses JHEP and axodraw. Extensively corrected to incorporate consistent perturbative expansion, additional pinch contributions, and running of delocalization parameter. Footnotes adde