Three-Nucleon Force and the Δ\Delta-Mechanism for Pion Production and Pion Absorption

The description of the three-nucleon system in terms of nucleon and $\Delta$ degrees of freedom is extended to allow for explicit pion production (absorption) from single dynamic $\Delta$ de-excitation (excitation) processes. This mechanism yields an energy dependent effective three-body hamiltonean. The Faddeev equations for the trinucleon bound state are solved with a force model that has already been tested in the two-nucleon system above pion-production threshold. The binding energy and other bound state properties are calculated. The contribution to the effective three-nucleon force arising from the pionic degrees of freedom is evaluated. The validity of previous coupled-channel calculations with explicit but stable $\Delta$ isobar components in the wavefunction is studied.Comment: 23 pages in Revtex 3.0, 9 figures (not included, available as postscript files upon request), CEBAF-TH-93-0

Divergent nematic susceptibility in an iron arsenide superconductor

Within the Landau paradigm of continuous phase transitions, ordered states of matter are characterized by a broken symmetry. Although the broken symmetry is usually evident, determining the driving force behind the phase transition is often a more subtle matter due to coupling between otherwise distinct order parameters. In this paper we show how measurement of the divergent nematic susceptibility of an iron pnictide superconductor unambiguously distinguishes an electronic nematic phase transition from a simple ferroelastic distortion. These measurements also reveal an electronic nematic quantum phase transition at the composition with optimal superconducting transition temperature.Comment: 8 pages, 8 figure

In Vitro Complement-Binding on Cytoplasmic Structures in Normal Human Skin: I. Immunofluorescence Studies

Incubation of cryostat sections of normal human skin with normal human serum (NHS) at 37°C followed by fluorescein isothiocyanate labeled rabbit antihuman C3 (FITC-R/Hu-C3) yields cytoplasmic staining of various cell types including keratinocytes of the upper epidermal layers, melanocytes, fibroblasts, smooth muscle cells, and cells lining vascular structures.Deposition of C3 on the respective cytoplasmic structures is most likely due to activation of the classical complement (C) cascade on these structures since no fluorescent staining is observed when serum of patients with hereditary C4-deficiency is used instead of NHS or when incubation with NHS is performed in the presence of EDTA or EGTA in concentrations known to inhibit classical C pathway activation. Further evidence suggesting the involvement of the classical C pathway comes from the finding that incubation of cryostat skin sections with NHS followed by FITC labeled rabbit antihuman Clq (FITC-R/Hu-Clq) results in a fluorescent staining pattern remarkably similar to that seen after exposure of cryostat skin sections to NHS and FITC-R/ Hu-C3.Although formal proof is lacking, our investigations strongly indicate that binding to and activation of C components on cytoplasmic structures occur independently of the presence of circulating antibodies. This assumption is based on the finding that in 17 out of 20 NHS we were not able to detect any skin reactive antibodies by indirect immunofluorescence (IF) techniques. More conclusive evidence for a direct, antibody-independent interaction between C components and cytoplasmic structures is provided by the observation that incubation of the substrate with purified Clq followed by FITC-R/ Hu-Clq results in cytoplasmic staining of some of the skin cell populations described above.The phenomenon of C-binding adn activation on cytoplasmic structures of normal human skin cells may be a critical event in the initiation of complement mediated pathopysiological reactions of the skin

A Systematic Extended Iterative Solution for QCD

An outline is given of an extended perturbative solution of Euclidean QCD which systematically accounts for a class of nonperturbative effects, while allowing renormalization by the perturbative counterterms. Proper vertices Gamma are approximated by a double sequence Gamma[r,p], with r the degree of rational approximation w.r.t. the QCD mass scale Lambda, nonanalytic in the coupling g, and p the order of perturbative corrections in g-squared, calculated from Gamma[r,0] - rather than from the perturbative Feynman rules Gamma(0)(pert) - as a starting point. The mechanism allowing the nonperturbative terms to reproduce themselves in the Dyson-Schwinger equations preserves perturbative renormalizability and is tied to the divergence structure of the theory. As a result, it restricts the self-consistency problem for the Gamma[r,0] rigorously - i.e. without decoupling approximations - to the superficially divergent vertices. An interesting aspect of the scheme is that rational-function sequences for the propagators allow subsequences describing short-lived excitations. The method is calculational, in that it allows known techniques of loop computation to be used while dealing with integrands of truly nonperturbative content.Comment: 48 pages (figures included). Scope of replacement: correction of a technical defect; no changes in conten

Geometry: The leading parameter for the Poisson’s ratio of bending-dominated cellular solids

Control over the deformation behaviour that a cellular structure shows in response to imposed external forces is a requirement for the effective design of mechanical metamaterials, in particular those with negative Poisson’s ratio. This article sheds light on the old question of the relationship between geometric microstructure and mechanical response, by comparison of the deformation properties of bar-and-joint-frameworks with those of their realisation as a cellular solid made from linear-elastic material. For ordered planar tessellation models, we find a classification in terms of the number of degrees of freedom of the framework model: first, in cases where the geometry uniquely prescribes a single deformation mode of the framework model, the mechanical deformation and Poisson’s ratio of the linearly-elastic cellular solid closely follow those of the unique deformation mode; the result is a bending-dominated deformation with negligible dependence of the effective Poisson’s ratio on the underlying material’s Poisson’s ratio and small values of the effective Young’s modulus. Second, in the case of rigid structures or when geometric degeneracy prevents the bending-dominated deformation mode, the effective Poisson’s ratio is material-dependent and the Young’s modulus View the MathML sourceE˜cs large. All analysed structures of this type have positive values of the Poisson’s ratio and large values of View the MathML sourceE˜cs. Third, in the case, where the framework has multiple deformation modes, geometry alone does not suffice to determine the mechanical deformation. These results clarify the relationship between mechanical properties of a linear-elastic cellular solid and its corresponding bar-and-joint framework abstraction. They also raise the question if, in essence, auxetic behaviour is restricted to the geometry-guided class of bending-dominated structures corresponding to unique mechanisms, with inherently low values of the Young’s modulus

Persistent detwinning of iron pnictides by small magnetic fields

Our comprehensive study on EuFe$_2$As$_2$ reveals a dramatic reduction of magnetic detwinning fields compared to other AFe$_2$As$_2$ (A = Ba, Sr, Ca) iron pnictides by indirect magneto-elastic coupling of the Eu$^{2+}$ ions. We find that only 0.1T are sufficient for persistent detwinning below the local Eu$^{2+}$ ordering; above $T_\text{Eu}$ = 19K, higher fields are necessary. Even after the field is switched off, a significant imbalance of twin domains remains constant up to the structural and electronic phase transition (190K). This persistent detwinning provides the unique possibility to study the low temperature electronic in-plane anisotropy of iron pnictides without applying any symmetrybreaking external force.Comment: accepted by Physical Review Letter

The pion-three-nucleon problem with two-cluster connected-kernel equations

It is found that the coupled piNNN-NNN system breaks into fragments in a nontrivial way. Assuming the particles as distinguishable, there are indeed four modes of fragmentation into two clusters, while in the standard three-body problem there are three possible two-cluster partitions and conversely the four-body problem has seven different possibilities. It is shown how to formulate the pion-three-nucleon collision problem through the integral-equation approach by taking into account the proper fragmentation of the system. The final result does not depend on the assumption of separability of the two-body t-matrices. Then, the quasiparticle method a' la Grassberger-Sandhas is applied and effective two-cluster connected-kernel equations are obtained. The corresponding bound-state problem is also formulated, and the resulting homogeneous equation provides a new approach which generalizes the commonly used techniques to describe the three-nucleon bound-state problem, where the meson degrees of freedom are usually suppressed.Comment: 20 pages, REVTeX, with 3 COLOR figures (PostScript

Geomorphology and paleoecology of the arid diagonal in Southern South America

Por medio de investigaciones geomorfológicas, pedológicas y palinológicas se ha reconstruido la localización, extensión y dinámica de la Diagonal Árida para diferentes fases del Cuaternario Superior. Los resultados se muestran a través de perfiles de los pisos altitudinales geoecológicos, por diagramas de variación, tanto temporal como espacial de diferentes indicadores y parámetros paleoclimáticos, y por bosquejos cartográficos de la extensión de la Diagonal Árida, así como de diferentes regímenes pluviométricos, y de la distribución de los sistemas atmosféricos actuales y del  pasado. Estas nvestigaciones paleoecológicas se han complementado en la Región de Cuyo con estudios de los cambios recientes - siglos XVI al XX - en la dinámica ambiental y las adaptaciones de los grupos humanos, enfocados principalmente al proceso de desertificación. estudios de los cambios recientes - siglos XVI al XX - en la dinámica ambiental y las adaptaciones de los grupos humanos, enfocados principalmente al proceso de desertificación.Position, extent and intensity of the Arid Diagonal at various time slices during the upper Quaternary were reconstructed. Geomorphic, pedologic and palynologic evidences indicate significant shifts of the geoecological belts with regard to altitude and extent. In this paper these are shown as N-S-transects, as elevationjtime graphs of various paleoclimatic indicators and as maps. The latter give an idea of the extent and the moisture conditions of the Arid Diagonal, as well as of the patterns of modern and past atmospheric circulation.Fil: Abraham, Elena Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Provincia de Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Universidad Nacional de Cuyo. Instituto Argentino de Investigaciones de las Zonas Áridas; ArgentinaFil: Garleff, K.. No especifíca;Fil: Liebricht, H.. No especifíca;Fil: Regairaz, A. C.. No especifíca;Fil: Schäbitz, Frank. No especifíca;Fil: Squeo, Francisco A.. No especifíca;Fil: Stingl, H.. No especifíca;Fil: Veit, H.. No especifíca;Fil: Villagrán, C.. No especifíca

Geomorphology and paleoecology of the arid diagonal in Southern South America

Por medio de investigaciones geomorfológicas, pedológicas y palinológicas se ha reconstruido la localización, extensión y dinámica de la Diagonal Árida para diferentes fases del Cuaternario Superior. Los resultados se muestran a través de perfiles de los pisos altitudinales geoecológicos, por diagramas de variación, tanto temporal como espacial de diferentes indicadores y parámetros paleoclimáticos, y por bosquejos cartográficos de la extensión de la Diagonal Árida, así como de diferentes regímenes pluviométricos, y de la distribución de los sistemas atmosféricos actuales y del  pasado. Estas nvestigaciones paleoecológicas se han complementado en la Región de Cuyo con estudios de los cambios recientes - siglos XVI al XX - en la dinámica ambiental y las adaptaciones de los grupos humanos, enfocados principalmente al proceso de desertificación. estudios de los cambios recientes - siglos XVI al XX - en la dinámica ambiental y las adaptaciones de los grupos humanos, enfocados principalmente al proceso de desertificación.Position, extent and intensity of the Arid Diagonal at various time slices during the upper Quaternary were reconstructed. Geomorphic, pedologic and palynologic evidences indicate significant shifts of the geoecological belts with regard to altitude and extent. In this paper these are shown as N-S-transects, as elevationjtime graphs of various paleoclimatic indicators and as maps. The latter give an idea of the extent and the moisture conditions of the Arid Diagonal, as well as of the patterns of modern and past atmospheric circulation.Fil: Abraham, Elena Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Provincia de Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Universidad Nacional de Cuyo. Instituto Argentino de Investigaciones de las Zonas Áridas; ArgentinaFil: Garleff, K.. No especifíca;Fil: Liebricht, H.. No especifíca;Fil: Regairaz, A. C.. No especifíca;Fil: Schäbitz, Frank. No especifíca;Fil: Squeo, Francisco A.. No especifíca;Fil: Stingl, H.. No especifíca;Fil: Veit, H.. No especifíca;Fil: Villagrán, C.. No especifíca

Confinement and the analytic structure of the one body propagator in Scalar QED

We investigate the behavior of the one body propagator in SQED. The self energy is calculated using three different methods: i) the simple bubble summation, ii) the Dyson-Schwinger equation, and iii) the Feynman-Schwinger represantation. The Feynman-Schwinger representation allows an {\em exact} analytical result. It is shown that, while the exact result produces a real mass pole for all couplings, the bubble sum and the Dyson-Schwinger approach in rainbow approximation leads to complex mass poles beyond a certain critical coupling. The model exhibits confinement, yet the exact solution still has one body propagators with {\it real} mass poles.Comment: 5 pages 2 figures, accepted for publication in Phys. Rev.