4,980 research outputs found

    Bi_2 Sr_2 CaCu_2 O_{8+delta} Bicrystal c-Axis Twist Josephson Junctions: A New Phase-Sensitive Test of Order Parameter Symmetry

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    Li {\it et al.} [Phys. Rev. Lett. {\bf 83}, 4160 (1999)] prepared atomically clean Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} (BSCCO) Josephson junctions between identical single crystal cleaves stacked and twisted an angle ϕ0\phi_0 about the c axis. For each bicrystal, the ratio JcJ/JcSJ_c^J/J_c^S of the c-axis twist junction critical current density to that across either single crystal part is unity, independent of ϕ0\phi_0 and the ratio AJ/ASA^J/A^S of junction areas. From extensive theoretical studies involving a variety of tunneling and superconducting order parameter (OP) forms, we conclude that the results provide strong evidence for incoherent c-axis tunneling and that the dominant OP is s-wave for TTcT\le T_c. Recently, Takano {\it et al.} [Phys. Rev. B {\bf 65}, 140315(R) (2002)] obtained results from BSCCO whisker twist junctions which also rule out a pure d-wave OP, but which are surprisingly suggestive of coherent c-axis tunneling form small Fermi surface hot spots.Comment: 5 pages, 11 figures, invited talk at LT23, to be published in Physica

    On the Monodromies of N=2 Supersymmetric Yang-Mills Theory with Gauge Group SO(2n)

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    We present families of algebraic curves describing the moduli-space of N ⁣= ⁣2N\!=\!2 supersymmetric Yang-Mills theory with gauge group SO(2n)SO(2n). We test our curves by computing the weak coupling monodromies and the number of N ⁣= ⁣1N\!=\!1 vacua.Comment: 14 pages, 5 Postscript figures, LaTeX file, uses epsf.st

    What has been the tax competition experience of the past 20 years?

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    This paper describes tax reforms in OECD countries over the last 20 years and how they are related to tax competition. Both individual countries? reforms and multilateralinitiatives and developments are covered. This is followed by an overview of theempirical evidence on tax competition. Our conclusion is that the evidence for someinterdependence in tax setting behaviour is strong, although the exact process driving thisremains unclear. While the most basic tax competition models fail to explain thedevelopment in OECD countries, there is more than one possible explanation for thereforms undertaken if more advanced models are considered. The multilateral initiativesthat were implemented however do not seem to be related to resource-based taxcompetition, instead they are about taxing rights. This paper describes tax reforms in OECD countries over the last 20 years and how they are related to tax competition. Both individual countries? reforms and multilateralinitiatives and developments are covered. This is followed by an overview of theempirical evidence on tax competition. Our conclusion is that the evidence for someinterdependence in tax setting behaviour is strong, although the exact process driving thisremains unclear. While the most basic tax competition models fail to explain thedevelopment in OECD countries, there is more than one possible explanation for thereforms undertaken if more advanced models are considered. The multilateral initiativesthat were implemented however do not seem to be related to resource-based taxcompetition, instead they are about taxing rights

    Mirror Maps and Instanton Sums for Complete Intersections in Weighted Projective Space

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    We consider a class of Calabi-Yau compactifications which are constructed as a complete intersection in weighted projective space. For manifolds with one K\"ahler modulus we construct the mirror manifolds and calculate the instanton sum.Comment: 10 page

    Enhanced Gauge Symmetry in Three-Moduli Models of Type-II String and Hypergeometric Series

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    The conifold singularities in the type-II string are considered as the points of phase transition. In some cases, these singularities can be understood in the framework of the conventional fields theores as the points of enhanced gauge symmetry. We consider a class of three moduli Type-II strings. It is shown the periods can be written in the form of hypergeometric series around the singular points in these models. The leading expansion around the conifold locus turns out to be described by Appell functions. In one singular point, we observe the enhanced gauge symmetry of SU(2)×SU(2)SU(2)\times SU(2) independent of the models. Around another conifold locus, however, the resulting expression of the Appell functions depends on the models. We examine the result by considering a relation between these Appell functions and underlying Riemann surfaces.Comment: 20 pages, Late

    General (Anti-)de Sitter Black Holes in Five Dimensions

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    We find a general class of rotating charged black hole solutions to N=2, D=5 gauged supergravity coupled to vector supermultiplets. The supersymmetry properties of these solutions are studied, and their mass and angular momenta are obtained. We also compute the stress tensor of the dual D=4, N=4 super Yang-Mills theory in the limit of strong `t Hooft coupling. It is shown that closed timelike curves occur outside the horizon, indicating loss of unitarity in the dual CFT. For imaginary coupling constant of the gravitini to the gauge fields, one can obtain multi-centered rotating charged de Sitter black holes. Some physical properties of these solutions are also discussed.Comment: 22 pages, LaTeX, some explanations concerning the appearance of closed timelike curves outside the horizon added in section 5.1, updated references, published versio

    Theory of Coherent cc-Axis Josephson Tunneling between Layered Superconductors

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    We calculate exactly the Josephson current for cc-axis coherent tunneling between two layered superconductors, each with internal coherent tight-binding intra- and interlayer quasiparticle dispersions. Our results also apply when one or both of the superconductors is a bulk material, and include the usually neglected effects of surface states. For weak tunneling, our results reduce to our previous results derived using the tunneling Hamiltonian. Our results are also correct for strong tunneling. However, the cc-axis tunneling results of Tanaka and Kashiwaya are shown to be incorrect in any limit. In addition, we consider the cc-axis coherent critical current between two identical layered superconductors twisted an angle ϕ0\phi_0 about the cc-axis with respect to each other. Regardless of the order parameter symmetry, our coherent tunneling results using a tight-binding intralayer quasiparticle dispersion are inconsistent with the recent cc-axis twist bicrystal Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} twist junction experiments of Li {\it et al.}Comment: 11 pages, 13 figures, submitted to Physical Review

    Radiation from a Josephson STAR-emitter

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    We calculate the angular dependence of the radiation-zone output power and electric polarization of stimulated terahertz amplified radiation (STAR) emitted from a dcdc voltage applied across cylindrical and rectangular stacks of intrinsic Josephson junctions. During coherent emission, a spatially uniform acac Josephson current density in the stack acts as a surface electric current density antenna source, leading to an harmonic radiation frequency spectrum, as in experiment, but absent in all cavity modesl of cylindrical mesas. Spatial fluctuations of the acac Josephson current cause its fundamental mode to lock onto the lowest finite energy cylindrical cavity mode, causing it to resonate, leading to a non-uniform magnetic surface current density radiation source, and a non-trivial combined fundamental frequency output power with linear polarization We also present a model of the superconducting substrate, and present results for rectangular mesas.Comment: 18 pages, 26 figures, submitted to PR

    On the Moduli Space of SU(3) Seiberg-Witten Theory with Matter

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    We present a qualitative model of the Coulomb branch of the moduli space of low-energy effective N=2 SQCD with gauge group SU(3) and up to five flavours of massive matter. Overall, away from double cores, we find a situation broadly similar to the case with no matter, but with additional complexity due to the proliferation of extra BPS states. We also include a revised version of the pure SU(3) model which can accommodate just the orthodox weak coupling spectrum.Comment: 32 pages, 25 figures, uses JHEP.cls, added references, deleted joke

    Zero-field Time Correlation Functions of Four Classical Heisenberg Spins on a Ring

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    A model relevant for the study of certain molecular magnets is the ring of N=4 classical spins with equal near-neighbor isotropic Heisenberg exchange interactions. Assuming classical Heisenberg spin dynamics, we solve explicitly for the time evolution of each of the spins. Exact triple integral representations are derived for the auto, near-neighbor, and next-nearest-neighbor time correlation functions for any temperature. At infinite temperature, the correlation functions are reduced to quadrature. We then evaluate the Fourier transforms of these functions in closed form, which are double integrals. At low temperatures, the Fourier transform functions explicitly demonstrate the presence of magnons. Our exact results for the infinite temperature correlation functions in the long-time asymptotic limit differ qualitatively from those obtained assuming diffusive spin dynamics. Whether such explicitly non-hydrodynamic behavior would be maintained for large-N rings is discussed.Comment: 18 pages, 21 figure
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