20,716 research outputs found

    The Spin Stiffness and the Transverse Susceptibility of the Half-filled Hubbard Model

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    The T=0T=0 spin stiffness ρs\rho _{s} and the transverse susceptibility χ\chi _{\perp } of the square lattice half-filled Hubbard model are calculated as a function of the Hubbard parameter ratio U/tU/t by series expansions around the Ising limit. We find that the calculated spin-stiffness, transverse susceptibility, and sublattice magnetization for the Hubbard model smoothly approach the Heisenberg values for large U/tU/t. The results are compared for different U/tU/t with RPA and other numerical studies.Comment: 9 Revtex pages, 3 Postscript figures, Europhys. Lett. in pres

    Linear relations among 4-point functions in the high energy limit of string theory

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    The decoupling of zero-norm states leads to linear relations among 4-point functions in the high energy limit of string theory. Recently it was shown that the linear relations uniquely determine ratios among 4-point functions at the leading order. The purpose of this paper is to extend the validity of the same approach to the next-to-leading order and higher orders.Comment: 17 pages, references added, minor modificatio

    Evolution of Surface Deformations of Weakly-Bound Nuclei in the Continuum

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    We study weakly-bound deformed nuclei based on the coordinate-space Skyrme Hartree-Fock-Bogoliubov approach, in which a large box is employed for treating the continuum and surface diffuseness. Approaching the limit of core-halo deformation decoupling, calculations found an exotic "egg"-like structure consisting of a spherical core plus a prolate halo in 38^{38}Ne, in which the resonant continuum plays an essential role. Generally the halo probability and the decoupling effect in heavy nuclei are reduced compared to light nuclei, due to denser level densities around Fermi surfaces. However, deformed halos in medium-mass nuclei are possible with sparse levels of negative parity, for example, in 110^{110}Ge. The surface deformations of pairing density distributions are also influenced by the decoupling effect and are sensitive to the effective pairing Hamiltonian.Comment: 5 pages and 5 figure

    Blandford-Znajek mechanism in black holes in alternative theories of gravity

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    According to the Blandford-Znajek mechanism, black hole jets are powered by the rotational energy of the compact object. In this work, we consider the possibility that the metric around black holes may not be described by the Kerr solution and we study how this changes the Blandford-Znajek model. If the Blandford-Znajek mechanism is responsible for the formation of jets, the estimate of the jet power in combination with another measurement can test the nature of black hole candidates and constrain possible deviations from the Kerr solution. However, this approach might become competitive with respect to other techniques only when it will be possible to have measurements much more precise than those available today.Comment: 12 pages, 5 figures. v2: refereed versio