2,372 research outputs found

    Ferromagnetism in the Hubbard model with Topological/Non-Topological Flat Bands

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    We introduce and study two classes of Hubbard models with magnetic flux or with spin-orbit coupling, which have a flat lowest band separated from other bands by a nonzero gap. We study the Chern number of the flat bands, and find that it is zero for the first class but can be nontrivial in the second. We also prove that the introduction of on-site Coulomb repulsion leads to ferromagnetism in both the classes.Comment: 6 pages, 5 figure

    On the chiral anomaly in non-Riemannian spacetimes

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    The translational Chern-Simons type three-form coframe torsion on a Riemann-Cartan spacetime is related (by differentiation) to the Nieh-Yan four-form. Following Chandia and Zanelli, two spaces with non-trivial translational Chern-Simons forms are discussed. We then demonstrate, firstly within the classical Einstein-Cartan-Dirac theory and secondly in the quantum heat kernel approach to the Dirac operator, how the Nieh-Yan form surfaces in both contexts, in contrast to what has been assumed previously.Comment: 18 pages, RevTe

    Inflation, bifurcations of nonlinear curvature Lagrangians and dark energy

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    A possible equivalence of scalar dark matter, the inflaton, and modified gravity is analyzed. After a conformal mapping, the dependence of the effective Lagrangian on the curvature is not only singular but also bifurcates into several almost Einsteinian spaces, distinguished only by a different effective gravitational strength and cosmological constant. A swallow tail catastrophe in the bifurcation set indicates the possibility for the coexistence of different Einsteinian domains in our Universe. This `triple unification' may shed new light on the nature and large scale distribution not only of dark matter but also on `dark energy', regarded as an effective cosmological constant, and inflation.Comment: 20 pages, 8 figures, Proceedings of the 11th Marcel Grossmann Meeting (MG11) in Berlin, Germany, July 23-29, 200

    Flat-Bands on Partial Line Graphs -- Systematic Method for Generating Flat-Band Lattice Structures

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    We introduce a systematic method for constructing a class of lattice structures that we call ``partial line graphs''.In tight-binding models on partial line graphs, energy bands with flat energy dispersions emerge.This method can be applied to two- and three-dimensional systems. We show examples of partial line graphs of square and cubic lattices. The method is useful in providing a guideline for synthesizing materials with flat energy bands, since the tight-binding models on the partial line graphs provide us a large room for modification, maintaining the flat energy dispersions.Comment: 9 pages, 4 figure

    Preserving and Enhancing Access to Non-Commercial Sound Recordings at The Harry Ransom Center

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    The Harry Ransom Center at The University of Texas at Austin requests funds to support a $35,132 one-year project to develop and complete a preservation survey of the Center’s archival sound recordings. This survey will establish, enhance, and document preservation digitization priorities, processes, and standards to ensure future access to a significant collection of primary research materials