169 research outputs found

    Mott Transition in the Hubbard model

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    In this article, I discuss W.Kohn's criterion for a metal insulator transition, within the framework of a one band Hubbard model. This and related ideas are applied to 1-dimensional Hubbard systems, and some intersting miscellaneous results discussed. The Jordan Wigner transformation converting the two species of fermions to two species of hardcore bosons is performed in detail, and the ``extra phases'' arising from odd-even effects are explicitly derived. Bosons are shown to prefer zero flux (i.e. diamagnetism), and the corresponding ``happy fluxes'' for the fermions identified. A curious result following from the interplay between orbital diamagnetism and spin polarization is highlighted. A ``spin-statistics'' like theorem, showing that the anticommutation relations between fermions of opposite spin are crucial to obtain the SU(2) invariance is pointed out.Comment: 15 page

    Spin-Ice and Other Frustrated Magnets on the Pyrochlore Lattice

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    The recent identification of the dysprosium titanate compound Dy2Ti2O7Dy_2 Ti_2 O_7 as a ``Spin-Ice'', i.e. the spin analog of regular entropic ice of Pauling, has created considerable excitement. The ability to manipulate spins using magnetic fields gives a unique advantage over regular ice in these systems, and has been used to study the recovery of entropy. Predicted magnetization plateaus have been observed, testing the underlying model consisting of a competition between short ranged super exchange, and long ranged dipolar interactions between spins. I discuss other compounds that are possibly spin ice like: Ho2Ti2O7Ho_2 Ti_2 O_7, and the two stannates Ho2Sn2O7Ho_2 Sn_2 O_7, Dy2Sn2O7Dy_2 Sn_2 O_7.Comment: 4 pages, Invited Contribution for Low Temperature Conference LT23, August 20-27, Hiroshima JAPAN, to be published in Physica B & C (2003

    SrCu2(BO3)2Sr Cu_2 (BO_3)_2: A Unique Mott Hubbard Insulator

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    We discuss the recently discovered system SrCu2(BO3)2Sr Cu_2 (BO_3)_2, a realization of an exactly solvable model proposed two decades earlier. We propose its interpretation as a Mott Hubbard insulator. The possible superconducting phase arising from doping is explored, and its nature as well as its importance for testing the RVB theory of superconductivity are discussed.Comment: 18 pages,7 figures, Based on Invited Talk at 16th Nishinomiya Yukawa memorial symposium, Nov 13-14, Nishinomiya. To appear in Progress in Theoretical Physics, Suppliment 145(2002

    Majorana Fermion Representation For An Antiferromagnetic Spin-1/2 Chain

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    We study the 1-dimensional Heisenberg antiferromagnet with s=1/2 using a Majorana representation of the s=1/2 spins. A simple Hartree-Fock approximation of the resulting model gives a bilinear fermionic description of the model. This description is rotationally invariant and gives power-law correlations in the ``ground state'' in a natural fashion. The excitations are a two-parameter family of particles, which are spin-1 objects. These are contrasted to the ``spinon'' spectrum, and the technical aspects of the representation are discussed, including the problem of redundant states.Comment: 24 pages in LaTeX, no figures; some clarifications/additions have been made following the referee's comments; to appear in Phys. Rev.

    Exact Spectral Functions of a Non Fermi Liquid in 1 Dimension

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    We study the exact one electron propagator and spectral function of a solvable model of interacting electrons due to Schulz and Shastry. The solution previously found for the energies and wave functions is extended to give the spectral functions, which turn out to be computable, interesting and non trivial. They provide one of the few examples of cases where the spectral functions are known asymptotically as well as exactly.Comment: 14 pages, 5 figures, revte