184 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

    Theory of extreme correlations using canonical Fermions and path integrals

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    The t-J model is studied using a novel and rigorous mapping of the Gutzwiller projected electrons, in terms of canonical electrons. The mapping has considerable similarity to the Dyson-Maleev transformation relating spin operators to canonical Bosons. This representation gives rise to a non Hermitean quantum theory, characterized by minimal redundancies. A path integral representation of the canonical theory is given. Using it, the salient results of the extremely correlated Fermi liquid (ECFL) theory, including the previously found Schwinger equations of motion, are easily rederived. Further a transparent physical interpretation of the previously introduced auxiliary Greens functions and the caparison factor is obtained. The low energy electron spectral function in this theory with a strong intrinsic asymmetry, is summarized in terms of a few expansion coefficients. These include an important emergent energy scale Δ0\Delta_0 that shrinks to zero on approaching the insulating state, thereby making it difficult to access the underlying low energy Fermi liquid behavior. The scaled low frequency ECFL spectral function is related simply to the Fano line shape. The resulting energy dispersion (EDC or MDC) is a hybrid of a massive and a massless Dirac spectrum EQγQΓ02+Q2 E^*_Q\sim \gamma\, Q- \sqrt{\Gamma_0^2 + Q^2} , where the vanishing of QQ, a momentum like variable, locates the kink. Therefore the quasiparticle velocity interpolates between (γ1)(\gamma \mp 1) over a width Γ0\Gamma_0 on the two sides of Q=0Q=0. The resulting kink strongly resembles a prominent low energy feature seen in angle resolved photoemission spectra (ARPES) of cuprate materials. We also propose novel ways of analyzing the ARPES data to isolate the predicted asymmetry between particle and hole excitations.Comment: 27 pages, 1 Figure, Updated figure and discussion of the "kink" featur

    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.