4,354 research outputs found

    Novel Mott Transitions in Non-Isotropic Two-Band Hubbard Model

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    The Mott transition in a two-band Hubbard model involving subbands of different widths is studied as a function of temperature using dynamical mean field theory combined with exact diagonalization. The phase diagram is shown to exhibit two successive first-order transitions if the full Hund's rule coupling is included. In the absence of spin-flip and pair-exchange terms the lower transition remains first-order while the upper becomes continuous.Comment: 4 pages, 4 figures improved results for n_s=

    Transition into a low temperature superconducting phase of unconventional pinning in Sr_2RuO_4

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    We have found a sharp transition in the vortex creep rates at a temperature T^\ast=0.05 T_c in a single crystal of Sr_2RuO_4 (T_{c}=1.03 K) by means of magnetic relaxation measurements. For T<T^\ast, the initial creep rates drop to undetectable low levels. One explanation for this transition into a phase with such extremely low vortex creep is that the low-temperature phase of Sr_2RuO_4 breaks time reversal symmetry. In that case, degenerate domain walls separating discreetly degenerate states of a superconductor can act as very strong pinning centers.Comment: 3 pages, 2 figure

    Prediction of Orbital Ordering in Single-Layered Ruthenates

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    The key role of the orbital degree of freedom to understand the magnetic properties of layered ruthenates is here discussed. In the G-type antiferromagnetic phase of Ca2_2RuO4_4, recent X-ray experiments reported the presence of 0.5 hole per site in the dxyd_{xy} orbital, while the dyzd_{\rm yz} and dzxd_{zx} orbitals contain 1.5 holes. This unexpected t2gt_{2g} hole distribution is explained by a novel state with orbital ordering (OO), stabilized by a combination of Coulomb interactions and lattice distortions. In addition, the rich phase diagram presented here suggests the possibility of large magnetoresistance effects, and predicts a new ferromagnetic OO phase in ruthenates.Comment: 4 pages, Revtex, with 2 figures embedded in the text. Submitted to Phys. Rev. Let

    Mechanism of hopping transport in disordered Mott insulators

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    By using a combination of detailed experimental studies and simple theoretical arguments, we identify a novel mechanism characterizing the hopping transport in the Mott insulating phase of Ca2x_{2-x}Srx_xRuO4_4 near the metal-insulator transition. The hopping exponent α\alpha shows a systematic evolution from a value of α=1/2\alpha=1/2 deeper in the insulator to the conventional Mott value α=1/3\alpha=1/3 closer to the transition. This behavior, which we argue to be a universal feature of disordered Mott systems close to the metal-insulator transition, is shown to reflect the gradual emergence of disorder-induced localized electronic states populating the Mott-Hubbard gap.Comment: 5 pages, 3 figures, To be published in Physical Review Letter

    Quasi-particle Density in Sr2RuO4 Probed by means of the Phonon Thermal Conductivity

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    The thermal conductivity of Sr2RuO4 along the least conducting direction perpendicular to the RuO2 plane has been studied down to 0.3 K. In this configuration the phonons remain the dominant heat carriers down to the lowest temperature, and their conductivity in the normal state is determined by the scattering on conduction electrons. We show that the phonon mean free path in the superconducting state is sensitive to the density of the quasi-particles in the bulk. An unusual magnetic field dependence of the phonon thermal conductivity is ascribed to the anisotropic superconducting gap structure in Sr2RuO4.Comment: 14 pages, 6 eps figures, Latex. This article corresponds to the reference 25 of Phys. Rev. Lett. vol.86 page2649-2652 (2001) and cond-mat/010449

    Observation of Andreev Surface Bound States in the 3-K phase Region of Sr_2RuO_4

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    The tunneling spectrum of the superconducting phase with T_c ~ 3.0 K has been measured in the Ru-embedded region of Sr_2RuO_4 using cleaved junctions. A sharp zero-bias conductance peak (ZBCP) has been observed below 3 K. All characteristics of this ZBCP suggest that it originates from Andreev surface bound states, indicating that the pairing in the 3-K phase is also non-s-wave. Below the bulk T_c of Sr_2RuO_4 (~1.5 K), a bell-shaped ZBCP was found. This supports that there is a phase transition in the 3-K phase region near the bulk T_c.Comment: 4 pages, to appear in Phys. Rev. Lett. 87 (2001