32,656 research outputs found

    Size and shape of Mott regions for fermionic atoms in a two-dimensional optical lattice

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    We investigate the harmonic-trap control of size and shape of Mott regions in the Fermi Hubbard model on a square optical lattice. The use of Lanczos diagonalization on clusters with twisted boundary conditions, followed by an average over 50-80 samples, drastically reduce finite-size effects in some ground state properties; calculations in the grand canonical ensemble together with a local-density approximation (LDA) allow us to simulate the radial density distribution. We have found that as the trap closes, the atomic cloud goes from a metallic state, to a Mott core, and to a Mott ring; the coverage of Mott atoms reaches a maximum at the core-ring transition. A `phase diagram' in terms of an effective density and the on-site repulsion is proposed, as a guide to maximize the Mott coverage. We also predict that the usual experimentally accessible quantities, the global compressibility and the average double occupancy (rather, its density derivative) display detectable signatures of the core-ring transition. Some spin correlation functions are also calculated, and predict the existence N\'eel ordering within Mott cores and rings.Comment: 5 pages, 6 figure

    Disordered two-dimensional superconductors: roles of temperature and interaction strength

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    We have considered the half-filled disordered attractive Hubbard model on a square lattice, in which the on-site attraction is switched off on a fraction ff of sites, while keeping a finite UU on the remaining ones. Through Quantum Monte Carlo (QMC) simulations for several values of ff and UU, and for system sizes ranging from 8×88\times 8 to 16×1616\times 16, we have calculated the configurational averages of the equal-time pair structure factor PsP_s, and, for a more restricted set of variables, the helicity modulus, ρs\rho_s, as functions of temperature. Two finite-size scaling {\it ansatze} for PsP_s have been used, one for zero-temperature and the other for finite temperatures. We have found that the system sustains superconductivity in the ground state up to a critical impurity concentration, fcf_c, which increases with UU, at least up to U=4 (in units of the hopping energy). Also, the normalized zero-temperature gap as a function of ff shows a maximum near f0.07f\sim 0.07, for 2U62\lesssim U\lesssim 6. Analyses of the helicity modulus and of the pair structure factor led to the determination of the critical temperature as a function of ff, for U=3,U=3, 4 and 6: they also show maxima near f0.07f\sim 0.07, with the highest TcT_c increasing with UU in this range. We argue that, overall, the observed behavior results from both the breakdown of CDW-superconductivity degeneracy and the fact that free sites tend to "push" electrons towards attractive sites, the latter effect being more drastic at weak couplings.Comment: 9 two-column pages, 14 figures, RevTe

    Performance and asset management effects of bank acquisitions

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    An analysis of how bank acquisitions affect the performance and asset management of the acquired bank, its acquirer, and the newly formed banking organization, showing that after the acquisition, the acquired bank is transformed along a wide variety of dimensions such that it becomes a replica of the acquirer.Asset-liability management ; Bank mergers
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