1000 resultados para Korrelierte Elektronensysteme, Festkörperphysik, Hubbard-Modell, Dynamische Molekularfeld-Theorie


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Mode of access: Internet.

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Separatabdruck aus der Metallarbeiter-Zeitung.

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Mode of access: Internet.

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Mode of access: Internet.

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Errata slip inserted.

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Approximately nine-tenths of bound vol. are blank leaves.

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We study a generalized Hubbard model on the two-leg ladder at zero temperature, focusing on a parameter region with staggered flux (SF)/d-density wave (DDW) order. To guide our numerical calculations, we first investigate the location of a SF/DDW phase in the phase diagram of the half-filled weakly interacting ladder using a perturbative renormalization group (RG) and bosonization approach. For hole doping 6 away from half-filling, finite-system density-matrix renormalizationgroup (DMRG) calculations are used to study ladders with up to 200 rungs for intermediate-strength interactions. In the doped SF/DDW phase, the staggered rung current and the rung electron density both show periodic spatial oscillations, with characteristic wavelengths 2/delta and 1/delta, respectively, corresponding to ordering wavevectors 2k(F) and 4k(F) for the currents and densities, where 2k(F) = pi(1 - delta). The density minima are located at the anti-phase domain walls of the staggered current. For sufficiently large dopings, SF/DDW order is suppressed. The rung density modulation also exists in neighboring phases where currents decay exponentially. We show that most of the DMRG results can be qualitatively understood from weak-coupling RG/bosonization arguments. However, while these arguments seem to suggest a crossover from non-decaying correlations to power-law decay at a length scale of order 1/delta, the DMRG results are consistent with a true long-range order scenario for the currents and densities. (c) 2005 Elsevier Inc. All rights reserved.

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The problem of strongly correlated electrons in one dimension attracted attention of condensed matter physicists since early 50’s. After the seminal paper of Tomonaga [1] who suggested the first soluble model in 1950, there were essential achievements reflected in papers by Luttinger [2] (1963) and Mattis and Lieb [3] (1963). A considerable contribution to the understanding of generic properties of the 1D electron liquid has been made by Dzyaloshinskii and Larkin [4] (1973) and Efetov and Larkin [5] (1976). Despite the fact that the main features of the 1D electron liquid were captured and described by the end of 70’s, the investigators felt dissatisfied with the rigour of the theoretical description. The most famous example is the paper by Haldane [6] (1981) where the author developed the fundamentals of a modern bosonisation technique, known as the operator approach. This paper became famous because the author has rigourously shown how to construct the Fermi creation/anihilation operators out of the Bose ones. The most recent example of such a dissatisfaction is the review by von Delft and Schoeller [7] (1998) who revised the approach to the bosonisation and came up with what they called constructive bosonisation.

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We modify a nonlinear σ model (NLσM) for the description of a granular disordered system in the presence of both the Coulomb repulsion and the Cooper pairing. We show that under certain controlled approximations the action of this model is reduced to the Ambegaokar-Eckern-Schön (AES) action, which is further reduced to the Bose-Hubbard (or “dirty-boson”) model with renormalized coupling constants. We obtain an effective action which is more general than the AES one but still simpler than the full NLσM action. This action can be applied in the region of parameters where the reduction to the AES or the Bose-Hubbard model is not justified. This action may lead to a different picture of the superconductor-insulator transition in two-dimensional systems.

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A dolgozatban a Neumann-modell lehetséges elméleti és módszertani rokonságát elemezzük annak fényében, hogy mind a neoklasszikusok, mind a klasszikus hagyományokat felélesztő neoricardiánusok a magukénak vallják. Ennek során megvizsgáljuk a klasszikus és a neoklasszikus gazdaságfelfogás, az ex post és az ex ante szemléletű modellek közötti különbségeket, és azt a forradalmi jelentőségű módszertani változást, amely a sok szempontból joggal bírálható modern matematikai közgazdaságtan kialakulásához vezetett. Összevetjük Neumann modelljét az osztrák iskola árbeszámítási elméletével, a Walras­Cassel- és a Schlesinger­Wald-féle modellekkel, illetve a Ricardo, Marx, Dmitriev, Leontief nevekkel fémjelezhető klasszikus vonulat eredményeivel. Rámutatunk arra, hogy Neumann voltaképpen az "igazságos és értelmes gazdaság" ősi ideáját öntötte kora modern fizikájában honos matematikai modell formájába. /===/ The paper investigates the potential theoretical and methodological sources of inspiration of the von Neumann model, in view of the fact that both the neoclassical and the neo-Ricardian economists claim heritage to it. In the course of that the author assesses the main differences of the classical and neoclassical, the ex post and ex ante modeling approaches. He also confronts the von Neumann model with the Walras–Cassel and the Schlesinger–Wald models, and with models worked out in the classical tradition a’la Ricardo, Marx, Dmitriev and Leontief. He concludes that the Neumann-model is, in fact, nothing but a reformulation of a very old belief in a “just and reasonable economic system” based on the modern modeling approach of contemporary physics and mathematics.