Repulsion-Sustained Supercurrent and Flux Quantization in Rings of Symmetric Hubbard Clusters

We test the response to a threading magnetic field of rings of 5-site $C_{4v}$-symmetric repulsive Hubbard clusters connected by weak intercell links; each 5-site unit has the topology of a CuO$_{4}$ cluster and a repulsive interaction is included on every site. In a numerical study of the three-unit ring with 8 particles, we take advantage of a novel exact-diagonalization technique which can be generally applied to many-fermion problems. For O-O hopping we find Superconducting Flux Quantization (SFQ), but for purely Cu-Cu links bound pair propagation is hindered by symmetry. The results agree with W=0 pairing theory.Comment: 4 pages, 2 figure

Time-dependent transport in graphene nanoribbons

We theoretically investigate the time-dependent ballistic transport in metallic graphene nanoribbons after the sudden switch-on of a bias voltage $V$. The ribbon is divided in three different regions, namely two semi-infinite graphenic leads and a central part of length $L$, across which the bias drops linearly and where the current is calculated. We show that during the early transient time the system behaves like a graphene bulk under the influence of a uniform electric field $E=V/L$. In the undoped system the current does not grow linearly in time but remarkably reaches a temporary plateau with dc conductivity $\sigma_{1}=\pi e^{2}/2h$, which coincides with the minimal conductivity of two-dimensional graphene. After a time of order $L/v_{F}$ ($v_{F}$ being the Fermi velocity) the current departs from the first plateau and saturates at its final steady state value with conductivity $\sigma_{2}=2e^{2}/h$ typical of metallic nanoribbons of finite width.Comment: 5 pages, 5 figure

Electronic screening and correlated superconductivity in carbon nanotubes

A theoretical analysis of the superconductivity observed recently in Carbon nanotubes is proposed. We argue that ultra-small (diameter $\sim 0.4 nm$) single wall carbon nanotubes (with transition temperature $T_c\sim 15 ^{o}K$) and entirely end-bonded multi-walled ones ($T_c\sim 12 ^{o}K$) can superconduct by an electronic mechanism, basically the same in both cases. By a Luttinger liquid -like approach, one finds enhanced superconducting correlations due to the strong screening of the long-range part of the Coulomb repulsion. Based on this finding, we perform a detailed analysis on the resulting Hubbard-like model, and calculate transition temperatures of the same order of magnitude as the measured ones.Comment: 6 pages, 1 figure, PACS: 71.10.Pm,74.50.+r,71.20.Tx, to appear in Phys. Rev.

Three-Body and One-Body Channels of the Auger Core-Valence-Valence decay: Simplified Approach

We propose a computationally simple model of Auger and APECS line shapes from open-band solids. Part of the intensity comes from the decay of unscreened core-holes and is obtained by the two-body Green's function $G^{(2)}_{\omega}$, as in the case of filled bands. The rest of the intensity arises from screened core-holes and is derived using a variational description of the relaxed ground state; this involves the two-holes-one-electron propagator $G_{\omega}$, which also contains one-hole contributions. For many transition metals, the two-hole Green's function $G^{(2)}_{\omega}$ can be well described by the Ladder Approximation, but the three-body Green's function poses serious further problems. To calculate $G_{\omega}$, treating electrons and holes on equal footing, we propose a practical approach to sum the series to all orders. We achieve that by formally rewriting the problem in terms of a fictitious three-body interaction. Our method grants non-negative densities of states, explains the apparent negative-U behavior of the spectra of early transition metals and interpolates well between weak and strong coupling, as we demonstrate by test model calculations.Comment: AMS-LaTeX file, 23 pages, 8 eps and 3 ps figures embedded in the text with epsfig.sty and float.sty, submitted to Phys. Rev.

Pairing in Cu-O Models: Clues of Joint Electron-Phonon and Electron-Electron Interactions

We discuss a many-electron Hamiltonian with Hubbard-like repulsive interaction and linear coupling to the phonon branches, having the Cu-O plane of the superconducting cuprates as a paradigm. A canonical transformation extracts an effective two-body problem from the many-body theory. As a prototype system we study the \cu cluster, which yields electronic pairing in the Hubbard model; moreover, a standard treatment of the Jahn-Teller effect predicts distortions that destroy electronic pairing. Remarkably, calculations that keep all the electronic spectrum into account show that vibrations are likely to be synergic with electronic pairing, if the coupling to half-breathing modes predominates, as experiments suggest.Comment: 4 pages, 3 figures, accepted by Phys. Rev.

W=0 pairing in Hubbard and related models of low-dimensional superconductors

Lattice Hamiltonians with on-site interaction $W$ have W=0 solutions, that is, many-body {\em singlet} eigenstates without double occupation. In particular, W=0 pairs give a clue to understand the pairing force in repulsive Hubbard models. These eigenstates are found in systems with high enough symmetry, like the square, hexagonal or triangular lattices. By a general theorem, we propose a systematic way to construct all the W=0 pairs of a given Hamiltonian. We also introduce a canonical transformation to calculate the effective interaction between the particles of such pairs. In geometries appropriate for the CuO$_{2}$ planes of cuprate superconductors, armchair Carbon nanotubes or Cobalt Oxides planes, the dressed pair becomes a bound state in a physically relevant range of parameters. We also show that W=0 pairs quantize the magnetic flux like superconducting pairs do. The pairing mechanism breaks down in the presence of strong distortions. The W=0 pairs are also the building blocks for the antiferromagnetic ground state of the half-filled Hubbard model at weak coupling. Our analytical results for the $4\times 4$ Hubbard square lattice, compared to available numerical data, demonstrate that the method, besides providing intuitive grasp on pairing, also has quantitative predictive power. We also consider including phonon effects in this scenario. Preliminary calculations with small clusters indicate that vector phonons hinder pairing while half-breathing modes are synergic with the W=0 pairing mechanism both at weak coupling and in the polaronic regime.Comment: 42 pages, Topical Review to appear in Journal of Physics C: Condensed Matte

Interacting resonant-level model with long-range interactions: Fast screening and suppression of the zero-bias conductance

The effects of long-range interactions in quantum transport are still largely unexplored, mainly due to the difficulty of devising efficient embedding schemes. In this work we present a substantial progress in the interacting resonant level model by reducing the problem to the solution of Kadanoff-Baym-like equations with a correlated embedding self-energy. The method allows us to deal with short- and long-range interactions and is applicable from the transient to the steady-state regime. Furthermore, memory effects are consistently incorporated and the results are not plagued by negative densities or nonconservation of the electric charge. We employ the method to calculate densities and currents with long-range interactions appropriate to low-dimensional leads, and show the occurrence of a jamming effect, which drastically reduces the screening time and suppresses the zero-bias conductance. None of these effects are captured by short-range model interactions

Modeling creep behaviour of boiler grade steels - Application to grade 92 steel

International audiencePower generation is now striving to improve the global efficiency of boiler plants, targeting 50% or higher. To accommodate the necessary increase in steam temperature, new materials may be developed and/or existing materials must be employed to their best. This is mandatory for the design of boiler components. One contribution by V&M in that direction is the setting up of numerical models for major boiler grades regarding their creep resistance. In coordination with Mines ParisTech, the first application has been made to Grade 92.The methodology is presented together with the first simulations. Typically, creep is modelled using two deformation mechanisms which correspond respectively to a low stress regime and a high stress regime. Damage is also taken into account. Calculations were carried out using ABAQUS ®. Numerical results are compared with a series of creep tests performed at Vallourec Research Aulnoye (VRA). Further development and applications of the model are finally discussed