24,661 research outputs found
Dominant Superconducting Fluctuations in the One-Dimensional Extended Holstein-Extended Hubbard model
The search for realistic one-dimensional (1D) models that exhibit dominant
superconducting (SC) fluctuations effects has a long history. In these 1D
systems, the effects of commensurate band fillings--strongest at
half-filling--and electronic repulsions typically lead to a finite charge gap
and the favoring of insulating density wave ordering over superconductivity.
Accordingly, recent proposals suggesting a gapless metallic state in the
Holstein-Hubbard (HH) model, possibly superconducting, have generated
considerable interest and controversy, with the most recent work demonstrating
that the putative dominant superconducting state likely does not exist. In this
paper we study a model with non-local electron-phonon interactions, in addition
to electron-electron interactions, this model unambiguously possesses dominant
superconducting fluctuations at half filling in a large region of parameter
space. Using both the numerical multi-scale functional renormalization group
for the full model and an analytic conventional renormalization group for a
bosonized version of the model, we demonstrate the existence of dominant
superconducting (SC) fluctuations. These dominant SC fluctuations arise because
the spin-charge coupling at high energy is weakened by the non-local
electron-phonon interaction and the charge gap is destroyed by the resultant
suppression of the Umklapp process. The existence of the dominant SC pairing
instability in this half-filled 1D system suggests that non-local
boson-mediated interactions may be important in the superconductivity observed
in the organic superconductors.Comment: 8 pages, 4 figure
Renormalization-group approach to superconductivity: from weak to strong electron-phonon coupling
We present the numerical solution of the renormalization group (RG) equations
derived in Ref. [1], for the problem of superconductivity in the presence of
both electron-electron and electron-phonon coupling at zero temperature. We
study the instability of a Fermi liquid to a superconductor and the RG flow of
the couplings in presence of retardation effects and the crossover from weak to
strong coupling. We show that our numerical results provide an ansatz for the
analytic solution of the problem in the asymptotic limits of weak and strong
coupling.Comment: 8 pages, 3 figures, conference proceedings for the Electron
Correlations and Materials Properties, in Kos, Greece, July 5-9, 200
On the Validity of the Tomonaga Luttinger Liquid Relations for the One-dimensional Holstein Model
For the one-dimensional Holstein model, we show that the relations among the
scaling exponents of various correlation functions of the Tomonaga Luttinger
liquid (LL), while valid in the thermodynamic limit, are significantly modified
by finite size corrections. We obtain analytical expressions for these
corrections and find that they decrease very slowly with increasing system
size. The interpretation of numerical data on finite size lattices in terms of
LL theory must therefore take these corrections into account. As an important
example, we re-examine the proposed metallic phase of the zero-temperature,
half-filled one-dimensional Holstein model without employing the LL relations.
In particular, using quantum Monte Carlo calculations, we study the competition
between the singlet pairing and charge ordering. Our results do not support the
existence of a dominant singlet pairing state.Comment: 7 page
Techniques for the Synthesis of Reversible Toffoli Networks
This paper presents novel techniques for the synthesis of reversible networks
of Toffoli gates, as well as improvements to previous methods. Gate count and
technology oriented cost metrics are used. Our synthesis techniques are
independent of the cost metrics. Two new iterative synthesis procedure
employing Reed-Muller spectra are introduced and shown to complement earlier
synthesis approaches. The template simplification suggested in earlier work is
enhanced through introduction of a faster and more efficient template
application algorithm, updated (shorter) classification of the templates, and
presentation of the new templates of sizes 7 and 9. A novel ``resynthesis''
approach is introduced wherein a sequence of gates is chosen from a network,
and the reversible specification it realizes is resynthesized as an independent
problem in hopes of reducing the network cost. Empirical results are presented
to show that the methods are effective both in terms of the realization of all
3x3 reversible functions and larger reversible benchmark specifications.Comment: 20 pages, 5 figure
b-quark decay in the collinear approximation
The semileptonic decay of a b-quark, b--> c l nu, is considered in the
relativistic limit where the decay products are approximately collinear.
Analytic results for the double differential lepton energy distributions are
given for finite charm-quark mass. Their use for the fast simulation of
isolated lepton backgrounds from heavy quark decays is discussed.Comment: 7 pages, 1 figure, submitted to Phys.Rev.
Phase Diagram of the Holstein-Hubbard Two-Leg Ladder
Using a functional renormalization group method, we obtain the phase diagram
of the two-leg ladder system within the Holstein-Hubbard model, which includes
both electron-electron and electron-phonon interactions. Our renormalization
group technique allows us to analyze the problem for both weak and strong
electron-phonon coupling. We show that, in contrast results from conventional
weak coupling studies, electron-phonon interactions can dominate
electron-electron interactions because of retardation effects.Comment: 4 page
Finite size effects on thermal denaturation of globular proteins
Finite size effects on the cooperative thermal denaturation of proteins are
considered. A dimensionless measure of cooperativity, Omega, scales as N^zeta,
where N is the number of amino acids. Surprisingly, we find that zeta is
universal with zeta = 1 + gamma, where the exponent gamma characterizes the
divergence of the susceptibility for a self-avoiding walk. Our lattice model
simulations and experimental data are consistent with the theory. Our finding
rationalizes the marginal stability of proteins and substantiates the earlier
predictions that the efficient folding of two-state proteins requires the
folding transition temperature to be close to the collapse temperature.Comment: 3 figures. Physical Review Letters (in press
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