699 research outputs found
Kondo effect in a one dimensional d-wave superconductor
We derive a solvable resonant-level type model, to describe an impurity spin
coupled to zero-energy bound states localized at the edge of a one dimensional
d-wave superconductor. This results in a two-channel Kondo effect with a quite
unusual low-temperature thermodynamics. For instance, the local impurity
susceptibility yields a finite maximum at zero temperature (but no
logarithmic-divergence) due to the splitting of the impurity in two Majorana
fermions. Moreover, we make comparisons with the Kondo effect occurring in a
two dimensional d-wave superconductor.Comment: 9 pages, final version; To be published in Europhysics Letter
Andreev scattering in the asymmetric ladder with preformed bosonic pairs
We discuss the phase coherence which emanates from the ladder-like proximity
effect between a ``weak superconductor'' with preformed bosonic pairs (here, a
single-chain Luther-Emery liquid with superconducting correlations that decay
approximately as ) and a Fermi gas with unpaired fermions. Carefully
studying tunneling mechanism(s), we show that the boson-mediated Cooper pairing
between remaining unpaired electrons results in a quasi long-range
superconductivity: Superconducting correlations decay very slowly as
with . This process is reminiscent of the coupling
of fermions to preformed bosonic pairs introduced in the context of high-Tc
cuprates.Comment: 5 pages, final version (To appear in PRB Rapid Communication
Quality of life in survivors after cervical artery dissection
Background and purpose : Little data exists about longterm outcome, quality of life (QOL) and its predictors after spontaneous cervical artery dissections (sCAD). Methods : Clinical and radiological data of 114 patients with sCAD were collected prospectively. Six patients died within 3 months, the remaining 108 were contacted after a mean of 1498 days (range: 379-3455), 99 survivors (92 %) replied. QOL, assessed with the stroke-specific QOL scale (SSQOL), and functional abilities, measured with modified Rankin Scale (mRS) were compared, and predictors of QOL were analyzed. Subgroup analyses were performed for patients with ischemic stroke, those with isolated local symptoms or transient ischemic symptoms and those without significant disabilities (mRS 0-1) at follow-up. Results : Seventy-one of 99 patients (72 %) had no significant disability, but only 53 (54 %) reported a good QOL (SS-QOL ≥ 4). Compared to the self-rated premorbid QOL of all patients, SS-QOL was impaired after sCAD (p 0.5). High National Institute of Health Stroke Scale score on admission and higher age were independent predictors of impaired QOL (p < 0.05). Conclusion : QOL is impaired in almost half of long-term survivors after sCAD, even in patients with local or transient symptoms or without functional disability. Impairment of QOL is a surprisingly frequent long-term sequela after sCAD and deserves attention as an outcome measure in these patient
Inhomogeneously doped two-leg ladder systems
A chemical potential difference between the legs of a two-leg ladder is found
to be harmful for Cooper pairing. The instability of superconductivity in such
systems is analyzed by compairing results of various analytical and numerical
methods. Within a strong coupling approach for the t-J model, supplemented by
exact numerical diagonalization, hole binding is found unstable beyond a
finite, critical chemical potential difference. The spinon-holon mean field
theory for the t-J model shows a clear reduction of the the BCS gaps upon
increasing the chemical potential difference leading to a breakdown of
superconductivity. Based on a renormalization group approach and Abelian
bosonization, the doping dependent phase diagram for the weakly interacting
Hubbard model with different chemical potentials was determined.Comment: Revtex4, 11 pages, 7 figure
Fermi surface renormalization in Hubbard ladders
We derive the one-loop renormalization equations for the shift in the
Fermi-wavevectors for one-dimensional interacting models with four Fermi-points
(two left and two right movers) and two Fermi velocities v_1 and v_2. We find
the shift to be proportional to (v_1-v_2)U^2, where U is the Hubbard-U. Our
results apply to the Hubbard ladder and to the t_1-t_2 Hubbard model. The
Fermi-sea with fewer particles tends to empty. The stability of a saddle point
due to shifts of the Fermi-energy and the shift of the Fermi-wavevector at the
Mott-Hubbard transition are discussed.Comment: 5 pages, 4 Postscript figure
Rhizobium determinants of rhizosphere persistence and root colonization
Bacterial persistence in the rhizosphere and colonization of root niches are critical for the establishment of many beneficial plant–bacteria interactions including those between Rhizobium leguminosarum and its host legumes. Despite this, most studies on R. leguminosarum have focused on its symbiotic lifestyle as an endosymbiont in root nodules. Here, we use random barcode transposon sequencing to assay gene contributions of R. leguminosarum during competitive growth in the rhizosphere and colonization of various plant species. This facilitated the identification of 189 genes commonly required for growth in diverse plant rhizospheres, mutation of 111 of which also affected subsequent root colonization (rhizosphere progressive), and a further 119 genes necessary for colonization. Common determinants reveal a need to synthesize essential compounds (amino acids, ribonucleotides, and cofactors), adapt metabolic function, respond to external stimuli, and withstand various stresses (such as changes in osmolarity). Additionally, chemotaxis and flagella-mediated motility are prerequisites for root colonization. Many genes showed plant-specific dependencies highlighting significant adaptation to different plant species. This work provides a greater understanding of factors promoting rhizosphere fitness and root colonization in plant-beneficial bacteria, facilitating their exploitation for agricultural benefit
Interaction-induced Fermi surface deformations in quasi one-dimensional electronic systems
We consider serious conceptual problems with the application of standard
perturbation theory, in its zero temperature version, to the computation of the
dressed Fermi surface for an interacting electronic system. In order to
overcome these difficulties, we set up a variational approach which is shown to
be equivalent to the renormalized perturbation theory where the dressed Fermi
surface is fixed by recursively computed counterterms. The physical picture
that emerges is that couplings that are irrelevant tend to deform the Fermi
surface in order to become more relevant (irrelevant couplings being those that
do not exist at vanishing excitation energy because of kinematical constraints
attached to the Fermi surface). These insights are incorporated in a
renormalization group approach, which allows for a simple approximate
computation of Fermi surface deformation in quasi one-dimensional electronic
conductors. We also analyze flow equations for the effective couplings and
quasiparticle weights. For systems away from half-filling, the flows show three
regimes corresponding to a Luttinger liquid at high energies, a Fermi liquid,
and a low-energy incommensurate spin-density wave. At half-filling Umklapp
processes allow for a Mott insulator regime where the dressed Fermi surface is
flat, implying a confined phase with vanishing effective transverse
single-particle coherence. The boundary between the confined and Fermi liquid
phases is found to occur for a bare transverse hopping amplitude of the order
of the Mott charge gap of a single chain.Comment: 38 pages, 39 figures. Accepted for publication in Phys. Rev.
From nodal liquid to nodal Mottness in a frustrated Hubbard model
We investigate the physics of frustrated 3-leg Hubbard ladders in the band
limit, when hopping across the ladder's rungs (t) is of the same
order as hopping along them (t) much greater than the onsite Coulomb repulsion
(U). We show that this model exhibits a striking electron-hole asymmetry close
to half-filling: the hole-doped system at low temperatures develops a
Resonating Valence Bond (RVB)-like d-wave gap (pseudogap close to (,0))
coinciding with gapless nodal excitations (nodal liquid); in contrast, the
electron-doped system is seen to develop a Mott gap at the nodes, whilst
retaining a metallic character of its majority Fermi surface. At lower
temperatures in the electron-doped case, d-wave superconducting correlations --
here, coexisting with gapped nodal excitations -- are already seen to arise.
Upon further doping the hole-doped case, the RVB-like state yields to d-wave
superconductivity. Such physics is reminiscent of that exhibited by the high
temperature cuprate superconductors--notably electron-hole asymmetry as noted
by Angle Resolved PhotoEmission Spectroscopy (ARPES) and the resistivity
exponents observed. This toy model also reinforces the importance of a more
thorough experimental investigation of the known 3-leg ladder cuprate systems,
and may have some bearing on low dimensional organic superconductors.Comment: 26 pages, 16 figure
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