Origin of Gap Anisotropy in Spin Fluctuation Models of the Fe-pnictides

We discuss the large gap anisotropy found for the A1g (s-wave) state in RPA spin-fluctuation and functional renormalization group calculations and show how the simple arguments leading to isotropic sign-switched s-wave states in these systems need to be supplemented by a consideration of pair scattering within Fermi surface sheets and between the individual electron sheets as well. In addition, accounting for the orbital makeup of the states on the Fermi surface is found to be crucial.Comment: 6 pages, 7 figure

Dopant-modulated pair interaction in cuprate superconductors

Comparison of recent experimental STM data with single-impurity and many-impurity Bogoliubov-de Gennes calculations strongly suggests that random out-of-plane dopant atoms in cuprates modulate the pair interaction locally. This type of disorder is crucial to understanding the nanoscale electronic structure inhomogeneity observed in BSCCO-2212, and can reproduce observed correlations between the positions of impurity atoms and various aspects of the local density of states such as the gap magnitude and the height of the coherence peaks. Our results imply that each dopant atom modulates the pair interaction on a length scale of order one lattice constant.Comment: 5 pages, 4 figure

Theory of Thermal Conductivity in YBa_2Cu_3O_{7-\delta}

We calculate the electronic thermal conductivity in a d-wave superconductor, including both the effect of impurity scattering and inelastic scattering by antiferromagnetic spin fluctuations. We analyze existing experiments, particularly with regard to the question of the relative importance of electronic and phononic contributions to the heat current, and to the influence of disorder on low-temperature properties. We find that phonons dominate heat transport near T_c, but that electrons are responsible for most of the peak observed in clean samples, in agreement with a recent analysis of Krishana et al. In agreement with recent data on YBa_2(Cu_1-xZn_x)_3O_7-\delta the peak position is found to vary nonmonotonically with disorder.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let

Local modulations of the spin-fluctuation mediated pairing interaction by impurities in d-wave superconductors

We present a self-consistent real space formulation of spin-fluctuation mediated d-wave pairing. By calculating all relevant inhomogeneous spin and charge susceptibilities in real space within the random phase approximation (RPA), we obtain the effective pairing interaction and study its spatial dependence near both local potential and hopping impurities. A remarkably large enhancement of the pairing interaction may be obtained near the impurity site. We discuss the relevance of our result to inhomogeneities observed by scanning tunneling spectroscopy on the surface of cuprate superconductors.Comment: 8 pages, 7 figure

Control of vegetative growth of grapevine shoots by ethylene-releasing substances Conditions and sites of action

Grapevine shoots of the cvs. Muscat Hamburg and Perlette were treated with two commercial ethylene-releasing substances, Ethrel (2-chloroethylphosphonic acid) and Alsol (2Tchloroethyl-tris-(2-methoxyethoxy)silane) at acid and neutral pH. Ethrel caused significant shoot growth inhibition, Alsol had no effect on the growth rate. Ethrel was more active at pH 6.9 than at pH 2.2, and its uptake was significantly higher at the low pH. In many cases the apical meristem of Ethrel-treated shoots was damaged. The relation of uptake to growth response was determined. Leaf blade treatments alone did not inhibit the growth of the apex.Die Kontrolle des vegetativen Wachstums von Rebtrieben durch Äthylen-liefernde Substanzen Einwirkungsbedingungen und -orteTriebe der Rebsorten Muskat Hamburg und Perlette wurden mit zwei Äthylen-freisetzenden Handelspräparaten behandelt: Ethrel (2-Chloräthylphosphonsäure) und Alsol (2-Chloräthyl-tris-(2-methoxyäthoxy)silan) bei saurem und neutralem pH. Ethrel verursachte eine signifikante Hemmung des Triebwachstums, Alsol hatte keinen Einfluß auf die Wachstumsgeschwindigkeit. Ethrel war bei pH 6,9 wirksamer als bei pH 2,2; im niedrigen pH-Bereich war seine Aufnahme signifikant erhöht. In den behandelten Sprossen war vielfach das apikale Meristem geschädigt. Die Beziehung zwischen Substanzaufnahme und Wachstumsreaktion wurde bestimmt. Bei alleiniger Blattbehandlung war das Wachstum der Triebspitze nicht gehemmt

Andreev states near short-ranged pairing potential impurities

We study Andreev states near atomic scale modulations in the pairing potential in both $s$- and d-wave superconductors with short coherence lengths. For a moderate reduction of the local gap, the states exist only close to the gap edge. If one allows for local sign changes of the order parameter, however, resonances can occur at energies close to the Fermi level. The local density of states (LDOS) around such pairing potential defects strongly resembles the patterns observed by tunneling measurements around Zn impurities in Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (BSCCO). We discuss how this phase impurity model of the Zn LDOS pattern can be distinguished from other proposals experimentally.Comment: 4 pages, 4 figure

Scattering by impurity-induced order parameter ``holes'' in d-wave superconductors

Nonmagnetic impurities in d-wave superconductors cause strong local suppressions of the order parameter. We investigate the observable effects of the scatterigng off such suppressions in bulk samples by treating the order parameter "hole" as a pointlike off-diagonal scatterer treated within a self-consistent t-matrix approximation. Strong scattering potentials lead to a finite-energy spectral feature in the d-wave "impurity band", the observable effects of which include enhanced low-temperature microwave power absorption and a stronger sensitivity of the London penetration depth to disorder than found previously in simpler ``dirty'' d-wave models.Comment: 5 pp. Revtex including 4 postscript figures, submitted to Phys. Rev.