Insect Phylogenetics: A Guided Tour of Insect Evolution

Interview with Dr. Noah Whitema

Effect of magnetic and non-magnetic impurities on highly anisotropic superconductivity

We generalize Abrikosov-Gor'kov solution of the problem of weakly coupled superconductor with impurities on the case of a multiband superconductor with arbitrary interband order parameter anisotropy, including interband sign reversal of the order parameter. The solution is given in terms of the effective (renormalized) coupling matrix and describes not only $T_c$ suppression but also renormalization of the superconducting gap basically at all temperatures. In many limiting cases we find analytical solutions for the critical temperature suppression. We illustrate our results by numerical calculations for two-band model systems.Comment: 18 pages (12pt) RevTeX, 4 postscript figure

On the T-dependence of the magnetic penetration depth in unconventional superconductors at low temperatures: can it be linear?

We present a thermodynamics argument against a strictly linear temperature dependence of the magnetic penetration depth, which applies to superconductors with arbitrary pairing symmetry at low temperatures.Comment: 5 pages, expanded version of cond-mat/971102

Evolution of the pairing pseudogap in the spectral function with interplane anisotropy

We study the pairing pseudogap in the spectral function as a function of interplane coupling. The analytical expressions for the self-energy in the critical regime are obtained for any degree of anisotropy. The frequency dependence of the self-energy is found to be qualitatively different in two and three dimensions, and the crossover from two to three dimensional behavior is discussed. In particular, by considering the anisotropy of the Fermi velocity and gap along the Fermi surface, we can qualitatively explain recent photoemission experiments on high temperature superconductors concerning the temperature dependent Fermi arcs seen in the pseudogap phase.Comment: 20 pages, revtex, 5 encapsulated postscript figures include

Transport Properties Calculation for a Quasi-Bidimensional System using T-Matrix Approximation

We performed a self-consistent calculation using T-Matrix approximation for a quasi-bidimensional system. We calculated the one particle spectrum function A(k,\omega) in the presence of strong d-wave attractive interaction. The c-axis charge dynamics was studied by considering incoherent interlayer hopping and ab-plane charge dynamics was studied in the coherent limit. It is shown that the c-axis charge dynamics is mainly governed by the scattering from the in plane fluctuations. We also present results for c-axis and ab-plane resistivity and for thermopower coefficient.Comment: 10 pages, 13 figures, accepted for publication in Physica

Plane - Chain coupling in YBa_{2}Cu_{3}O_{7} : temperature dependence of the penetration depth

We have studied the penetration depth for a model of $YBa_{2}Cu_{3}O_{7}$ involving pairing both in the $CuO_{2}$ planes and in the CuO chains. In this model pairing in the planes is due to an attractive interaction, while Coulomb repulsion induces in the chains an order parameter with opposite sign. Due to the anticrossing produced by hybridization between planes and chains, one obtains a d-wave like order parameter which changes sign on a single sheet of the Fermi surface and has nodes in the gap. We find that our model accounts quite well for the anisotropy of the penetration depth and for the absolute values. We reproduce fairly well the whole temperature dependence for both the a and the b directions, including the linear dependence at low temperature. We use a set of parameters which are all quite reasonable physically. Our results for the c direction are also satisfactory, although the situation is less clear both experimentally and theoretically.Comment: 11 pages, revtex, 6 figure

Effect of spatial variations of superconducting gap on suppression of the transition temperature by impurities

We calculate correction to the critical temperature of a dirty superconductor, which results from the local variations of the gap function near impurity sites. This correction is of the order of T_c/E_F and becomes important for short-coherence length superconductors. It generally reduces a pair-breaking effect. In s-wave superconductors small amounts of nonmagnetic impurities can increase the transition temperature.Comment: 5 pages, ReVTE

Vortex Matter Transition in Bi2{}_2Sr2{}_2CaCu2{}_2O8+y{}_{8+y} under Tilted Fields

Vortex phase diagram under tilted fields from the $c$ axis in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+y}$ is studied by local magnetization hysteresis measurements using Hall probes. When the field is applied at large angles from the $c$ axis, an anomaly ($H_p^\ast$) other than the well-known peak effect ($H_p$) are found at fields below $H_p$. The angular dependence of the field $H_p^\ast$ is nonmonotonic and clearly different from that of $H_p$ and depends on the oxygen content of the crystal. The results suggest existence of a vortex matter transition under tilted fields. Possible mechanisms of the transition are discussed.Comment: Revtex, 4 pages, some corrections are adde