Spin-orbit scattering in d-wave superconductors

When non-magnetic impurities are introduced in a d-wave superconductor, both thermodynamic and spectral properties are strongly affected if the impurity potential is close to the strong resonance limit. In addition to the scalar impurity potential, the charge carriers are also spin-orbit coupled to the impurities. Here it is shown that (i) close to the unitarity limit for the impurity scattering, the spin-orbit contribution is of the same order of magnitude than the scalar scattering and cannot be neglected, (ii) the spin-orbit scattering is pair-breaking and (iii) induces a small id_xy component to the off-diagonal part of the self-energy.Comment: 9 pages, 3 postscript figures, euromacr.tex-europhys.sty, submitted to Europhysics Letter

Anomalous impurity effects in nonadiabatic superconductors

We show that, in contrast with the usual electron-phonon Migdal-Eliashberg theory, the critical temperature Tc of an isotropic s-wave nonadiabatic superconductor is strongly reduced by the presence of diluted non-magnetic impurities. Our results suggest that the recently observed Tc-suppression driven by disorder in K3C60 [Phys. Rev. B vol.55, 3866 (1997)] and in Nd(2-x)CexCuO(4-delta) [Phys. Rev. B vol.58, 8800 (1998)] could be explained in terms of a nonadiabatic electron-phonon coupling. Moreover, we predict that the isotope effect on Tc has an impurity dependence qualitatively different from the one expected for anisotropic superconductors.Comment: 10 pages, euromacr.tex, europhys.sty, 6 figures. Replaced with accepted version (Europhysics Letters

Zeeman response of d-wave superconductors: Born approximation for impurity and spin-orbit scattering potentials

The effects of impurity and spin-orbit scattering potentials can strongly affect the Zeeman response of a d-wave superconductor. Here, both the phase diagram and the quasiparticle density of states are calculated within the Born approximation and it is found that the spin-orbit interaction influences in a qualitatively different way the Zeeman response of d-wave and s-wave superconductors.Comment: 19 pages, 6 eps figures, submitted to Physica

Electron-phonon effects on spin-orbit split bands of two dimensional systems

The electronic self-energy is studied for a two dimensional electron gas coupled to a spin-orbit Rashba field and interacting with dispersionless phonons. For the case of a momentum independent electron-phonon coupling (Holstein model) we solve numerically the self-consistent non-crossing approximation for the self-energy and calculate the electron mass enhancement $m^*/m$ and the spectral properties. We find that, even for nominal weak electron-phonon interaction, for strong spin-orbit couplings the electrons behave as effectively strongly coupled to the phonons. We interpret this result by a topological change of the Fermi surface occurring at sufficiently strong spin-orbit coupling, which induces a square-root divergence in the electronic density of states at low energies. We provide results for $m^*/m$ and for the density of states of the interacting electrons for several values of the electron filling and of the spin-orbit interaction.Comment: 9 pages, 6 figures. Version as printe

Anisotropic random resistor networks: a model for piezoresistive response of thick-film resistors

A number of evidences suggests that thick-film resistors are close to a metal-insulator transition and that tunneling processes between metallic grains are the main source of resistance. We consider as a minimal model for description of transport properties in thick-film resistors a percolative resistor network, with conducting elements governed by tunneling. For both oriented and randomly oriented networks, we show that the piezoresistive response to an applied strain is model dependent when the system is far away from the percolation thresold, while in the critical region it acquires universal properties. In particular close to the metal-insulator transition, the piezoresistive anisotropy show a power law behavior. Within this region, there exists a simple and universal relation between the conductance and the piezoresistive anisotropy, which could be experimentally tested by common cantilever bar measurements of thick-film resistors.Comment: 7 pages, 2 eps figure

Relevance of multiband Jahn-Teller effects on the electron-phonon interaction in A3A_3C60_{60}

Assessing the effective relevance of multiband effects in the fullerides is of fundamental importance to understand the complex superconducting and transport properties of these compounds. In this paper we investigate in particular the role of the multiband effects on the electron-phonon (el-ph) properties of the $t_{1u}$ bands coupled with the Jahn-Teller intra-molecular $H_g$ vibrational modes in the C$_{60}$ compounds. We show that, assuming perfect degeneracy of the electronic bands, vertex diagrams arising from the breakdown of the adiabatic hypothesis, are one order of magnitude smaller than the non-crossing terms usually retained in the Migdal-Eliashberg (ME) theory. These results permit to understand the robustness on ME theory found by numerical calculations. The effects of the non degeneracy of the $t_{1u}$ in realistic systems are also analyzed. Using a tight-binding model we show that the el-ph interaction is mainly dominated by interband scattering within a single electronic band. Our results question the reliability of a degenerate band modeling and show the importance of these combined effects in the $A_3$C$_{60}$ family.Comment: 5 pages, 3 eps figure

Pauli susceptibility of nonadiabatic Fermi liquids

The nonadiabatic regime of the electron-phonon interaction leads to behaviors of some physical measurable quantities qualitatively different from those expected from the Migdal-Eliashberg theory. Here we identify in the Pauli paramagnetic susceptibility $\chi$ one of such quantities and show that the nonadiabatic corrections reduce $\chi$ with respect to its adiabatic limit. We show also that the nonadiabatic regime induces an isotope dependence of $\chi$, which in principle could be measured.Comment: 7 pages, 3 figures, euromacr.tex, europhys.sty. Replaced with accepted version (Europhysics Letters

Brachycera in Cretaceous amber, part 9.

97 pages : illustrations (some color) ; 26 cm.Diverse new basal (aschizan) Cyclorrhapha fossilized in amber are described from the Tertiary and Cretaceous, and their relationships are examined with character-based phylogenetic hypotheses for each family or family group. There are 18 new species in 15 genera (11 of them new) and four families plus the Syrphoidea. Fossils are from the Early Cretaceous of Lebanon, Late Cretaceous of New Jersey (United States) and Alberta (Canada), Eocene of the eastern Baltic coast, and Miocene of the Dominican Republic, but predominantly from the mid-Cretaceous of Myanmar. Stem-group Lonchopteroidea are Alonchoptera lebanica, n. gen., n.sp., and Lonchopterites burmensis, n. sp. Platypezidae include the stem groups Burmapeza radicis, n. gen., n. sp., Canadopeza biacrosticha, n. gen., n. sp., and Calvopeza divergens, n. gen., n. sp. An unnamed Microsania sp. is the first definitive Platypezidae in Baltic amber; Lebanopeza azari, n. gen., n. sp., is a stem group to the Microsaniinae and Melanderomyiinae. Chandleromyia anomala, n. gen., n. sp., is an anomalously derived Platypezinae from the Cretaceous, and two new species of the diverse Recent genus Lindneromyia are in Dominican amber (L. neomedialis and L. dominicana). Fossils of the relict family Ironomyiidae (with 3 living species from eastern Australia) include two stem-group genera with two new species each, all in Burmese amber: Palaeopetia dorsalis and P. terminus, Proironia (n. gen.) gibbera and P. burmitica. All other species of Palaeopetia are compression fossils from the Cretaceous of Asia and Eurasia. For Phoridae, a new defining feature is a stridulatum on the procoxa and profemur in both sexes, occu[r]ring in most fossil taxa where observable. New sciadocerines include Eosciadocera pauciseta, n. sp., a very large species in Baltic amber, and two stem groups in Burmese amber, Prophora dimorion, n. gen., n. sp., and a very small, undescribed taxon. Archiphora pria Grimaldi and Cumming in Turonian-aged New Jersey amber is transferred to Hennigophora Brown, based on evidence from a new specimen. Prioriphorinae (not taxonomically treated here) is a paraphyletic, Cretaceous grade to the very diverse, crown-group radiation of Euphorida that occurred in the Cenozoic. Two syrphoids occur in Burmese amber: Prosyrphus thompsoni, n. gen., n. sp. (an apparent stem group to the Syrphidae), and Aschizomyia burmensis, n. gen., n. sp. (with more ambiguous affinities). Several immatures of undetermined family are reported, one a probable phorid larva. No definitive Schizophora are yet known from the Cretaceous

Superconductivity of Rb3_3C60_{60}: breakdown of the Migdal-Eliashberg theory

In this paper, through an exhaustive analysis within the Migdal-Eliashberg theory, we show the incompatibility of experimental data of Rb$_3$C$_{60}$ with the basic assumptions of the standard theory of superconductivity. For different models of the electron-phonon spectral function $\alpha^2F(\Omega)$ we solve numerically the Eliashberg equations to find which values of the electron-phonon coupling $\lambda$, of the logarithmic phonon frequency $\Omega_{ln}$ and of the Coulomb pseudopotential $\mu^*$ reproduce the experimental data of Rb$_3$C$_{60}$. We find that the solutions are essentially independent of the particular shape of $\alpha^2F(\Omega)$ and that, to explain the experimental data of Rb$_3$C$_{60}$, one has to resort to extremely large couplings: $\lambda=3.0\pm 0.8$. This results differs from the usual partial analyses reported up to now and we claim that this value exceeds the maximum allowed $\lambda$ compatible with the crystal lattice stability. Moreover, we show quantitatively that the obtained values of $\lambda$ and $\Omega_{ln}$ strongly violate Migdal's theorem and consequently are incompatible with the Migdal-Eliashberg theory. One has therefore to consider the generalization of the theory of superconductivity in the nonadiabatic regime to account for the experimental properties of fullerides.Comment: 9 pages, 8 eps figure encloses, epjb style, to appear on Eur. Phys. J.