Preeminent role of the Van Hove singularity in the strong-coupling analysis of scanning tunneling spectroscopy for two-dimensional cuprates

In two dimensions the non-interacting density of states displays a Van Hove singularity (VHS) which introduces an intrinsic electron-hole asymmetry, absent in three dimensions. We show that due to this VHS the strong-coupling analysis of tunneling spectra in high-$T_c$ superconductors must be reconsidered. Based on a microscopic model which reproduces the experimental data with great accuracy, we elucidate the peculiar role played by the VHS in shaping the tunneling spectra, and show that more conventional analyses of strong-coupling effects can lead to severe errors.Comment: 5 pages, 4 figure

Strong-coupling analysis of scanning tunneling spectra in Bi2_2Sr2_2Ca2_2Cu3_3O10+δ_{10+\delta}

We study a series of spectra measured in the superconducting state of optimally-doped Bi-2223 by scanning tunneling spectroscopy. Each spectrum, as well as the average of spectra presenting the same gap, is fitted using a strong-coupling model taking into account the band structure, the BCS gap, and the interaction of electrons with the spin resonance. After describing our measurements and the main characteristics of the strong-coupling model, we report the whole set of parameters determined from the fits, and we discuss trends as a function of the gap magnitude. We also simulate angle-resolved photoemission spectra, and compare with recent experimental results.Comment: Published versio

Breakup of the Fermi surface near the Mott transition in low-dimensional systems

We investigate the Mott transition in weakly-coupled one-dimensional (1d) fermionic chains. Using a generalization of Dynamic Mean Field Theory, we show that the Mott gap is suppressed at some critical hopping $t_{\perp}^{c2}$. The transition from the 1d insulator to a 2d metal proceeds through an intermediate phase where the Fermi surface is broken into electron and hole pockets. The quasiparticle spectral weight is strongly anisotropic along the Fermi surface, both in the intermediate and metallic phases. We argue that such pockets would look like `arcs' in photoemission experiments.Comment: REVTeX 4, 5 pages, 4 EPS figures. References added; problem with figure 4 fixed; typos correcte

Tunneling spectra of strongly coupled superconductors: Role of dimensionality

We investigate numerically the signatures of collective modes in the tunneling spectra of superconductors. The larger strength of the signatures observed in the high-Tc superconductors, as compared to classical low-Tc materials, is explained by the low dimensionality of these layered compounds. We also show that the strong-coupling structures are dips (zeros in the d2I/dV2 spectrum) in d-wave superconductors, rather than the steps (peaks in d2I/dV2) observed in classical s-wave superconductors. Finally we question the usefulness of effective density of states models for the analysis of tunneling data in d-wave superconductors.Comment: 8 pages, 6 figure

Étude de la formation de la peau en coulée continue sans solidification au ménisque. II. Mécanismes de formation des marques

International audienceIn vertical continuous casting, the formation of linear marks observed all around the products – “the ripples” – is known to be due to the partial solidification of the meniscus of the free surface of the liquid metal. On the contrary, in the case of horizontal continuous casting or meniscusfree continuous casting, the presence of ripples or marks on the surface of the cast products still remains mysterious. In fact, the initial solidification against the cold mould occurs far from the free surface of the liquid metal. In order to better identify the solidification mechanisms that are responsible of the ripples appearance on the surface of the horizontal continuous casting products we have built two experimental devices. With the so-called «hot» device, it is possible to obtain spheroidal graphite cast iron (SG iron) samples that have been solidified in the same manner as the products in meniscus-free continuous casting (as far as the formation of the skin is concerned). The “cold” device allows the direct observation of the solidification of organic compounds that are transparent in the liquid state and opaque in the solid state.Pour identifier l'origine des marques de surface en coulée continue sans solidification au ménisque, un travail expérimental a été mené avec deux maquettes de simulation utilisant l'une des alliages métalliques, l'autre des alliages transparents. Après avoir exposé dans un premier article les résultats de l'observation de divers échantillons réalisés, nous présentons ici les résultats de l'observation directe des phénomènes. L'ensemble de ces données permet de proposer un premier modèle microscopique pour la formation des marques

Hall effect in strongly correlated low dimensional systems

We investigate the Hall effect in a quasi one-dimensional system made of weakly coupled Luttinger Liquids at half filling. Using a memory function approach, we compute the Hall coefficient as a function of temperature and frequency in the presence of umklapp scattering. We find a power-law correction to the free-fermion value (band value), with an exponent depending on the Luttinger parameter $K_{\rho}$. At high enough temperature or frequency the Hall coefficient approaches the band value.Comment: 7 pages, 3 figure

Real space finite difference method for conductance calculations

We present a general method for calculating coherent electronic transport in quantum wires and tunnel junctions. It is based upon a real space high order finite difference representation of the single particle Hamiltonian and wave functions. Landauer's formula is used to express the conductance as a scattering problem. Dividing space into a scattering region and left and right ideal electrode regions, this problem is solved by wave function matching (WFM) in the boundary zones connecting these regions. The method is tested on a model tunnel junction and applied to sodium atomic wires. In particular, we show that using a high order finite difference approximation of the kinetic energy operator leads to a high accuracy at moderate computational costs.Comment: 13 pages, 10 figure

Imaging the essential role of spin-fluctuations in high-Tc superconductivity

We have used scanning tunneling spectroscopy to investigate short-length electronic correlations in three-layer Bi2Sr2Ca2Cu3O(10+d) (Bi-2223). We show that the superconducting gap and the energy Omega_dip, defined as the difference between the dip minimum and the gap, are both modulated in space following the lattice superstructure, and are locally anti-correlated. Based on fits of our data to a microscopic strong-coupling model we show that Omega_dip is an accurate measure of the collective mode energy in Bi-2223. We conclude that the collective mode responsible for the dip is a local excitation with a doping dependent energy, and is most likely the (pi,pi) spin resonance.Comment: 4 pages, 4 figure

Node-like excitations in superconducting PbMo6S8 probed by scanning tunneling spectroscopy

We present the first scanning tunneling spectroscopy study on the Chevrel phase PbMo6S8, an extreme type II superconductor with a coherence length only slightly larger than in high-Tc cuprates. Tunneling spectra measured on atomically flat terraces are spatially homogeneous and show well-defined coherence peaks. The low-energy spectral weight, the zero bias conductance and the temperature dependence of the gap are incompatible with a conventional isotropic s-wave interpretation, revealing the presence of low-energy excitations in the superconducting state. We show that our data are consistent with the presence of nodes in the superconducting gap.Comment: To appear in PRB; 5 pages, 4 figure

Heterovalent interlayers and interface states: an ab initio study of GaAs/Si/GaAs (110) and (100) heterostructures

We have investigated ab initio the existence of localized states and resonances in abrupt GaAs/Si/GaAs (110)- and (100)-oriented heterostructures incorporating 1 or 2 monolayers (MLs) of Si, as well as in the fully developed Si/GaAs (110) heterojunction. In (100)-oriented structures, we find both valence- and conduction-band related near-band edge states localized at the Si/GaAs interface. In the (110) systems, instead, interface states occur deeper in the valence band; the highest valence-related resonances being about 1 eV below the GaAs valence-band maximum. Using their characteristic bonding properties and atomic character, we are able to follow the evolution of the localized states and resonances from the fully developed Si/GaAs binary junction to the ternary GaAs/Si/GaAs (110) systems incorporating 2 or 1 ML of Si. This approach also allows us to show the link between the interface states of the (110) and (100) systems. Finally, the conditions for the existence of localized states at the Si/GaAs (110) interface are discussed based on a Koster-Slater model developed for the interface-state problem.Comment: REVTeX 4, 14 pages, 15 EPS figure