Interplay of the pseudogap and the BCS gap for heteropairs in 40^{40}K-6^6Li mixture

The description of heteropairs like $^{40}$K-$^6$Li near and in the superconducting state requires a fully selfconsistent theory [see Hanai and Ohashi, Phys. Rev. A 90, 043622 (2014)]. We derive analytic pseudogap Green's functions for the "normal" and superconducting states from the Luttinger-Ward theory with the T-matrix in the static separable approximation. The self-consistency in the closing loop of self-energy has two pronounced effects on the single-particle spectrum. First, the single-particle excitations decay before the asymptotic quasiparticle propagation is established, therefore the normal state is not a Fermi liquid. Second, the pseudogap has a V shape even for s-wave pairing. The V-shaped pseudogap and the U-shaped BCS gap interfere resulting in slope breaks of the gap walls and the in-gap states in the density of states. Various consequences of an incomplete self-consistency are demonstrated.Comment: Published versio

Influence of the Cr Content on the Corrosion Properties of a Series of Binary Cobalt-Chromium Alloys in Acidic Artificial Saliva

International audienceCobalt-based alloys are more and more considered for dental applications, instead the expensive noble alloys and the other predominantly based alloys based on the allergen element nickel. These alloys frequently contain chromium to allow good resistance against corrosion as this may take place in the buccal milieu. The most often chromium is present in high quantity, close to 30wt. %, but it is maybe possible to decrease a little its contents without losing corrosion resistance. The aim of this study is exploring how the corrosion behavior of a cobalt alloy may vary versus its chromium content. In order to simplify this investigation only Co-xCrbinary alloys were considered, with x decreasing from 30 to 0wt.% by slices of 5wt.%. A series of seven alloys was synthesized by foundry under inert atmosphere and their behavior in corrosion in an artificial saliva of simple composition (NaCl 9g/L) acidified to pH=2.3 was specified using various classical electrochemical techniques (open circuit potential follow-up, Stern-Geary, Tafel, cyclic polarization). It appeared that passivation of the alloys occurred during the first two hours of immersion when the chromium content was 10 wt. % or higher, with as result high corrosion potentials and low corrosion current densities

Thermal expansion of chromium-rich iron-based or iron/nickel-based alloys reinforced by tantalum carbides

Six alloys reinforced by TaC carbides based on iron (ferritic) or both iron and nickel (austenitic) were studied in thermal expansion between 100 and 1200°C for two microstructure orientations. The heating, isothermal and cooling parts of the dilatometry curves were characterized. The thermal expansion of the ferritic alloys is less important than that of the austenitic alloys. A compressive deformation of the matrix subjected to stresses applied by the carbides network was observed. The importance of this phenomenon seems depending more on the matrix nature than on the microstructure orientation.Досліджено шість сплавів на основі заліза (феритні та аустенітні) і нікелю, зміцнені карбідами TaC, та їх термічне розширення в інтервалі 100...1200°C для двох мікроструктурних орієнтацій. Проаналізовано ізотермічну та охолоджувальну ділянки на дилатометричних кривих. Термічне розширення феритних сплавів виражене не так чітко, як аустенітних. Стискальні деформації матриці призводять до напружень, які поширюються на колонії карбідів. Важливість цього явища вбачається у впливі фази матриці на мікроструктурні особливості сплаву загалом.Исследовано шесть сплавов на основе железа (ферритные и аустенитные) и никеля, упрочненных карбидами ТаС, и их термическое расширение в интервале 100... 1200°C для двух микроструктурных ориентаций. Проанализированы изотермический и охлаждающим участки на дилатометрических кривых. Термическое расширение ферритных сплавов выражено менее четко, нежели аустенитных. Сжимающие деформации матриц обуславливают напряжениям, которые распространяются на колонии карбидов. Важность этого явления видится во влиянии матричной фазы на микроструктурные особенности сплава в целом

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

State of the Sub-surface Microstructure of Carbides strengthened cast Superalloys after High Temperature Oxidation -Use of Thermodynamic Modelling for a better Understanding

International audienceSeveral cast strengthened superalloys, Ni base and Co base, were exposed to high temperature oxidation for long times and metallographically examined. Different phenomena occurred in the sub-surface microstructure, depending on both alloy and temperature. Thermodynamic modelling was used to know what it happened for carbon during oxidation, then to explain the observed microstructural changes. It appears that carbon atoms either quit the alloy probably after its oxidation into gases, or on the contrary go deeper into the bulk where they promote the precipitation of new carbides by solid state transformation. Thereafter, thermodynamic modelling allowed to know the new local refractoriness of the zones affected by oxidation, then to appreciate the new mechanical properties in the sub-surface

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

Influence of the Microstructural Texture of Cast Superalloys on their High-Temperature Oxidation Behaviour

International audienceThe high-temperature oxidation behaviours of Ni base and Co base cast superalloys were studied to determine the effect of different grain sizes and different surface dendritic orientations. These microstructural characteristics were obtained by varying solidification rate and cutting orientation with regard to the external surface. Thermogravimetry tests were run at 1000 and 1100°C and parabolic oxidation constants were considered. It appears that oxidation is faster for fine microstructures than for coarser ones for Ni or Co alloys including tungsten in their chemical composition, while it is the contrary when W is not present. When the sample surface is mainly parallel to the dendritic network, the oxidation rate is greater than for a surface perpendicular to dendrites, for the studied Ni alloy. The same phenomenon is observed for the studied Co base alloy at 1100°C but the order is inverted at 1000°C. These different behaviours can be explained by the grain boundary densities and orientations obtained on surface, since they can influence the diffusion of species involved in the oxidation phenomena. It is also possible that the characteristics of the chromia scale, such as grain size and general quality, depend on the microstructural texture of the alloy

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

Multi-band Superconductivity in the Chevrel Phases SnMo6S8 and PbMo6S8

Sub-Kelvin scanning tunnelling spectroscopy in the Chevrel Phases SnMo6S8 and PbMo6S8 reveals two distinct superconducting gaps with Delta_1 = 3 meV, Delta_2 ~ 1.0 meV and Delta_1 = 3.1 meV, Delta_2 ~ 1.4 meV respectively. The gap distribution is strongly anisotropic, with Delta_2 predominantly seen when scanning across unit-cell steps on the (001) sample surface. The spectra are well-fitted by an anisotropic two-band BCS s-wave gap function. Our spectroscopic data are confirmed by electronic heat capacity measurements which also provide evidence for a twin-gap scenario.Comment: 5 pages, 4 figure