The metallic transport of (TMTSF)_2X organic conductors close to the superconducting phase

Comparing resistivity data of quasi-one dimensional superconductors (TMTSF)_2PF_6 and (TMTSF)_2ClO_4 along the least conducting c*-axis and along the high conductivity a -axis as a function of temperature and pressure, a low temperature regime is observed in which a unique scattering time governs transport along both directions of these anisotropic conductors. However, the pressure dependence of the anisotropy implies a large pressure dependence of the interlayer coupling. This is in agreement with the results of first-principles DFT calculations implying methyl group hyperconjugation in the TMTSF molecule. In this low temperature regime, both materials exhibit for rc a temperature dependence aT + bT^2. Taking into account the strong pressure dependence of the anisotropy, the T-linear rc is found to correlate with the suppression of the superconducting Tc, in close analogy with ra data. This work is revealing the domain of existence of the 3D coherent regime in the generic (TMTSF)_2X phase diagram and provides further support for the correlation between T-linear resistivity and superconductivity in non-conventional superconductors

Towards a consistent picture for quasi-1D organic superconductors

The electrical resistivity of the quasi-1D organic superconductor (TMTSF)2PF6 was recently measured at low temperature from the critical pressure needed to suppress the spin-density-wave state up to a pressure where superconductivity has almost disappeared. This data revealed a direct correlation between the onset of superconductivity at Tc and the strength of a non-Fermi-liquid linear term in the normal-state resistivity, going as r(T) = r0 + AT + BT2 at low temperature, so that A goes to 0 as Tc goes to 0. Here we show that the contribution of low-frequency antiferromagnetic fluctuations to the spin-lattice relaxation rate is also correlated with this non-Fermi-liquid term AT in the resistivity. These correlations suggest that anomalous scattering and pairing have a common origin, both rooted in the low-frequency antiferromagnetic fluctuations measured by NMR. A similar situation may also prevail in the recently-discovered iron-pnictide superconductors.Comment: ISCOM'09 proceedings to be published in Physica

From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba,K)Fe2As2

The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes

Linear-T scattering and pairing from antiferromagnetic fluctuations in the (TMTSF)_2X organic superconductors

An exhaustive investigation of metallic electronic transport and superconductivity of organic superconductors (TMTSF)_2PF_6 and (TMTSF)_2ClO_4 in the Pressure-Temperature phase diagram between T=0 and 20 K and a theoretical description based on the weak coupling renormalization group method are reported. The analysis of the data reveals a high temperature domain (T\approx 20 K) in which a regular T^2 electron-electron Umklapp scattering obeys a Kadowaki-Woods law and a low temperature regime (T< 8 K) where the resistivity is dominated by a linear-in temperature component. In both compounds a correlated behavior exists between the linear transport and the extra nuclear spin-lattice relaxation due to antiferromagnetic fluctuations. In addition, a tight connection is clearly established between linear transport and T_c. We propose a theoretical description of the anomalous resistivity based on a weak coupling renormalization group determination of electron-electron scattering rate. A linear resistivity is found and its origin lies in antiferromagnetic correlations sustained by Cooper pairing via constructive interference. The decay of the linear resistivity term under pressure is correlated with the strength of antiferromagnetic spin correlations and T_c, along with an unusual build-up of the Fermi liquid scattering. The results capture the key features of the low temperature electrical transport in the Bechgaard salts

The "Protocole d'Examen Cognitif de la Personne Agee - Lausanne" (PECPA-L): a cognitive assessment tool for the French-speaking elderly in Switzerland

OBJECTIVE: The main objective of this study was to validate and normalize, in the French-speaking population of Switzerland, an empirically derived structured cognitive assessment scale for the elderly. METHOD: 237 healthy elderly and 115 elderly with mild-to-moderate dementia as well as a preliminary sample of 27 subjects with MCI, all community-dwelling, were assessed using the PECPA-L for its validation and normalization using Crohnbach's alpha, rank comparisons, and AUROC measures. RESULTS: The internal consistency of the PECPA-L was good (Crohnbach's Alpha=0.79). The discriminating power of the PECPA-L between the normal elderly and the mildly to moderately demented (AUROC=0.940) and those with MCI (AUROC=0.925) was high. Normal aging does not influence all cognitive domains equally; therefore, the subscores of the PECPA-L vary in their discriminating power between the normal and the demented elderly, with gnosis abilities having the least (AUROC=0.719) and delayed memory performance having the highest (AUROC=0.927) discriminating power. Normative data according to age and education are provided. CONCLUSIONS: The PECPA-L is a highly appropriate tool for the detection and documentation of early cognitive impairment in the French-speaking population in Switzerland while accounting for age and education. Copyright (c) 2006 S. Karger AG, Base

Thiazolyl Phosphine Ligands for Copper-Catalyzed Arylation and Vinylation of Nucleophiles in Organic and Aqueous Media

International audienceA straightforward and efficient synthetic route to thiazolyl phosphines is reported. Moreover, the first application of these molecules in catalysis is described. These phosphines are excellent ligands in the copper-catalyzed arylation and Vinylation of nucleophiles. The reactions could be performed in acetonitrile and Various mixed aqueous/organic solvents and even in pure water in the presence of a phase transfer catalyst. In each case, coupling products are obtained with high yields at very mild temperatures