The relationship between the normal state Fermi liquid scattering rate and the superconducting state

International audienceMany superconductors show a low temperature electrical resistivity of Fermi liquid type ρ =AT2. We show empirically that there exists a relationship between A and Tc when both vary under an external parameter, such as pressure. The more resistive the compound the higher the Tc. Through the analysis of Landau theory of FL, we find that it is a general feature of FL, due to the fact that the scattering that is the main cause of τ is the same one that bounds the pairs that condensed at Tc. We devise a method that allows the determination of the coupling constant λ, which is validated through application to 3He-'s superfluid transitions and τ's extracted from different properties. This method works for conventional superconductors, but fails with heavy fermions

Quantum Topological Excitations: from the Sawtooth Lattice to the Heisenberg Chain

The recently elucidated structure of the delafossite YCuO$_{2.5}$ reveals a Cu-O network with nearly independent $\Delta$ chains having different interactions between the $s=1/2$ spins. Motivated by this result, we study the $\Delta$ chain for various ratios $J_{\rm bb}/J_{\rm bv}$ of the base-base and base-vertex interactions. By exact diagonalization and extrapolation, we show that the elementary excitation spectrum, which (within numerical error) is the same for total spins $S_{\rm tot}=0$ and 1, has a gap only in the interval $0.4874(1) \leq J_{\rm bb}/J_{\rm bv} \leq 1.53(1)$. The gap is dispersionless for $J_{\rm bb}/J_{\rm bv}=1$, but has increasing $k$-dependence as $J_{\rm bb}/J_{\rm bv}$ moves away from unity, related to the instability of dimers in the ground state.Comment: 4 pages, 6 figures (revtex twocolumn

Extension of Bilbro-McMillan Charge Density Wave-Superconductivity coexistence relation to quantum régimes: application to superconducting domes around quantum critical points

International audienceQuantum critical points (QCP) accompanied by superconductivity are ubiquitous in condensed matter physics. In general, the transition temperature T_O of an ordered state, e.g. antiferromagnetic, goes to zero under the influence of an external parameter, e.g. pressure. Superconductivity appears before the disappearance of the ordered state, but reaches its maximum when T_O=0. Presently, the implications of the QCP's on superconductivity are a subject of debate. It is propose here that both transition temperatures satisfy the relation 〖T ̃_O〗^2+〖T ̃_c〗^2=1, where the tilde indicates normalization to the maximum values. Inspired from the basic postulate of theories of superconductors, it is proposed as an extension from Bilbro-McMillan relation

Absence of a structural transition up to 40 Gpa in MgB2 and the relevance of magnesium non-stoichiometry

We report measurements on MgB2 up to ~40GPa. Increasing pressure yields a monotonous decrease of the lattice parameters and of the c/a ratio, but no structural transition down to parameters smaller than those of AlB2. The transition superconducting temperature also decreases with temperature in a sample dependent way. The results are explained by an increase of the filling of the 2D pxy bands with pressure, the Mg stoichiometry determining the starting position of the Fermi level. Our measurements indicate that these hole bands are the relevant ones for superconductivity.Comment: submitted March 9th 2001, PRB accepte

Electrical resistivity of the Ti4O7 Magneli phase under high pressure

We have measured resistivity as a function of temperature and pressure of Ti4O7 twinned crystals using different contact configurations. Pressures over 4kbar depress the localization of bipolarons and allow the study of the electrical conduction of the bipolaronic phase down to low temperatures. For pressures P > 40 kbar the bipolaron formation transition is suppressed and a nearly pressure independent behavior is obtained for the resistivity. We observed an anisotropic conduction. When current is injected parallel to the principal axis, a metallic conduction with interacting carrier effects is predominant. A superconducting state was not obtained down to 1.2 K, although evidences of the proximity of a quantum critical point were noticed. While when current is injected non-parallel to the crystal's principal axis, we obtained a logarithmic divergence of the resistivity at low temperatures. For this case, our results for the high pressure regime can be interpreted in the framework of interacting carriers (polarons or bipolarons) scattered by Two Level Systems.Comment: 9 Revtex pages, 12 EPS figures included, submitted to The European Physical Journal B. Contact author: C. Acha (e-mail address: [email protected]

Effects of bioenergy on biodiversity arising from land-use change and crop type

Understanding how the world's flora and fauna will respond to bioenergy expansion is critical. This issue is particularly pronounced considering bioenergy's potential role as a driver of land‐use change, the variety of production crops being considered and currently used for biomass, and the diversity of ecosystems that can potentially supply land for bioenergy across the planet. We conducted 2 global meta‐analyses to determine how 8 of the most commonly used bioenergy crops may affect site‐level biodiversity. One search was directed at finding data on biodiversity in different production land uses and the other at extracting energy‐yield estimates of potential bioenergy crops. We used linear mixed‐effect models to test whether effects on biodiversity varied with different individual bioenergy crop species, estimated energy yield, first‐ or second‐generation crops, type of reference ecosystem considered, and magnitude of vertical change in habitat structure between any given crop and the reference ecosystem. Species diversity and abundance were generally lower in crops considered for bioenergy relative to the natural ecosystems they may replace. First‐generation crops, derived from oils, sugars, and starches, tended to have greater effects than second‐generation crops, derived from lignocellulose, woody crops, or residues. Crop yield had nonlinear effects on abundance and, to a lesser extent, overall biodiversity; biodiversity effects were driven by negative yield effects for birds but not other taxa. Our results emphasize that replacing natural ecosystems with bioenergy crops across the planet will largely be detrimental for biodiversity, with first generation and high‐yield crops having the strongest negative effects. We argue that meeting energy goals with bioenergy using existing marginal lands or biomass extraction within existing production landscapes may provide more biodiversity‐friendly alternatives than conversion of natural ecosystems for biofuel production.Fil: Nunez Regueiro, Mauricio Manuel. University of Florida. Department of Wildlife Ecology and Conservation; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; Argentina. Universidad Católica de Salta; ArgentinaFil: Siddiqui, Sharmin F.. University of Florida. Department of Wildlife Ecology and Conservation; Estados UnidosFil: Fletcher, Robert J.. University of Florida. Department of Wildlife Ecology and Conservation; Estados Unido

Homework motivation and engagement throughout compulsory education

The present investigation examines changes in students' homework engagement and motivation as they advance to higher grade levels in Spanish compulsory education. The study takes into account the possible effect of prior academic achievement on students' homework engagement and motivation. Participants included 1257 students (ranging in age from 9 to 16 years) from four regions in northern Spain. Results show that: (a) There are statistically significant differences in students' homework engagement and motivation as they advance to higher grade levels; and (b) Students' prior academic achievement is related to their homework engagement and motivation.info:eu-repo/semantics/publishedVersio

Effect of pressure on the superconductivity of Rb0.19WO3

International audienceWe have performed electrical resistivity measurements under pressures up to 20GPa between 1 and 300K on monocrystalline hexagonal Rb0.19WO3. For pressures lower than ~5GPa, we observe a decrease of the metallic-like resistivity at room temperature as well as a small decrease of Tc. At this pressure, the resistivity starts to increase slowly up to 10GPa accompanied by a sharper decrease of Tc .The resistivity curves above 10GPa denote an activated behaviour and a Tc lower than 3K indicating that there is a phase transition that takes place gradually between 5 and 10GPa. We interpret our measurements as the result of structural transformations under high pressure