η\eta-pairing as a mechanism of superconductivity in models of strongly correlated electrons

We consider extended versions of the Hubbard model which contain additional interactions between nearest neighbours. In this letter we show that a large class of these models has a superconducting ground state in arbitrary dimensions. In some special cases we are able to find the complete phase diagram. The superconducting phase exist even for moderate repulsive values of the Hubbard interaction $U$.Comment: 9 pages, RevTex, ITP-SB-94-18, 1 PS figure appende

TRY plant trait database – enhanced coverage and open access

Plant traits—the morphological, anatomical, physiological, biochemical and phenological characteristics of plants—determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of trait‐based plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traits—almost complete coverage for ‘plant growth form’. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and trait–environmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives

Characterizing the phylogenetic structure of communities by an additive partitioning of phylogenetic diversity

1 Analysing the phylogenetic structure of natural communities may illuminate the processes governing the assembly and coexistence of species in ecological communities. 2 Unifying previous works, we present a statistical framework to quantify the phylogenetic structure of communities in terms of average divergence time between pairs of individuals or species, sampled from different sites. This framework allows an additive partitioning of the phylogenetic signal into alpha (within-site) and beta (among-site) components, and is closely linked to Simpson diversity. It unifies the treatment of intraspecific (genetic) and interspecific diversity, leading to the definition of differentiation coefficients among community samples (e.g. I-ST, P-ST) analogous to classical population genetics coefficients expressing differentiation among populations (e.g. F-ST, N-ST). 3 Two coefficients which express community differentiation among sites from species identity (I-ST) or species phylogeny (P-ST) require abundance data (number of individuals per species per site), and estimators that are unbiased with respect to sample size are given. Another coefficient (Pi(ST)) expresses the gain of the mean phylogenetic distance between species found in different sites compared with species found within sites, and requires only incidence data (presence/absence of each species in each site). 4 We present tests based on phylogenetic tree randomizations to detect community phylogenetic clustering (P-ST > I-ST or Pi(ST) > 0) or phylogenetic overdispersion (P-ST < I-ST or Pi(ST) < 0). In addition, we propose a novel approach to detect phylogenetic clustering or overdispersion in different clades or at different evolutionary time depths using partial randomizations. 5 I-ST, P-ST or Pi(ST) can also be used as distances between community samples and regressed on ecological or geographical distances, allowing us to investigate the factors responsible for the phylogenetic signal and the critical scales at which it appears. 6 We illustrate the approach on forest tree communities in Equatorial Guinea, where a phylogenetic clustering signal was probably due to phylogenetically conserved adaptations to the elevation gradient and was mostly contributed to by ancient clade subdivisions. 7 The approach presented should find applications for comparing quantitatively phylogenetic patterns of different communities, of similar communities in different regions or continents, or of populations (within species) vs. communities (among species).FLWINinfo:eu-repo/semantics/publishe

Insulating phases of the one-dimensional t-J model with nearest-neighbor repulsive interaction

Using the deformed Hubbard operator approach, we analytically study weak-coupling phase diagram of the one-dimensional t-J-V model at half filling. In the case of small deformed parameter ζ(≪1), the interactions induced by the no double occupancy constraint are softened, accessible by the bosonization field theory and the renormalization group technique. The ground state exhibits insulating behavior of density-wave correlations. The bond-spin-density-wave (BSDW) and bond-charge-density-wave (BCDW) phases are realized in the whole weak-coupling regime while the charge-density-wave (CDW) and spin-density-wave (SDW) phases depend on V/J > (V/J) c or V/J > (V/J) c , where (V/J) c =1/4. Furthermore, our results are expected to adiabatically continue back to ζ=1. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2011