A model describing the low-density carrier state in an organic single-crystal field-effect transistor (FET) with high-κ gate dielectrics is studied. The interplay between charge-carrier coupling with ...intermolecular vibrations in the bulk of the organic material and the long-range interaction induced at the interface with a polar dielectric is investigated. This interplay is responsible for the stabilization of a polaronic state with an internal structure extending on few lattice sites, at much lower coupling strengths than expected from the polar interaction alone. This effect could drive the carriers close to self-trapping in high-κ organic FETs without invoking unphysically large values of the carrier-interface interaction.
Testudo hermanni hermanni is becoming seriously endangered throughout its range. It has a scattered distribution, with a small number of residual populations found in Spain, France and Italy. In this ...study we sampled a population of T. h. hermanni from Southern Tuscany (Massa Marittima, Grosseto), composed of native and introduced individuals (recognizable due to residual signs of previous marking on the carapace). Overall, 95% of the captured individuals were adults and the sex ratio was slightly, but not significantly, biased in favour of females. Population density was relatively high in comparison with other Italian populations, although it was doubled by previous reinforcement. Genetic analysis performed on six polymorphic microsatellite loci revealed a high level of genetic variability and heterozygosity, with no evidence of current inbreeding processes. Moreover, introduced individuals presented genotypes similar to those of the native individuals, thus suggesting that the reinforcement intervention did not cause a significant change in the original genetic pool. Nevertheless, long-term monitoring of the population is necessary to ensure its stability and vitality. Furthermore, to preserve the genetic identity of the local population in the future, uncontrolled translocation events should be avoided.
We study a system of electrons interacting through long--range Coulomb forces on a one--dimensional lattice, by means of a variational ansatz which is the strong--coupling counterpart of the ...Gutzwiller wave function. Our aim is to describe the quantum analogue of Hubbard\'s classical ``generalized Wigner crystal\'\'. We first analyse charge ordering in a system of spinless fermions, with particular attention to the effects of lattice commensurability. We argue that for a general (rational) number of electrons per site $n$ there are three regimes, depending on the relative strength $V$ of the long--range Coulomb interaction (as compared to the hopping amplitude $t$). For very large $V$ the quantum ground state differs little from Hubbard\'s classical solution, for intermediate to large values of $V$ we recover essentially the Wigner crystal of the continuum model, and for small $V$ the charge modulation amounts to a small--amplitude charge--density wave. We then include the spin degrees of freedom and show that in the Wigner crystal regimes (i.e. for large $V$) they are coupled by an antiferromagnetic kinetic exchange $J$, which turns out to be smaller than the energy scale governing the charge degrees of freedom. Our results shed new light on the insulating phases of organic quasi--1D compounds where the long--range part of the interaction is unscreened, and magnetic and charge orderings coexist at low temperatures.
Although the role of macroinvertebrates in the nutrient cycling of the Indo-Pacific mangrove ecosystems is now a paradigm of ecological research, most of the research and scientific assumptions deal ...with decapod crustaceans alone. However, decapods are not the only representatives of mangrove macrobenthos and in certain mangals they are surely outnumbered by gastropods, especially by potamidids of the genus Terebralia. This paper aims at bridging the gap in the knowledge of the ecological role of the largest and most abundant potamidid species of the Indo-Pacific mangals, the semiterrestrial T. palustris. In particular we studied size structure and distribution in relation to micro-environmental factors and feeding ecology, both in terms of diet composition and grazing rate, of two populations of this gastropod colonising a Kenyan mangrove creek. In contrast to what has been described by various authors concerning other mangals, in our study site juveniles and adults of T. palustris were present throughout the whole forest, from the landward belt to the seaward fringe, locally reaching very high densities at various levels. Both life stages showed a preference for patches characterised by soil with high organic content; thus the habitat of juveniles largely overlapped with that of the adults, although they are known to occupy different ecological niches. Indeed, gut-content analysis confirmed that juveniles are detritivorous and adults are mainly leaf-litter consumers. In situ experiments on the grazing activity of the adult snails showed that, in contrast to the litter-consuming crabs, they feed in every light and tidal condition, for an hourly consumption rate of ~0.65 g per snail. Considering an average density of about 10.5 adults m-2, it is estimated that a total of 10.5 leaves m-2 are removed by adult snails in each tidal cycle (i.e. about every 6 h). T. palustris is the only leaf consumer capable of searching for food under water during high tide and is responsible for the removal of a great quantity of mangrove litter. The results of the present study strongly suggest that T. palustris plays a major role in the food web, mangrove litter degradation and nutrient cycling of the Kenyan mangal. PUBLICATION ABSTRACT
Recent experiments have demonstrated that the performances of organic FETs strongly depend on the dielectric properties of the gate insulator. In particular, it has been shown that the temperature ...dependence of the mobility evolves from a metallic-like to an insulating behavior upon increasing the dielectric constant of the gate material. This phenomenon can be explained in terms of the formation of small polarons, due to the polar interaction of the charge carriers with the phonons at the organic/dielectric interface. Building on this model, the possible consequences of the Coulomb repulsion between the carriers at high concentrations are analyzed.
Optical and spectral properties of carriers in the presence of strong antiferromagnetic correlations and interacting with optical phonon modes are analyzed using Dynamical Mean Field Theory. We ...interpret the mid-infrared band in
σ(
ω) in term of mixed spin lattice polaronic excitations which arise from the stabilization of the lattice polaron due to the antiferromagnetic correlations. We compare our results with experimental data in NCCO showing that the doping and temperature dependences of the optical conductivity in this compound is naturally reproduced within a spin/lattice polaronic model.
The gastropod Cerithidea decollata typically feeds on mud at the mangrove soil surface at low tide and rests on Avicennia marina trunks throughout high tide. Tens of individuals can be observed ...climbing the trunks and clustering, 2-3 hr before the incoming high tide, approximately 40 cm higher than the level that the tide will reach. As soon as the water disappears, snails descend and disperse on the ground again. Signals able to tell the snails the height of the incoming tide are still unknown but we wanted to investigate whether or not some information may be transferred from resident snails to snails translocated from areas undergoing a different tidal regime. Snails from sites where tides never exceed 5 cm above ground level (translocated) were transported to sites flooded by higher tides (up to 80 cm above the ground), and their behaviour was compared with local snails (residents). At the beginning of the experiment, sea water occasionally made contact with translocated snails as they ascended the trunks later than resident snails and clustered closer to the water. After several tides, translocated snails started to behave similarly to resident snails and, by the 10th tide, the behaviour of the two groups was indistinguishable. However, the behaviour of translocated C. decollata was not affected by being in contact or not with resident snails; thus, it appears that no information was transferred from resident to translocated snails.
We calculate the optical conductivity of small polarons in the Holstein and Holstein-
t-
J models, by applying the dynamical mean field theory. We show that the antiferromagnetic correlations tend to ...increase the region of the parameters where polaronic signatures are found in the optical spectra, and shift the polaronic absorption band to higher frequencies compared to the case of purely lattice polarons. On the other hand, the electron–lattice interaction is essential in order to have polaronic features in the optical absorption.
The crystallization of electrons in quasi low-dimensional solids is studied in a model which retains the full three-dimensional nature of the Coulomb interactions. We show that restricting the ...electron motion to layers (or chains) gives rise to a rich sequence of structural transitions upon varying the particle density. In addition, the concurrence of low-dimensional electron motion and isotropic Coulomb interactions leads to a sizeable stabilization of the Wigner crystal, which could be one of the mechanisms at the origin of the charge ordered phases frequently observed in such compounds.
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