Intraspecific niche models for the invasive ambrosia beetle Xylosandrus crassiusculus suggest contrasted responses to climate change.

Xylosandrus crassiusculus is an invasive ambrosia beetle comprising two differentiated genetic lineages, named cluster 1 and cluster 2. These lineages invaded different parts of the world at different periods of time. We tested whether they exhibited different climatic niches using Schoener's D and Hellinger's I indices and modeled their current potential geographical ranges using the Maxent algorithm. The resulting models were projected according to future and recent past climate datasets for Europe and the Mediterranean region. The future projections were performed for the periods 2041-2070 and 2071-2100 using 3 SSPs and 5 GCMs. The genetic lineages exhibited different climate niches. Parts of Europe, the Americas, Sub-Saharan Africa, Asia, and Oceania were evaluated as suitable for cluster 1. Parts of Europe, South America, Central and South Africa, Asia, and Oceania were considered as suitable for cluster 2. Models projection under future climate scenarios indicated a decrease in climate suitability in Southern Europe and an increase in North Eastern Europe in 2071-2100. Most of Southern and Western Europe was evaluated as already suitable for both clusters in the early twentieth century. Our results show that large climatically suitable regions still remain uncolonized and that climate change will affect the geographical distribution of climatically suitable areas. Climate conditions in Europe were favorable in the twentieth century, suggesting that the recent colonization of Europe is rather due to an increase in propagule pressure via international trade than to recent environmental changes.

The worldwide invasion history of a pest ambrosia beetle inferred using population genomics.

Xylosandrus crassiusculus, a fungus-farming wood borer native to Southeastern Asia, is the most rapidly spreading invasive ambrosia species worldwide. Previous studies focusing on its genetic structure suggested the existence of cryptic genetic variation in this species. Yet, these studies used different genetic markers, focused on different geographical areas and did not include Europe. Our first goal was to determine the worldwide genetic structure of this species based on both mitochondrial and genomic markers. Our second goal was to study X. crassiusculus' invasion history on a global level and identify the origins of the invasion in Europe. We used a COI and RAD sequencing design to characterize 188 and 206 specimens worldwide, building the most comprehensive genetic data set for any ambrosia beetle to date. The results were largely consistent between markers. Two differentiated genetic clusters were invasive, albeit in different regions of the world. The markers were inconsistent only for a few specimens found exclusively in Japan. Mainland USA could have acted as a source for further expansion to Canada and Argentina through stepping stone expansion and bridgehead events. We showed that Europe was only colonized by Cluster II through a complex invasion history including several arrivals from multiple origins in the native area, and possibly including bridgehead from the United States. Our results also suggested that Spain was colonized directly from Italy through intracontinental dispersion. It is unclear whether the mutually exclusive allopatric distribution of the two clusters is due to neutral effects or due to different ecological requirements.

Variable temperature FTIR spectra of polycrystalline purine nucleobases and estimating strengths of individual hydrogen bonds.

In the first part of this work, we report the FTIR spectra of pure NH and isotopically substituted ND (10-15% D and 80-90% D) polycrystalline hypoxanthine, xanthine, adenine and guanine recorded in the 400-4000 cm-1 range, as a function of temperature (10-300 K). We provide assignments of the stretching and out-of-plane bending amine (NH2) and imine (NH) bands to the distinct H-bonds present in the crystal, based on the temperature sensitivity and isotopic exchange behavior. Empirical correlations between spectral and thermodynamic or structural parameters enabled us to estimate the energies and lengths of H-bonds in the studied nucleobase crystals and to correlate them with literature data. The empirical H-bonding energies are compared with H-bonding and stacking energies computed for hypoxanthine. In the second part, strategies for using the empirical correlations together with information extracted from quantum mechanical data (in particular from the Bader's quantum theory of atoms in molecules, QTAIM) for the evaluation of hydrogen bonding properties are discussed, and their advantages and drawbacks pointed out. The justification for a cooperative use of quantum-mechanical calculations with empirical spectra-energy correlations is discussed.

Climate change impact on the potential geographical distribution of two invading Xylosandrus ambrosia beetles.

Xylosandrus compactus and X. crassiusculus are two polyphagous ambrosia beetles originating from Asia and invasive in circumtropical regions worldwide. Both species were recently reported in Italy and further invaded several other European countries in the following years. We used the MaxEnt algorithm to estimate the suitable areas worldwide for both species under the current climate. We also made future projections for years 2050 and 2070 using 11 different General Circulation Models, for 4 Representative Concentration Pathways (2.6, 4.5, 6.0 and 8.5). Our analyses showed that X. compactus has not been reported in all potentially suitable areas yet. Its current distribution in Europe is localised, whereas our results predicted that most of the periphery of the Mediterranean Sea and most of the Atlantic coast of France could be suitable. Outside Europe, our results also predicted Central America, all islands in Southeast Asia and some Oceanian coasts as suitable. Even though our results when modelling its potential distribution under future climates were more variable, the models predicted an increase in suitability poleward and more uncertainty in the circumtropical regions. For X. crassiusculus, the same method only yielded poor results, and the models thus could not be used for predictions. We discuss here these results and propose advice about risk prevention and invasion management of both species.

An ultra-compact leaky-integrate-and-fire model for building spiking neural networks.

We introduce an ultra-compact electronic circuit that realizes the leaky-integrate-and-fire model of artificial neurons. Our circuit has only three active devices, two transistors and a silicon controlled rectifier (SCR). We demonstrate the implementation of biologically realistic features, such as spike-frequency adaptation, a refractory period and voltage modulation of spiking rate. All characteristic times can be controlled by the resistive parameters of the circuit. We built the circuit with out-of-the-shelf components and demonstrate that our ultra-compact neuron is a modular block that can be associated to build multi-layer deep neural networks. We also argue that our circuit has low power requirements, as it is normally off except during spike generation. Finally, we discuss the ultimate ultra-compact limit, which may be achieved by further replacing the SCR circuit with Mott materials.

Imaging the nanoscale phase separation in vanadium dioxide thin films at terahertz frequencies.

Vanadium dioxide (VO2) is a material that undergoes an insulator-metal transition upon heating above 340 K. It remains debated as to whether this electronic transition is driven by a corresponding structural transition or by strong electron-electron correlations. Here, we use apertureless scattering near-field optical microscopy to compare nanoscale images of the transition in VO2 thin films acquired at both mid-infrared and terahertz frequencies, using a home-built terahertz near-field microscope. We observe a much more gradual transition when THz frequencies are utilized as a probe, in contrast to the assumptions of a classical first-order phase transition. We discuss these results in light of dynamical mean-field theory calculations of the dimer Hubbard model recently applied to VO2, which account for a continuous temperature dependence of the optical response of the VO2 in the insulating state.

How a dc Electric Field Drives Mott Insulators Out of Equilibrium.

Out of equilibrium phenomena are a major issue of modern physics. In particular, correlated materials such as Mott insulators experience fascinating long-lived exotic states under a strong electric field. Yet, the origin of their destabilization by the electric field is not elucidated. Here we present a comprehensive study of the electrical response of canonical Mott insulators GaM_{4}Q_{8} (M=V, Nb, Ta, Mo; Q=S, Se) in the context of a microscopic theory of electrical breakdown where in-gap states allow for a description in terms of a two-temperature model. Our results show how the nonlinearities and the resistive transition originate from a massive creation of hot electrons under an electric field. These results give new insights for the control of the long-lived states reached under an electric field in these systems which has recently open the way to new functionalities used in neuromorphic applications.

Bad-Metal Behavior Reveals Mott Quantum Criticality in Doped Hubbard Models.

Bad-metal (BM) behavior featuring linear temperature dependence of the resistivity extending to well above the Mott-Ioffe-Regel (MIR) limit is often viewed as one of the key unresolved signatures of strong correlation. Here we associate the BM behavior with the Mott quantum criticality by examining a fully frustrated Hubbard model where all long-range magnetic orders are suppressed, and the Mott problem can be rigorously solved through dynamical mean-field theory. We show that for the doped Mott insulator regime, the coexistence dome and the associated first-order Mott metal-insulator transition are confined to extremely low temperatures, while clear signatures of Mott quantum criticality emerge across much of the phase diagram. Remarkable scaling behavior is identified for the entire family of resistivity curves, with a quantum critical region covering the entire BM regime, providing not only insight, but also quantitative understanding around the MIR limit, in agreement with the available experiments.

Manganite-based memristive heterojunction with tunable non-linear I-V characteristics.

A resistive random access memory (ReRAM) based on the memristive effect allows high-density integration through a cross-point array (CPA) structure. However, a significant common drawback of the CPA configuration is the crosstalk between cells. Here, we introduce a solution based on a novel heterojunction stack solely made of members of the perovskite manganite family Pr(1-x)Ca(x)MnO3 (PCMO) and CaMnO(3-δ) (CMO) which show electroforming-free bipolar resistive switching. The heterojunction consists of rectifying interfaces and shows a symmetrical and tunable non-linear current-voltage curve. The spectromicroscopic measurements support the scenario of specialized roles, with the memristive effect taking place at the active Al-PCMO interface via a redox mechanism, while non-linearity was achieved by adopting a rectifying double interface PCMO-CMO-PCMO.

Memristive and neuromorphic behavior in a Li(x)CoO2 nanobattery.

The phenomenon of resistive switching (RS), which was initially linked to non-volatile resistive memory applications, has recently also been associated with the concept of memristors, whose adjustable multilevel resistance characteristics open up unforeseen perspectives in cognitive computing. Herein, we demonstrate that the resistance states of Li(x)CoO2 thin film-based metal-insulator-metal (MIM) solid-state cells can be tuned by sequential programming voltage pulses, and that these resistance states are dramatically dependent on the pulses input rate, hence emulating biological synapse plasticity. In addition, we identify the underlying electrochemical processes of RS in our MIM cells, which also reveal a nanobattery-like behavior, leading to the generation of electrical signals that bring an unprecedented new dimension to the connection between memristors and neuromorphic systems. Therefore, these LixCoO2-based MIM devices allow for a combination of possibilities, offering new perspectives of usage in nanoelectronics and bio-inspired neuromorphic circuits.

First-order insulator-to-metal Mott transition in the paramagnetic 3D system GaTa4Se8.

The nature of the Mott transition in the absence of any symmetry breaking remains a matter of debate. We study the correlation-driven insulator-to-metal transition in the prototypical 3D Mott system GaTa(4)Se(8), as a function of temperature and applied pressure. We report novel experiments on single crystals, which demonstrate that the transition is of first order and follows from the coexistence of two states, one insulating and one metallic, that we toggle with a small bias current. We provide support for our findings by contrasting the experimental data with calculations that combine local density approximation with dynamical mean-field theory, which are in very good agreement.

Epidemiology of asexuality induced by the endosymbiotic Wolbachia across phytophagous wasp species: host plant specialization matters.

Among eukaryotes, sexual reproduction is by far the most predominant mode of reproduction. However, some systems maintaining sexuality appear particularly labile and raise intriguing questions on the evolutionary routes to asexuality. Thelytokous parthenogenesis is a form of spontaneous loss of sexuality leading to strong distortion of sex ratio towards females and resulting from mutation, hybridization or infection by bacterial endosymbionts. We investigated whether ecological specialization is a likely mechanism of spread of thelytoky within insect communities. Focusing on the highly specialized genus Megastigmus (Hymenoptera: Torymidae), we first performed a large literature survey to examine the distribution of thelytoky in these wasps across their respective obligate host plant families. Second, we tested for thelytoky caused by endosymbionts by screening in 15 arrhenotokous and 10 thelytokous species for Wolbachia, Cardinium, Arsenophonus and Rickettsia endosymbionts and by performing antibiotic treatments. Finally, we performed phylogenetic reconstructions using multilocus sequence typing (MLST) to examine the evolution of endosymbiont-mediated thelytoky in Megastigmus and its possible connections to host plant specialization. We demonstrate that thelytoky evolved from ancestral arrhenotoky through the horizontal transmission and the fixation of the parthenogenesis-inducing Wolbachia. We find that ecological specialization in Wolbachia's hosts was probably a critical driving force for Wolbachia infection and spread of thelytoky, but also a constraint. Our work further reinforces the hypothesis that community structure of insects is a major driver of the epidemiology of endosymbionts and that competitive interactions among closely related species may facilitate their horizontal transmission.

Two-dimensional electron gas with six-fold symmetry at the (111) surface of KTaO3.

Two-dimensional electron gases (2DEGs) at transition-metal oxide (TMO) interfaces, and boundary states in topological insulators, are being intensively investigated. The former system harbors superconductivity, large magneto-resistance, and ferromagnetism. In the latter, honeycomb-lattice geometry plus bulk spin-orbit interactions lead to topologically protected spin-polarized bands. 2DEGs in TMOs with a honeycomb-like structure could yield new states of matter, but they had not been experimentally realized, yet. We successfully created a 2DEG at the (111) surface of KTaO3, a strong insulator with large spin-orbit coupling. Its confined states form a network of weakly-dispersing electronic gutters with 6-fold symmetry, a topology novel to all known oxide-based 2DEGs. If those pertain to just one Ta-(111) bilayer, model calculations predict that it can be a topological metal. Our findings demonstrate that completely new electronic states, with symmetries not realized in the bulk, can be tailored in oxide surfaces, promising for TMO-based devices.

Avalanche breakdown in GaTa4Se(8-x)Te(x) narrow-gap Mott insulators.

Mott transitions induced by strong electric fields are receiving growing interest. Recent theoretical proposals have focused on the Zener dielectric breakdown in Mott insulators. However, experimental studies are still too scarce to conclude about the mechanism. Here we report a study of the dielectric breakdown in the narrow-gap Mott insulators GaTa4Se(8-x)Te(x). We find that the I-V characteristics and the magnitude of the threshold electric field (Eth) do not correspond to a Zener breakdown, but rather to an avalanche breakdown. Eth increases as a power law of the Mott-Hubbard gap (Eg), in surprising agreement with the universal law Eth is proportional to Eg(2.5) reported for avalanche breakdown in semiconductors. However, the delay time for the avalanche that we observe in Mott insulators is over three orders of magnitude greater than in conventional semiconductors. Our results suggest that the electric field induces local insulator-to-metal Mott transitions that create conductive domains that grow to form filamentary paths across the sample.

Inferences on population history of a seed chalcid wasp: invasion success despite a severe founder effect from an unexpected source population.

Most invasive species established in Europe originate from either Asia or North America, but little is currently known about the potential of the Anatolian Peninsula (Asia Minor) and/or the Near East to constitute invasion sources. Mediterranean forests are generally fragile ecosystems that can be threatened by invasive organisms coming from different regions of the Mediterranean Basin, but for which historical data are difficult to gather and the phylogeographic patterns are still poorly understood for most terrestrial organisms. In this study, we characterized the genetic structure of Megastigmus schimitscheki, an invasive seed-feeding insect species originating from the Near East, and elucidated its invasion route in South-eastern France in the mid 1990s. To disentangle the evolutionary history of this introduction, we gathered samples from the main native regions (Taurus Mountains in Turkey, Lebanon and Cyprus) and from the invaded region that we genotyped using five microsatellite markers and for which we sequenced the mitochondrial Cytochrome Oxidase I gene. We applied a set of population genetic statistics and methods, including approximate Bayesian computation. We proposed a detailed phylogeographic pattern for the Near East populations, and we unambiguously showed that the French invasive populations originated from Cyprus, although the available historical data strongly suggested that Turkey could be the most plausible source area. Interestingly, we could show that the introduced populations were founded from an extremely restricted number of individuals that realized a host switch from Cedrus brevifolia to C. atlantica. Evolutionary hypotheses are discussed to account for this unlikely scenario.

Electronic subband reconfiguration in a d0-perovskite induced by strain-driven structural transformations.

It is well known that transport in lightly n-doped SrTiO(3) involves light and heavy electron bands. We have found that upon application of moderate quasi-isotropic pressures, the relative positions of these subbands are changed by a few meV and, eventually, a band inversion occurs at ~1 kbar. Such effects are, however, suppressed in the closely related KTaO(3) perovskite. We show that the extremely subtle electronic reconfiguration in SrTiO(3) is triggered by strain-induced structural transformations that are accompanied by remarkable mobility enhancements up to about Δµ/µ≈300%. Our results provide a microscopic rationale for the recently discovered transport enhancement under strain and underscore the role of the internal structural degrees of freedom in the modulation of the perovskite electronic properties.

Two-dimensional electron gas with universal subbands at the surface of SrTiO(3).

As silicon is the basis of conventional electronics, so strontium titanate (SrTiO(3)) is the foundation of the emerging field of oxide electronics. SrTiO(3) is the preferred template for the creation of exotic, two-dimensional (2D) phases of electron matter at oxide interfaces that have metal-insulator transitions, superconductivity or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs), which is crucial to understanding their remarkable properties, remains elusive. Here we show, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO(3) (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades. This 2DEG is confined within a region of about five unit cells and has a sheet carrier density of ∼0.33 electrons per square lattice parameter. The electronic structure consists of multiple subbands of heavy and light electrons. The similarity of this 2DEG to those reported in SrTiO(3)-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO(3) lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO(3)-based devices and a novel means of generating 2DEGs at the surfaces of transition-metal oxides.

Matrix isolation infrared spectrum of the sulfuric acid-monohydrate complex: new assignments and resolution of the "missing H-Bonded v(OH) band" issue.

The matrix isolation infrared spectra of "dry" and "wet" vapors of sulfuric acid have been investigated as trapped in solid argon matrices. The availability of a spectrum of trapped anhydrous acid vapor and its comparison with the spectra of trapped water containing vapors of the acid allowed the identification of the hydrogen-bonding shifted hydroxyl bands for both the acid and the water moieties of the monohydrated H(2)SO(4).H(2)O complex. The experimental results are compared to the various theoretically calculated wavenumber values of the acid and its monohydrated complex. The complex stabilization energies, as obtained from calculations and empirical correlations, are compared.

Dynamical mean field theory of an effective three-band model for NaxCoO2.

We derive an effective Hamiltonian for highly correlated t_{2g} states centered at the Co sites of NaxCoO2. The essential ingredients of the model are an O mediated hopping, a trigonal crystal-field splitting, and on-site effective interactions derived from the exact solution of a multiorbital model in a CoO6 cluster, with parameters determined previously. The effective model is solved by dynamical mean field theory. We obtain a Fermi surface and electronic dispersion that agrees well with angle-resolved photoemission spectra. Our results also elucidate the origin of the "sinking pockets" in different doping regimes.

A reliable confirmation of the chemical structure of synthetic oligonucleotides: detection of active protons in DNA oligomers by low-temperature FT infrared spectroscopy.

Cooling the samples allowed us to characterize solid oligonucleotides such as dimers, trimers and pentamers of cytidine, for the first time, in the IR range of the out-of-plane bending molecular modes (1000-400 cm(-1)) at 20K. Especially interesting are the narrow IR bands of the out-of-plane bending nu(4) NH(2) proton mode, which are apparently invisible at room temperature. This unequivocally defined and well-resolved NH(2) bending band should provide important information on the exact chemical form and hydrogen bonding interactions of cytidine amine groups. As such, this unique IR spectroscopy is suggested as a practical analytical tool to validate and characterize synthetic DNA bases and oligonucleotides. Using an approach of this type it was found that desalted oligonucleotide samples of the same nominal composition, but which had been produced by three different manufacturers, differ significantly in their IR spectra. These data suggest that the presumably identical oligonucleotides are in fact different, at least with respect to the content and nature of their NH protons.