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International audience; Visible nanocrystal-based light-emitting diodes (LEDs) are about to become commercially available. However, their infrared counterparts suffer from two key limitations. First, III–V semiconductor technologies are strong competitors. Second, their potential for operation beyond 1.7 µm remains unexplored. The range from 1.5 to 4 µm corresponds to a technological gap in which the efficiency of interband quantum-well-based devices vanishes and quantum cascade lasers are not efficient enough. Powerful infrared LEDs in this range are needed for applications such as active imaging, organic molecule sensing and airfield lighting. Here we report the design of a HgTe nanocrystal-based LED with luminescence between 2 and 2.3 µm. With an external quantum efficiency of 0.3% and radiance up to 3 W Sr−1 m−2, these HgTe LEDs already present a competitive performance for emission above 2 µm.

We introduce new $U_q\mathfrak{sl}_2$-invariant boundary conditions for the open XXZ spin chain. For generic values of $q$ we couple the bulk Hamiltonian to an infinite-dimensional Verma module on one or both boundaries of the spin chain, and for $q=e^{\frac{i\pi}{p}}$ a $2p$-th root of unity $ - $ to its $p$-dimensional analogue. Both cases are parametrised by a continuous "spin" $\alpha\in\mathbb{C}$. To motivate our construction, we first specialise to $q=i$, where we obtain a modified XX Hamiltonian with unrolled quantum group symmetry, whose spectrum and scaling limit is computed explicitly using free fermions. In the continuum, this model is identified with the $(\eta,\xi)$ ghost CFT on the upper-half plane with a continuum of conformally invariant boundary conditions on the real axis. The different sectors of the Hamiltonian are identified with irreducible Virasoro representations. Going back to generic $q$ we investigate the algebraic properties of the underlying lattice algebras. We show that if $q^\alpha\notin\pm q^{\mathbb{Z}}$, the new boundary coupling provides a faithful representation of the blob algebra which is Schur-Weyl dual to $U_q\mathfrak{sl}_2$. Then, modifying the boundary conditions on both the left and the right, we obtain a representation of the universal two-boundary Temperley-Lieb algebra. The generators and parameters of these representations are computed explicitly in terms of $q$ and $\alpha$. Finally, we conjecture the general form of the Schur-Weyl duality in this case. This paper is the first in a series where we will study, at all values of the parameters, the spectrum and its continuum limit, the representation content of the relevant lattice algebras and the fusion properties of these new spin chains., Comment: 54 pages

International audience; This paper presents a model of capital accumulation for a large number of heterogenous producer-consumers in an exchange space in which interactions depend on agents' positions. Each agent is described by his production, consumption, stock of capital, as well as the position he occupies in this abstract space. Each agent produces one differentiated good whose price is fixed by market clearing conditions. Production functions are Cobb-Douglas, and capital stocks follow the standard capital accumulation dynamic equation. Agents consume all goods but have a preference for goods produced by their closest neighbors. Agents in the exchange space are subject both to attractive and repulsive forces. Exchanges drive agents closer, but beyond a certain level of proximity, agents will tend to crowd out more distant agents. The present model uses a formalism based on statistical field theory developed earlier by the authors. This approach allows the analytical treatment of economic models with an arbitrary number of agents, while preserving the system's interactions and complexity at the individual level. Our results show that the dynamics of capital accumulation and agents' position in the exchange space are correlated. Interactions in the exchange space induce several phases of the system. A first phase appears when attractive forces are limited. In this phase, an initial central position in the exchange space favors capital accumulation in average and leads to a higher level of capital, while agents far from the center will experience a slower accumulation process. A high level of initial capital drives agents towards a central position, i.e. improve the terms of their exchanges: they experience a higher demand and higher prices for their product. As usual, high capital productivity favors capital accumulation, while higher rates of capital depreciation reduce capital stock. In a second phase, attractive forces are predominant. The previous results remain, but an additional threshold effect appears. Even though no restriction was imposed initially on the system, two types of agents emerge, depending on their initial stock of capital. One type of agents will remain above the capital threshold and occupy and benefit from a central position. The other type will remain below the threshold, will not be able to break it and will remain at the periphery of the exchange space. In this phase, capital distribution is less homogenous than in the first phase.

The simplest picture of excitons in materials with atomic-like localization of electrons is that of Frenkel excitons, where electrons and holes stay close together, which is associated with a large binding energy. Here, using the example of the layered oxide V2O5, we show how localized charge-transfer excitations combine to form excitons that also have a huge binding energy but, at the same time, a large electron-hole distance, and we explain this seemingly contradictory finding. The anisotropy of the exciton delocalization is determined by the local anisotropy of the structure, whereas the exciton extends orthogonally to the chains formed by the crystal structure. Moreover, we show that the bright exciton goes together with a dark exciton of even larger binding energy and more pronounced anisotropy. These findings are obtained by combining first principles many-body perturbation theory calculations, ellipsometry experiments, and tight binding modelling, leading to very good agreement and a consistent picture. Our explanation is general and can be extended to other materials.

The attractor Hausdorff dimension is an important quantity bridging information theory and dynamical systems, as it is related to the number of effective degrees of freedom of the underlying dynamical system. By using the link between extreme value theory and Poincare recurrences, it is possible to estimate this quantity from time series of high-dimensional systems without embedding the data. In general $$d \le n$$ , where n is the dimension of the full phase-space, as the dynamics freezes some of the available degrees of freedom. This is equivalent to constraining trajectories on a compact object in phase space, namely the attractor. Information theory shows that the equality $$d=n$$ holds for random systems. However, applying extreme value theory, we show that this result cannot be recovered and that $$d

International audience; We present the complete theory for the decay rates of non-equilibrium fluctuations in a ternary liquid mixture subjected to a stationary temperature gradient, when the quiescent non-convective state is stable. In the most general case, within Boussinesq approximation, four fluctuating modes exist. Depending on the parameter values, propagative modes may be present, and we discuss numerically some cases where that is so. We complete the work with a discussion of symmetry upon changes in concentration representation, as well as examination of some limiting cases with practical relevance for which analytical progress is possible. We make contact with previous publications, which were based on some kind of approximation.

arXiv:1907.11916v1, The discovery of superconductivity at 200 kelvin in the hydrogen sulfide system at high pressures demonstrated the potential of hydrogen-rich materials as high-temperature superconductors. Recent theoretical predictions of rare-earth hydrides with hydrogen cages and the subsequent synthesis of LaH10 with a superconducting critical temperature (Tc) of 250 kelvin have placed these materials on the verge of achieving the long-standing goal of room-temperature superconductivity. Electrical and X-ray diffraction measurements have revealed a weakly pressure-dependent Tc for LaH10 between 137 and 218 gigapascals in a structure that has a face-centred cubic arrangement of lanthanum atoms. Here we show that quantum atomic fluctuations stabilize a highly symmetrical Fm3¯¯¯m crystal structure over this pressure range. The structure is consistent with experimental findings and has a very large electron–phonon coupling constant of 3.5. Although ab initio classical calculations predict that this Fm3¯¯¯m structure undergoes distortion at pressures below 230 gigapascals, yielding a complex energy landscape, the inclusion of quantum effects suggests that it is the true ground-state structure. The agreement between the calculated and experimental Tc values further indicates that this phase is responsible for the superconductivity observed at 250 kelvin. The relevance of quantum fluctuations calls into question many of the crystal structure predictions that have been made for hydrides within a classical approach and that currently guide the experimental quest for room-temperature superconductivity. Furthermore, we find that quantum effects are crucial for the stabilization of solids with high electron–phonon coupling constants that could otherwise be destabilized by the large electron–phonon interaction9, thus reducing the pressures required for their synthesis., This research was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 802533); the Spanish Ministry of Economy and Competitiveness (FIS2016-76617-P); Grant-in-Aid for Scientific Research (number 16H06345, 18K03442 and 19H05825) from the Ministry of Education, Culture, Sports, Science and Technology, Japan; and NCCR MARVEL funded by the Swiss National Science Foundation. Computational resources were provided by the Barcelona Superconducting Center (project FI-2019-1-0031) and the Swiss National Supercomputing Center (CSCS) with project s970.

International audience; Optical properties of polymerized polyethylene layers deposited on glass substrates by PECVD technique at different radio frequency powers are studied through transmission and reflection spectra. Analysis of the experimental spectra by using conventional classic models which permit the determination of optical constants as the refractive index n, the dielectric permittivity \epsilon, the extinction k and absorption coefficients α which leads to the optical parameters characterizing the layers such as static refractive index ns,permittivity \epsilons, plasmon frequency ωp, carrier density to effective mass ratio N/m*e,optical conductivity σop, optical band gap Eg and dispersion diffusion energies E0 and Ed. These different values permit to select the suitable power for elaborate layers with performant properties for various applications in the field of communication, electrical isolation, and miniaturized optoelectronics and sensors devices. \textcopyright 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.

In this work, we investigate the possibility of controlling the acoustic damping in a liquid when nanoparticles are suspended in it. To shed light on this topic, we performed Inelastic X-Ray Scattering (IXS) measurements of the terahertz collective dynamics of aqueous suspensions of nanospheres of various materials, size, and relative concentration, either charged or neutral. A Bayesian analysis of measured spectra indicates that the damping of the two acoustic modes of water increases upon nanoparticle immersion. This effect seems particularly pronounced for the longitudinal acoustic mode, which, whenever visible at all, rapidly damps off when increasing the exchanged wavevector. Results also indicate that the observed effect strongly depends on the material the immersed nanoparticles are made of.

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Cooling down nanomechanical force probes is a generic strategy to enhance their sensitivities through the concomitant reduction of their thermal noise and mechanical damping rates. However, heat conduction becomes less efficient at low temperatures, which renders difficult to ensure and verify their proper thermalization. Here we implement optomechanical readout techniques operating in the photon counting regime to probe the dynamics of suspended silicon carbide nanowires in a dilution refrigerator. Readout of their vibrations is realized with sub-picowatt optical powers, in a situation where less than one photon is collected per oscillation period. We demonstrate their thermalization down to 32 ± 2 mK, reaching very large sensitivities for scanning probe force sensors, 40 zN Hz−1/2, with a sensitivity to lateral force field gradients in the fN m−1 range. This opens the road toward explorations of the mechanical and thermal conduction properties of nanoresonators at minimal excitation level, and to nanomechanical vectorial imaging of faint forces at dilution temperatures., Optical readout techniques for nanomechanical force probes usually generate more heat than what can be dissipated through the nanoresonators. Here, the authors use an interferometric readout scheme, achieving large force sensitivity using suspended silicon carbide nanowires at dilution temperatures.

The use of natural sustainable resources such as wood in green industrial processes is currently limited by our poor understanding of the impact of moisture on their thermodynamic and mechanical behaviors. Here, a molecular dynamics approach is used to investigate the physical response of a typical hydrophilic biopolymer in softwood hemicellulose—xylan—when subjected to moisture adsorption. A unique moisture-induced crossover is found in the thermodynamic and mechanical properties of this prototypical biopolymer with many quantities such as the heat of adsorption, heat capacity, thermal expansion and elastic moduli exhibiting a marked evolution change for a moisture content about 30 wt%. By investigating the microscopic structure of the confined water molecules and the polymer–water interfacial area, the molecular mechanism responsible for this crossover is shown to correspond to the formation of a double-layer adsorbed film along the amorphous polymeric chains. In addition to this moisture-induced crossover, many properties of the hydrated biopolymer are found to obey simple material models., Cellulose, 27 (1), ISSN:1572-882X, ISSN:0969-0239

International audience; We present an efficient method to compute the eigenvalues and eigenmodes of the diffusion operator ∇(D∇) on one-dimensional heterogeneous structures with multiple semi-permeable barriers. This method allows us to calculate the diffusion propagator and related quantities such as diffusion MRI signal or first exit time distribution analytically for regular geometries and numerically for arbitrary ones. The effect of the barriers and the transition from infinite permeability (no barriers) to zero permeability (impermeable barriers) are investigated.

While spintronics has traditionally relied on ferromagnetic metals as spin generators and detectors, spin–orbitronics exploits the efficient spin–charge interconversion enabled by spin–orbit coupling in non-magnetic systems. Although the Rashba picture of split parabolic bands is often used to interpret such experiments, it fails to explain the largest conversion effects and their relationship with the electronic structure. Here, we demonstrate a very large spin-to-charge conversion effect in an interface-engineered, high-carrier-density SrTiO3 two-dimensional electron gas and map its gate dependence on the band structure. We show that the conversion process is amplified by enhanced Rashba-like splitting due to orbital mixing and in the vicinity of avoided band crossings with topologically non-trivial order. Our results indicate that oxide two-dimensional electron gases are strong candidates for spin-based information readout in new memory and transistor designs. Our results also emphasize the promise of topology as a new ingredient to expand the scope of complex oxides for spintronics. A very large spin-to-charge conversion arising from a combination of the Rashba effect and topologically non-trivial states is realized at the interface of strontium titanate and aluminium, with implications for the role of topology in memory and transistor designs.

(111)-oriented CdZnTe semiconductor material exhibits a crystal polarity and hence, a face terminated by Cd atoms, conventionally called (111) A face with an opposite face terminated by Te atoms, called (111) B face. These A and B faces have different properties. Their mechanical properties have been studied by mechanical indentation. Sample surfaces were carefully prepared and mechanically and chemically polished, while the crystalline orientation and material defectivity was monitored on both faces using x-ray rocking curve measurements around the (222) orientation in order to evaluate faces with similar quality. The elastic moduli and hardness values of the A and B faces of CdZnTe were similar. Hardness increased from 0.60 GPa to 0.90 GPa with Zn concentration on the B face as a result of solution strengthening. Crack formation and propagation was carefully studied. Both toughness value and crack resistance were affected by polarity. A model involving plastic dislocation flow is proposed to interpret the results.

The capacitor is a key element of electronic devices and is characterized by positive capacitance. However, a negative capacitance (NC) behaviour may occur in certain cases and implies a local voltage drop opposed to the overall applied bias. Therefore, a local NC response results in voltage enhancement across the rest of the circuit. Within a suitably designed heterostructure, ferroelectrics display such an NC effect, and various ferroelectric-based microelectronic and nanoelectronic devices have been developed, showing improved performance attributed to NC. However, the exact physical nature of the NC response and direct experimental evidence remain elusive or controversial thus far. In this Review, we discuss the physical mechanisms responsible for ferroelectric NC, tackling static and transient NC responses. We examine ferroelectric responses to voltage and charge, as well as ferroelectric switching, and discuss proof-of-concept experiments and possibilities for device implementation. Finally, we highlight different approaches for the optimization of the intrinsic NC response to maximize voltage amplification. Ferroelectrics-based materials can display a negative capacitance (NC) effect, providing an opportunity to implement NC in electronic circuits to improve their performance. In this Review, the authors discuss static and transient NC responses in ferroelectrics and highlight proof-of-concept experiments and possibilities for device implementation.

Seven scientists share their views on some of the latest developments in attosecond science and X-ray free electron lasers (XFELs) and highlight exciting new directions.

International audience; CuO is an attractive photocatalytic material for water splitting due to its high earth abundance and low cost. In this paper, we report the deposition of CuO thin films by sol–gel dip-coating process. Sol deposition has attractive advantages including low-cost solution processing and uniform film formation over large areas with a fairly good control of the film stoichiometry and thickness. Pure CuO phase was obtained for calcination temperatures higher than 360 °C in air. The CuO photocurrents for hydrogen evolution depend on the crystallinity and the microstructure of the film. Values of −0.94 mA cm−2 at pH = 8 and 0 V vs. RHE were achieved for CuO photoelectrodes annealed at 400 °C under air. More interestingly, the stability of the photoelectrode was enhanced upon the sol–gel deposition of a TiO2 protective layer. In this all sol–gel CuO/TiO2 photocathode, a photocurrent of −0.5 mA cm−2 is achieved at pH = 7 and 0 V vs. RHE with a stability of ~100% over 600 s.

Granular multiparticle ensembles are of interest from fundamental statistical viewpoints as well as for the understanding of collective processes in industry and in nature. Extraction of physical data from optical observations of three-dimensional (3D) granular ensembles poses considerable problems. Particle-based tracking is possible only at low volume fractions, not in clusters. We apply shadow-based and feature-tracking methods to analyze the dynamics of granular gases in a container with vibrating side walls under microgravity. In order to validate the reliability of these optical analysis methods, we perform numerical simulations of ensembles similar to the experiment. The simulation output is graphically rendered to mimic the experimentally obtained images. We validate the output of the optical analysis methods on the basis of this ground truth information. This approach provides insight in two interconnected problems: the confirmation of the accuracy of the simulations and the test of the applicability of the visual analysis. The proposed approach can be used for further investigations of dynamical properties of such media, including the granular Leidenfrost effect, granular cooling, and gas-clustering transitions.

The promise enabled by boron arsenide’s (BAs) high thermal conductivity (κ) in power electronics cannot be assessed without taking into account the reduction incurred when doping the material. Using first principles calculations, we determine the κ reduction induced by different group IV impurities in BAs as a function of concentration and charge state. We unveil a general trend, where neutral impurities scatter phonons more strongly than the charged ones. CB and GeAs impurities show by far the weakest phonon scattering and retain BAs κ values of over ~1000 W⋅K−1⋅m−1 even at high densities. Both Si and Ge achieve large hole concentrations while maintaining high κ. Furthermore, going beyond the doping compensation threshold associated to Fermi level pinning triggers observable changes in the thermal conductivity. This informs design considerations on the doping of BAs, and it also suggests a direct way to determine the onset of compensation doping in experimental samples.

Applications relying on mid-infrared radiation (λ ~ 3-30 μm) have progressed at a very rapid pace in recent years, stimulated by scientific and technological breakthroughs like mid-infrared cameras and quantum cascade lasers. On the other side, standalone and broadband devices allowing control of the beam amplitude and/or phase at ultra-fast rates (GHz or more) are still missing. Here we show a free-space amplitude modulator for mid-infrared radiation (λ ~ 10 μm) that can operate at room temperature up to at least 1.5 GHz (−3dB cutoff at ~750 MHz). The device relies on a semiconductor heterostructure enclosed in a judiciously designed metal–metal optical resonator. At zero bias, it operates in the strong light-matter coupling regime up to 300 K. By applying an appropriate bias, the device transitions towards the weak-coupling regime. The large change in reflectance is exploited to modulate the intensity of a mid-infrared continuous-wave laser up to 1.5 GHz., Broadband integrated electrical modulators are key components for photonic systems. Here, the authors present a room temperature mid-IR free-space amplitude modulator based on a semiconductor heterostructure that exploits the change in reflectance occurring at the change between weak and strong coupling.

Extending recent work on stress fluctuations in complex fluids and amorphous solids we describe in general terms the ensemble average $v(\Delta t)$ and the standard deviation $\delta v(\Delta t)$ of the variance $v[\mathbf{x}]$ of time series $\mathbf{x}$ of a stochastic process $x(t)$ measured over a finite sampling time $\Delta t$. Assuming a stationary, Gaussian and ergodic process, $\delta v$ is given by a functional $\delta v_G[h]$ of the autocorrelation function $h(t)$. $\delta v(\Delta t)$ is shown to become large and similar to $v(\Delta t)$ if $\Delta t$ corresponds to a fast relaxation process. Albeit $\delta v = \delta v_G[h]$ does not hold in general for non-ergodic systems, the deviations for common systems with many microstates are merely finite-size corrections. Various issues are illustrated for shear-stress fluctuations in simple coarse-grained model systems., Comment: 19 pages, 18 figures, submitted to EPJE

We report on spin transport in state-of-the-art epitaxial monolayer graphene based 2D-magnetic tunnel junctions (2D-MTJs). In our measurements, supported by ab-initio calculations, the strength of interaction between ferromagnetic electrodes and graphene monolayers is shown to fundamentally control the resulting spin signal. In particular, by switching the graphene/ferromagnet interaction, spin transport reveals magneto-resistance signal MR > 80% in junctions with low resistance × area products. Descriptions based only on a simple K-point filtering picture (i.e. MR increase with the number of layers) are not sufficient to predict the behavior of our devices. We emphasize that hybridization effects need to be taken into account to fully grasp the spin properties (such as spin dependent density of states) when 2D materials are used as ultimately thin interfaces. While this is only a first demonstration, we thus introduce the fruitful potential of spin manipulation by proximity effect at the hybridized 2D material / ferromagnet interface for 2D-MTJs., 2D materials are foreseen as an opportunity to tailor spintronics devices interfaces, a.k.a spinterfaces. Here, using state-of-the-art large-scale integration in spin-valves, authors demonstrate that hybridization of graphene with a metallic spin source results in strong spin filtering effects.

Spin-wave and exchange stiffness constants of 22 ferromagnetic compounds have been deduced from their spontaneous magnetization, $$M_{\mathrm{s}}$$ , by using an improved technique. The improvement consists in utilizing the entire $$M_{\mathrm{s}}(T)$$ curve, up to the Curie point, rather than just its low-temperature part, with $$T\ll T_{\mathrm{C}}$$ . For 17 of the 22 ferromagnets, literature data have been used, while 5 compounds have been studied anew, on single crystals grown for the purpose.

The physical properties of metals change when their dimensions are reduced to the nano-scale and new phenomena such as the localized surface-plasmon resonance (LSPR) appear. This collective electronic excitation can be tuned over a large spectral range by adapting the material, size and shape. The existing literature is as rich as it is controversial—for example, size-dependent spectral shifts of the LSPR in small metal nanoparticles, induced by quantum effects, are reported to the red, to the blue or entirely absent. Here we report how complementary experiments on size-selected small silver nanoparticles embedded in silica can yield inconsistent results on the same system: whereas optical absorption shows no size effect in the range between only a few atoms and ~10 nm, a clear spectral shift is observed in single-particle electron spectroscopy. Our quantitative interpretation, based on a mixed classical/quantum model, resolves the apparent contradictions, not only within our experimental data, but also in the literature. Our comprehensive model describes how the local environment is the crucial parameter controlling the manifestation or absence of quantum size effects. The origin of size-dependent shifts of surface plasmon resonances in metal nanoparticles has been controversial for decades. A combined experimental and theoretical study on silver samples and their environments now provides a quantitative picture.

Spin-resonance modes (SRM) are taken as evidence for magnetically driven pairing in Fe-based superconductors, but their character remains poorly understood. The broadness, the splitting and the spin-space anisotropies of SRMs contrast with the mostly accepted interpretation as spin excitons. We study hole-doped Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ that displays a spin reorientation transition. This reorientation has little impact on the overall appearance of the resonance excitations with a high-energy isotropic and a low-energy anisotropic mode. However, the strength of the anisotropic low-energy mode sharply peaks at the highest doping that still exhibits magnetic ordering resulting in the strongest SRM observed in any Fe-based superconductor so far. This remarkably strong SRM is accompanied by a loss of about half of the magnetic Bragg intensity upon entering the SC phase. Anisotropic SRMs thus can allow the system to compensate for the loss of exchange energy arising from the reduced antiferromagnetic correlations within the SC state., 10 pages, 5 figures, 1 table

The prospect of combining both magnetic and plasmonic properties in a single nanoparticle promises both valuable insights on the properties of such systems from a fundamental viewpoint and numerous possibilities for technological applications. However, the combination of two of the most prominent metallic candidates—iron and silver—has presented numerous experimental difficulties because their thermodynamic properties impede miscibility and even coalescence. Herein, we present the thorough characterization of physically prepared Fe50Ag50 nanoparticles embedded in carbon and silica matrices via electron microscopy, optical spectroscopy, magnetometry and synchrotron-based X-ray spectroscopy. Iron and silver segregate completely into structures resembling fried eggs, with a nearly spherical, crystallized silver part surrounded by an amorphous structure of iron carbide or oxide, depending on the environment of the particles. Consequently, the particles exhibit both plasmonic absorption corresponding to the silver nanospheres in an oxide environment and a reduced but measurable magnetic response. The suitability of such nanoparticles for technological applications is discussed from the viewpoint of their high chemical reactivity with their environment.

Material development has played a crucial role in modern civilization and IT. The importance of high-density and high-performance memory in modern computer systems and IT is ever increasing. This trend will be more obvious as computational architectures shift from being processing-centric to memory- (or data-) centric. The need for emerging and new memory technologies with nonvolatility and low power-consuming performance is rapidly increasing, while improvements in current dynamic random-access memory and NAND flash are being pursued. In both new and current memories, material innovation is of central importance. In this issue of MRS Bulletin, recent improvements in these two critical fields are reviewed with a focus on emerging and novel materials for the disruptive memory concept. Recent progress in scanning probe-based memory devices is also described.

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The family of III-Nitride semiconductors has been under intensive research for almost 30 years and has revolutionized lighting applications at the dawn of the 21st century. However, besides the developments and applications achieved, nitride alloys continue to fuel the quest for novel materials and applications. We report on the synthesis of a new nitride-based compound by using annealing of AlN heteroepitaxial layers under a Si-atmosphere at temperatures between 1350 °C and 1550 °C. The structure and stoichiometry of this compound are investigated by high resolution transmission electron microscopy (TEM) techniques and energy dispersive X-Ray (EDX) spectroscopy. Results are supported by density functional theory (DFT) calculations. The identified structure is a derivative of the parent wurtzite AlN crystal where the anion sublattice is fully occupied by N atoms and the cation sublattice is the stacking of 2 different planes along : The first one exhibits a ×3 periodicity along with 1/3 of the sites being vacant. The rest of the sites in the cation sublattice are occupied by an equal number of Si and Al atoms. Assuming a semiconducting alloy, a range of stoichiometries is proposed, Al5+αSi5+δN12 with α being between −2/3 and 1/4 and δ between 0 and 3/4.

The present work focuses on the study of exchange-biased magnetic core/shell nanoparticles at the atomic scale. The nanoparticles (NPs) consist of a perfectly epitaxial crystalline cobalt oxide (CoO) shell on a magnetite (Fe3O4) core. The numerical core/shell is built by taken into account the spinel structure of the core (Fe3O4) and the face-centered cubic structure of the shell (CoO). Two different configurations of core/shell NPs were examined: homogeneous and inhomogeneous core/shell. Our magnetic simulations are based on a 3D classical Heisenberg model. Monte Carlo simulations performing single spin rotation are used to investigate the effect of exchange bias on the spin configurations and hysteresis loops of core/shell nanoparticles. The numerical results reveal, as expected, a significant hysteresis loop shift obtained for a weak interface coupling. In addition, the magnetization reversal is not perfectly uniform in space when the interfacial coupling is different from zero. Finally, the increase in the magnetic interfacial coupling produces an increase in coercive field and a decrease in the exchange bias field.

International audience; The origin of stabilization of complex microstructures along macrotwin boundaries in martensites is explained by comparing two models based on Ginzburg-Landau theory. The first model incorporates a geometrically nonlinear strain tensor to ensure that the Landau energy is invariant under rigid body rotations, while the second model uses a linearized strain tensor under the assumption that deformations and rotations are small. We show that the approximation in the second model does not always hold for martensites and that the experimental observations along macrotwin boundaries can only be reproduced by the geometrically nonlinear (exact) theory.

The pressure and temperature effects on the optical responses involving the $$1s-1p$$ intersubband transition of an exciton in a spherical quantum dot are investigated. Calculations are performed in the framework of the effective mass approximation and the energies are obtained by using a Ritz variational method. Our approach is based on the Hylleraas formalism were the correlations between the electron and hole are taken into account. Temperature, pressure and the size effects on the linear and third nonlinear optical properties are analyzed. Our results show that the temperature and pressure provide important effects on linear and nonlinear parts of the absorption coefficient (AC) and the relative refractive index change (RI) associated to the $$1s-1p$$ transition. We found that by increasing the temperature and pressure the AC and RI curves shift to lower and higher energies respectively. Calculations show also that the dot size affects considerably the AC and RI and their corresponding amplitude.

This article reviews post-analysis processing methods for data acquired using atom probe tomography (APT). Field-induced aberrations of APT images arise from distorted ion flight trajectories and differences in ion evaporation rates. Addressing this issue requires the development of image processing tools that yield three-dimensionally reconstructed images that reliably reflect the original specimens. One of the biggest advantages of the APT technique is its ability to collect information about millions of individual atoms. Understanding these data requires the development of mathematical and statistical data mining tools, involving disciplines beyond the basic physics of APT. The above issues have important implications for addressing materials science-related questions.

In the adiabatic approximation the values of the spring constants of the springs contained in a molecule depend on its electronic state. We consider molecules with two electronic energy levels separated by. For a crystal of such molecules, the phonon branches depend therefore on the electronic states of the molecules. One can ask if that dependence does not introduce a coupling between the molecules via the optical and the acoustic branches. It is known that for a one-dimensional chain of N identical diatomic molecules there are two phonon branches, an optical branch and an acoustic one. In this study we introduce in the hamiltonian of the chain two assumptions: i) each molecule has two electronic energy levels separated by and the spring constant of the spring contained in the molecule has a value which depends on its electronic state; ii) the spring constant of the spring which links two molecules nearest neighbours has a value which depends on the electronic states of both molecules linked. One can show that phonons create on each molecule a …eld-like which favours the excited level and create between two molecules nearest neighbours an exchange-like interaction which can be ferro-like, antiferro-like and which can be equal to zero. For some values of T and the chain can display a …rst order phase transition with the presence of a thermal hysteresis loop. The phase transition takes place between the phase where all the molecules are in the fundamental level and that where they are in the excited one. PACS 63.20.Kr Phonon-electron and phonon-phonon interactions-63.50.+x Vi-brational states in disordered systems-64.60.-i General studies of phase transitions 2

International audience; This work reports the synthesis and studies of semi-soft ferrimagnetic CoFe 2 O 4 nanoparticles using sol-gel method. The X-ray diffraction patterns confirm the formation of cubic spinel CoFe 2 O 4 nanoparticles. The average crystallite size was found from XRD data is about 30 nm. The high resolution transmission electron microscopy analysis shows that nanoparticles are highly crystalline. The magnetic properties reveal that the particles are ferrimagnetically ordered soft magnetic materials with coercive field of 620 Oe and saturation magnetization M s = 60 emug − 1. The higher value of saturation magnetization is due to the ordered single-domains magnetic nanoparticles and lower magnitude of coercivity is attributed to the decrease of inter-particle interactions and magneto-elastic anisotropy. The value of remanence ratio R < 0.5 is responsible for magnetostatic interactions of the particles. The nanoparticles possess low values of dielectric constant which decreased with increasing frequency. The low dielectric constant makes the nanoparticles as a promising candidate for high frequency magnetic devices.

Organic heterostructures based on zinc phthalocyanine (ZnPc) and perylene tetracarboxylic dianhydride (PTCDA) were deposited by matrix-assisted pulsed laser evaporation (MAPLE) technique on conductive flexible substrate (ITO/PET) in three configurations: ZnPc/PTCDA (stacked layers), ZnPc:PTCDA (blend) and ZnPc/ZnPc:PTCDA/PTCDA. The effect of the configuration on the optical and electrical properties of the obtained heterostructures was investigated. For all heterostructures was observed an improved optical absorption in visible domain. The I–V characteristics recorded under illumination, revealed higher short circuit current (ISC) values for the ZnPc:PTCDA and ZnPc/ZnPc:PTCDA/PTCDA structures in comparison with that of the ZnPc/PTCDA structure. The results proved that by MAPLE can be obtained flexible organic heterostructures (in different configurations) with properties adequate for applications in flexible electronics and solar cell fields.

We present evidence of plastic avalanches in the so-called elastic regime of a glassy three dimensional Cu50Zr50 system. By means of extensive molecular Dynamics simulations we investigate this mechanical response of this system, subjected to tensile deformation, under finite strain rates at room temperature. We observe that the plastic avalanches, depicted as stress drops in the stress signal, occur not only in the well described and analyzed plastic regime, but even at the very early stage of the protocol, the global linear response regime. Several parameters such as different strain rates and system sizes were tested and the distributions of avalanche sizes and durations for each case were statistically analyzed. It turns out that in all cases the distributions follow a power law scaling indicating the existence of scale free avalanches of plastic events even at the very early deformation stage.

In 1861, Maxwell conceived the idea of the displacement current, which then made laws of electrodynamics more complete and also resulted in the realization of devices exploiting such displacement current. Interestingly, it is presently unknown if such displacement current can result in large intrinsic ac current in ferroic systems possessing domains, despite the flurry of recent activities that have been devoted to domains and their corresponding conductivity in these compounds. Here, we report first-principles-based atomistic simulations that predict that the transverse (polarization-related) displacement currents of 71° and 109° domains in the prototypical BiFeO3 multiferroic material are significant at the walls of such domains and in the GHz regime, and, in fact, result in currents that are at least of the same order of magnitude than previously reported dc currents (that are likely extrinsic in nature and due to electrons). Such large, localized and intrinsic ac currents are found to originate from low-frequency vibrations at the domain walls, and may open the door to the design of novel devices functioning in the GHz or THz range and in which currents would be confined within the domain wall. Domain walls in bismuth ferrite are predicted to have high a.c. conductivity, which could be exploited to develop giga- and terahertz devices. The observation of conducting domain walls in otherwise insulating functional materials has raised the prospect of forming reconfigurable electronic circuits. Unfortunately, it has been difficult to definitively identify the mechanism driving d.c. domain wall conduction, although it appears to arise from material defects, making precisely controlled devices difficult to fabricate. Sergey Prosandeev and co-workers from the University of Arkansas, USA, and Southern Federal University, Russia, have used numerical modeling to show that high frequency domain wall conductivity can occur in multiferroic bismuth ferrite due to the intrinsic physics of the walls. By avoiding reliance on extrinsic factors this mechanism should provide a more robust platform for developing gigahertz regime electronics.

International audience; Optical Tamm states are 2D electromagnetic modes propagating at the interface between a Bragg mirror and a metallic film. When a thin (a few tens nm) metallic micron—radius disk is deposited on a Bragg mirror, optical Tamm states can be confined below the disk surface, creating a Tamm-states cavity. We describe here the photoluminescence properties of colloidal semiconductor nanocrystals embedded in a Tamm cavity. Tamm states confinement effects are demonstrated and analysed as a function of the disk diameter, and compared with finite-elements simulations.

Nanostructured Fe65Si20Cr15 powders were prepared by mechanical alloying in a planetary ball mill. Morphological, microstructural, structural, and magnetic characterizations of the milled powders were investigated by scanning electron microscopy, X-ray diffraction, Mossbauer spectroscopy and vibrating sample magnetometer. A mixture of substitutional bcc Fe(Si) (˜29 nm) and bcc Cr(Si) (˜45 nm) is obtained after 3 h of milling. Upon prolonged milling, all the as-milled powders exhibit non-equilibrium α-Fe(Si,Cr) solid solution with an average crystallite size of 19–23 nm. The lattice parameter decreases initially up to 3 h and then increases with further increase in milling time due to dissolution of Cr and Si into Fe host lattice. The variation of microstructural parameters such as average crystallite size, r.m.s. microstrain, static Debye–Waller parameter, and dislocation density as a function of milling time show good correlations. Mossbauer spectra of the milled powders, recorded at room temperature, reveal the decrease of the average hyperfine field suggesting a random distribution of atoms during milling and point out the formation of the disordered bcc Fe(Si,Cr) solid solution. After 15 h of milling, the coercivity and magnetization values are 81.7 Oe and 120.3 emu/g, respectively.

The critical behavior of perovskite manganite La0.67Ba0.33Mn0.95Fe0.05O3 at the ferromagnetic–paramagnetic has been analyzed. The results show that the sample exhibited the second-order magnetic phase transition. The estimated critical exponents derived from the magnetic data using various such as modified d’Arrott plot Kouvel–Fisher method and critical magnetization M(T C, H). The critical exponents values for the La0.67Ba0.33Mn0.95Fe0.05O3 are close to those expected from the mean field model β = 0.504 ± 0.01 with T C = 275661 ± 0.447 (from the temperature dependence of the spontaneous magnetization below T C ), γ = 1.013 ± 0.017 with T C = 276132 ± 0.452 (from the temperature dependence of the inverse initial susceptibility above T C ), and δ = 3.0403 ± 0.0003. Moreover, the critical exponents also obey the single scaling equation of M(H, e) = |e| β f ±(H/|e| β+γ ).

We report the structural, magnetoelectric (ME), magnetic and electric control of magnetic properties in Co4Nb2O9 (CNO) single crystal. A detailed ME measurement reveals a nonlinear ME effect instead of a linear ME effect in CNO single crystal. By fitting the magnetization-electric field (M-E) curve, it can be found that the linear ($${\alpha }_{e}$$ α e ) and quadratic (γ) coefficients equal to ~8.27 ps/m and ~−6.46 ps/MV for upper branch, as well as ~8.38 ps/m and ~6.75 ps/MV for the lower branch. More importantly, a pronounced response was observed under a small cooling magnetic field, which cannot even cause the spin flop. This suggests a magnetoelectric effect can occur at paraelectric state for CNO single crystal. Furthermore, we also found that the magnetization of every axis responds to electric field applied along a-axis, but fails to do so when the electric field is applied c-axis. Such findings supply a direct evidence to the magnetic structure and ME coupling mechanism indirectly reflected by our neutron experiment.

A microfluidic on-chip imaging cell sorter has several advantages over conventional cell sorting methods, especially to identify cells with complex morphologies such as clusters. One of the remaining problems is how to efficiently discriminate targets at the species level without labelling. Hence, we developed a label-free microfluidic droplet-sorting system based on image recognition of cells in droplets. To test the applicability of this method, a mixture of two plankton species with different morphologies (Dunaliella tertiolecta and Phaeodactylum tricornutum) were successfully identified and discriminated at a rate of 10 Hz. We also examined the ability to detect the number of objects encapsulated in a droplet. Single cell droplets sorted into collection channels showed 91 ± 4.5% and 90 ± 3.8% accuracy for D. tertiolecta and P. tricornutum, respectively. Because we used image recognition to confirm single cell droplets, we achieved highly accurate single cell sorting. The results indicate that the integrated method of droplet imaging cell sorting can provide a complementary sorting approach capable of isolating single target cells from a mixture of cells with high accuracy without any staining.

International audience; A silica monolayer has been introduced between tow artificial silica opals using Langmuir Blodgett and inverse Schaefer transfer techniques. The structure and optical properties of the sandwich structure have been characterized by atomic force microscopy, scanning electron microscopy and specular reflection and transmission spectra. The quality of the defect mode inside the stopband made by inverse Schaefer technique is as good as the one obtained by the most commonly used Langmuir-Blodgett technique. Finite difference time domain simulations have been performed and show very good agreement with experimental result.

A generalization of the Kapitza–Dirac effect enables new ways of controlling free electrons with ultrashort light pulses.

International audience; Experimental results on the light-to-sound transduction by a superlattice embedded in an optical microcavity, using picosecond ultrasonics, are presented. Superlattices based on a periodic stacking of AlAs/GaAs layers are known to be good generators of quasimonochromatic phonons, but also are very sensitive phonon detectors. Selection rules for both processes are different but theory predicts the possibility to detect the same phonons generated by the SL as a result of the presence of light reflections on the substrate/superlattice interface. For this purpose, a superlattice embedded in an optical microcavity, where the electric field of the probe light can be stationary, is studied. Experiments on the ability to generate and detect phonons at 0.3 THz and 0.6 THz were performed at low temperature. It is shown that such phonons are generated, propagated, and can be detected by the device after a round trip through the substrate.

The Josephson effect describes the flow of supercurrent in a weak link, such as a tunnel junction, nanowire, or molecule, between two superconductors. It is the basis for a variety of circuits and devices, with applications ranging from medicine to quantum information. Currently, experiments using Josephson circuits that behave like artificial atoms are revolutionizing the way we probe and exploit the laws of quantum physics. Microscopically, the supercurrent is carried by Andreev pair states, which are localized at the weak link. These states come in doublets and have energies inside the superconducting gap. Existing Josephson circuits are based on properties of just the ground state of each doublet and so far the excited states have not been directly detected. Here we establish their existence through spectroscopic measurements of superconducting atomic contacts. The spectra, which depend on the atomic configuration and on the phase difference between the superconductors, are in complete agreement with theory. Andreev doublets could be exploited to encode information in novel types of superconducting qubits., Submitted to Nature

International audience; A formaldehyde-free adhesive consisting of a corn flour/NaOH adhesive mixture and a mimosa tannin/hexamine intermediate component was developed and evaluated for application to wood panels such as particleboards. The main ingredients of this adhesive include corn flour, NaOH, mimosa tannin and hexamine. This study investigated the physical properties (rheological and thermal analysis) of corn flour/NaOH and mimosa tannin/hexamine adhesives as well as mechanical properties of particleboards produced with these adhesives. Thermomechanical (TMA) experiments indicate the best performance of the adhesives to be around the relative mass proportions of 50:50 between corn flour/NaOH and mimosa tannin/hexamine. Former NMR measurements on corn starch and mimosa tannin proved that the two components behave as a polymer blend rather than co-reacting. The laboratory results show that particleboards bonded with the adhesive at this optimal ratio show good mechanical properties. Moreover, the formaldehyde emission levels obtained from boards bonded with the optimal adhesive were considerably lower to those obtained from boards made with control urea formaldehyde. © 2013 Springer-Verlag Berlin Heidelberg.