Groupe d’Étude de la Matière Condensée

This paper presents a nonlinear dynamic model for a quadrotor helicopter in a form suited for backstepping control design. Due to the under-actuated property of quadrotor helicopter, the controller can set the helicopter track three Cartesian positions (x,y,z) and the yaw angle to their desired values and stabilize the pitch and roll angles. The system has been presented into three interconnected subsystems. The first one representing the under-actuated subsystem, gives the dynamic relation of the horizontal positions (x,y) with the pitch and roll angles. The second fully-actuated subsystem gives the dynamics of the vertical position z and the yaw angle. The last subsystem gives the dynamics of the propeller forces. A backstepping control is presented to stabilize the whole system. The design methodology is based on the Lyapunov stability theory. Various simulations of the model show that the control law stabilizes a quadrotor with good tracking

We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results.

The all-electron GW approximation energy band gap of bulk hexagonal boron nitride is shown to be of indirect type. The resulting computed in-plane polarized optical spectrum, obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function, is in excellent agreement with experiment and has a strong anisotropy compared to out-of-plane polarized spectrum. A detailed analysis of the excitonic structures within the band gap shows that the low-lying excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe et al. [Nat. Mater. 3, 404 (2004).] based on a Wannier model assuming h-BN to be a direct-band-gap semiconductor.

Iron(II) spin crossover molecular materials are made of coordination centres switchable between two states by temperature, pressure or a visible light irradiation. The relevant macroscopic parameter which monitors the magnetic state of a given solid is the high-spin (HS) fraction denoted n(HS), i.e., the relative population of HS molecules. Each spin crossover material is distinguished by a transition temperature T(1/2) where 50% of active molecules have switched to the low-spin (LS) state. In strongly interacting systems, the thermal spin switching occurs abruptly at T(1/2). Applying pressure induces a shift from HS to LS states, which is the direct consequence of the lower volume for the LS molecule. Each material has thus a well defined pressure value P(1/2). In both cases the spin state change is easily detectable by optical means thanks to a thermo/piezochromic effect that is often encountered in these materials. In this contribution, we discuss potential use of spin crossover molecular materials as temperature and pressure sensors with optical detection. The ones presenting smooth transitions behaviour, which have not been seriously considered for any application, are spotlighted as potential sensors which should stimulate a large interest on this well investigated class of materials.

The performance of hybrid organic perovskite (HOP) for solar energy conversion is driving a renewed interest in their light emitting properties. The recent observation of broad visible emission in layered HOP highlights their potential as white-light emitters. Improvement of the efficiency of the material requires a better understanding of its photophysical properties. We present in-depth experimental investigations of white-light (WL) emission in thin films of the (C6H11NH3)2PbBr4. The broadband, strongly Stokes shifted emission presents a maximum at 90 K when excited at 3.815 eV, and below this temperature coexists with an excitonic edge emission. X-rays and calorimetry measurements exclude the existence of a phase transition as an origin of the thermal behavior of the WL luminescence. The free excitonic emission quenches at low temperature, despite a binding energy estimated to 280 meV. Time-resolved photoluminescence spectroscopy reveals the multicomponent nature of the broad emission. We analyzed the dependence of these components as a function of temperature and excitation energy. The results are consistent with the existence of self-trapped states. The quenching of the free exciton and the thermal evolution of the WL luminescence decay time are explained by the existence of an energy barrier against self-trapping, estimated to ∼10 meV.

When compared to standard colloidal nanocrystals, individual CdSe-CdS core-shell nanocrystals with thick shells exhibit strongly reduced blinking. Analyzing the photon statistics and lifetime of the on state, we first demonstrate that bright periods correspond to single photon emission with a fluorescence quantum efficiency of the monoexcitonic state greater than 95%. We also show that low intensity emitting periods are not dark but correspond to a grey state, with a fluorescence quantum efficiency of 19%. From these measurements, we deduce the radiative lifetime (45 ns) and the Auger lifetime (10.5 ns) of the grey state.

Abstract A detailed study of the IR spectrum of goethite is given with the aim of relating variations to crystalline order and particle size. The OH stretching vibrations are split into two active components at high frequency, plus two inactive ones at low frequency. Two different bending modes exist from site group splitting. Their active modes from factor group splitting are at lower frequencies than the uncoupled ones. The lattice bands at 630 and 400 cm −1 correspond to Fe-O or Fe-OH stretching, approximately parallel to a and c , and thus respectively sensitive and not sensitive to the particle shape, as long as it remains elongated along c .

An implementation of the GW approximation (GWA) based on the all-electron projector-augmented-wave (PAW) method is presented, where the screened Coulomb interaction is computed within the random-phase approximation (RPA) instead of the plasmon-pole model. Two different ways of computing the self-energy are reported. The method is used successfully to determine the quasiparticle energies of six semiconducting or insulating materials: Si, SiC, AlAs, InAs, NaH, and KH. To illustrate the method the real and imaginary part of the frequency-dependent self-energy together with the spectral function of silicon are computed. Finally, the GWA results are compared with other calculations, highlighting that all-electron GWA results can differ markedly from those based on pseudopotential approaches.

ISSN:1476-1122

In foggy weather, the contrast of images grabbed by in-vehicle cameras in the visible light range is drastically degraded, which makes the current applications very sensitive to weather conditions. An onboard vision system should take fog effects into account. The effects of fog varies across the scene and are exponential with respect to the depth of scene points. Because it is not possible in this context to compute the road scene structure beforehand contrary to fixed camera surveillance, a new scheme is proposed. Weather conditions are first estimated and then used to restore the contrast according to a scene structure which is inferred a priori and refined during the restoration process. Based on the aimed application, different algorithms with increasing complexities are proposed. Results are presented using sample road scenes under foggy weather and assessed by computing the contrast before and after restoration.

We study the origin of the cooperative nature of spin crossover (SC) between low-spin and high-spin (HS) states from the viewpoint of elastic interactions among molecules. As the size of each molecule changes depending on its spin state, the elastic interaction among the lattice distortions provides the cooperative interaction of the spin states. We develop a simple model of SC with intra and intermolecular potentials which accounts for the elastic interaction including the effect of the inhomogeneity of the spin states and apply constant temperature molecular dynamics based on the Nosé-Hoover formalism. We demonstrate that, with increase of the strength of the intermolecular interactions, the temperature dependence of the HS component changes from a gradual crossover to a first-order transition.

This paper presents a new controller based on backstepping and sliding mode techniques for miniature quadrotor helicopter. The system is formed by three inter-connected subsystems. The first one which represents the under-actuated subsystem, gives the dynamic relation of the horizontal positions with the tilts angles. The second one, fully-actuated subsystem, gives the dynamics of the vertical position and the yaw angle. The last subsystem gives the dynamics of the propeller forces. The design methodology of the controller is based on the Lyapunov stability. To show the effectiveness of the proposed trajectory tracking control, simulation results are performed on the quadrotor model. The application of the proposed controller to a real miniature helicopter is also presented, some results are included to demonstrate the good performance of the proposed controller

Cathodoluminescence and photoluminescence spectroscopies have been performed on hexagonal boron nitride powders. The combination of these techniques allows us to analyze the two observed luminescence bands. A deep-level UV emission at about $4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is attributed to defects or impurities, and a near-band-gap UV emission is observed at about $5.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The deep-level band is composed of four peaks, which are attributed to phonon replica due to localized vibrations. In the near-band-gap region, six components are observed between 5.2 and $5.96\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, in agreement with the recent experiments performed on h-BN single crystals by Watanabe et al. [Nat. Mater. 3, 404 (2004)], but they are assigned here to Frenkel excitons.

Two-step and multistep spin transitions are frequently observed in switchable cooperative molecular solids. They present the advantage to open the way for three- or several-bit electronics. Despite extensive experimental studies, their theoretical description was to date only phenomenological, based on Ising models including competing ferro- and antiferro-magnetic interactions, even though it is recognized that the elastic interactions are at the heart of the spin transition phenomenon, due to the volume change between the low- and high-temperature phases. To remedy this shortcoming, we designed the first consistent elastic model, taking into account both volume change upon spin transition and elastic frustration. This ingredient was revealed to be powerful, since it was able to obtain all observed experimental configurations in a consistent way. Thus, according to the strength of the elastic frustration, the system may undergo first-order transition with hysteresis, gradual, hysteretic two-step or multistep transitions, and incomplete transitions. Furthermore, the analysis of the spatial organization of the HS and LS species in the plateau regions revealed the emergence of complex antiferro-elastic patterns going from simple antiferro-magnetic-like order to long-range spatial modulations of the high-spin fraction. These results enabled us to identify the elastic frustration as the fundamental mechanism at the origin of the very recent experimental observations showing the existence of organized spatial modulations of the high-spin fraction inside the plateau of two-step spin transitions.

Several acquisitions of x-ray microtomography have been performed on a beads packing while it compacts under vertical vibrations. An image analysis allows to study the evolution of the packing structure during its progressive densification. In particular, the volume distribution of the pores reveals a large tail, compatible to an exponential law, which slowly reduces as the system gets more compact. This is quite consistent, for large pores, with the free volume theory. These results are also in very good agreement with those obtained by a previous numerical model of granular compaction.

A 20 MW/5 GHz lower hybrid current drive (LHCD) system was initially due to be commissioned and used for the second mission of ITER, i.e. the Q = 5 steady state target. Though not part of the currently planned procurement phase, it is now under consideration for an earlier delivery. In this paper, both physics and technology conceptual designs are reviewed. Furthermore, an appropriate work plan is also developed. This work plan for design, R&D, procurement and installation of a 20 MW LHCD system on ITER follows the ITER Scientific and Technical Advisory Committee (STAC) T13-05 task instructions. It gives more details on the various scientific and technical implications of the system, without presuming on any work or procurement sharing amongst the possible ITER partners b The LHCD system of ITER is not part of the initial cost sharing.. This document does not commit the Institutions or Domestic Agencies of the various authors in that respect.

We report a two-dimensional Hofmann-like spin-crossover (SCO) material, [Fe(trz-py)2{Pt(CN)4}]·3H2O, built from [FePt(CN)4] layers separated by interdigitated 4-(2-pyridyl)-1,2,4,4H-triazole (trz-py) ligands with two symmetrically inequivalent FeII sites. This compound exhibits an incomplete first-order spin transition at 153 K between fully high-spin (HS–HS) and intermediate high-spin low-spin (HS–LS) ordered states. At low temperature, it undergoes a bidirectional photoswitching to HS–HS and fully low-spin (LS–LS) states with green and near-IR light irradiation, respectively, with associated T(LIESST = Light-Induced Excited Spin-State Trapping) and T(reverse-LIESST) values of 52 and 85 K, respectively. Photomagnetic investigations show that the reverse-LIESST process, performed from either HS-HS or HS-LS states, enables access to a hidden stable LS-LS state, revealing the existence of a hidden thermal hysteresis. Crystallographic investigations allowed to identify that the strong metastability of the HS-LS state originates from the existence of a strong elastic frustration causing antiferroelastic interactions within the [FePt(CN)4] layers, through the rigid NC-Pt-CN bridges connecting the inequivalent FeII sites. The existence of the stable LS-LS state paves the way for a multidirectional photoswitching and allows potential applications for electronic devices based on ternary digits.

We have investigated the cathodoluminescence (CL) emission and the Raman spectra along individual ZnO nanorods grown by a catalyst-free method. The spatial correlation between the CL emission and the defect related Raman modes permits establishing a correspondence between the non-radiative recombination centres (NRRCs) and the defects responsible for the 275 cm −1 Raman band. According to this relation, the NRRCs in these nanorods are tentatively associated with complexes of zinc interstitials.

We report on experimental studies of the collision process between an incident bead and a three-dimensional granular packing (made of particles identical to the impacting one). The understanding of such a process and the resulting ejection of particles is, in particular, crucial to describe eolian sand transport. We present here an extensive experimental analysis of the collision and ejection process. The analysis is two dimensional in the sense that we determined only the vertical component V{z} of the ejection velocity of the splashed particles and the horizontal component V{x} lying in the incident plane. We extracted in particular the distribution of the ejection velocities for a wide range of impact angles theta{i} and incident velocity V{i} . We show that the mean quadratic horizontal velocity of the splashed particles is almost insensitive to changes in the impact angle and velocity, while the mean quadratic vertical velocity slightly increases with increasing impact velocity (as V{i}{1/2}). Moreover, the mean number of splashed particles per collision is found to be dependent on both the impact angle and velocity, and to scale with the impact speed as V{i}{3/2}. A consequence of these outcomes is that the sum of the kinetic energy of the splashed particles is directly proportional to the kinetic energy of the incident particle. Finally, we provide the bivariate probability distribution function P(V{x},V{z}) of the ejection velocities and show that it can be approximated by the product of a log-normal distribution and a circular normal one.

The family of spinel compounds is a large and important class of multifunctional materials of general formulation AB2X4 with many advanced applications in energy and optoelectronic areas such as fuel cells, batteries, catalysis, photonics, spintronics, and thermoelectricity. In this work, it is demonstrated that the ternary ultrawide-band-gap (∼5 eV) spinel zinc gallate (ZnGa2O4) arguably is the native p-type ternary oxide semiconductor with the largest Eg value (in comparison with the recently discovered binary p-type monoclinic β-Ga2O3 oxide). For nominally undoped ZnGa2O4 the high-temperature Hall effect hole concentration was determined to be as large as p = 2 × 1015 cm–3, while hole mobilities were found to be μh = 7–10 cm2/(V s) (in the 680–850 K temperature range). An acceptor-like small Fermi level was further corroborated by X-ray spectroscopy and by density functional theory calculations. Our findings, as an important step toward p-type doping, opens up further perspectives for ultrawide-band-gap bipolar spinel electronics and further promotes ultrawide-band-gap ternary oxides such as ZnGa2O4 to the forefront of the quest of the next generation of semiconductor materials for more efficient energy optoelectronics and power electronics.