Centre de Recherche sur les Ions, les Matériaux et la Photonique

Following a description used to explain a phase transformation observed after pulsed femtosecond laser irradiation, a transient thermal process is used to describe latent-track formation after high electronic excitation induced by energetic (GeV) heavy ions. The transient thermal calculation is restricted to the amorphous materials a-Ge, a-Si, and a-${\mathrm{Fe}}_{85}$${\mathrm{B}}_{15}$, for which nearly all latent-track radii and/or macroscopic thermodynamic properties are known. The heat-flow equation is solved numerically in cylindrical geometry. The time-dependent heat-generation term is assumed to be due to the electron-atom interaction. The characteristic length \ensuremath{\lambda} of the energy transport by secondary electrons is taken as the only free parameter and the maximum diameter of the cylinder of liquid matter is considered as the diameter of the observed latent track. Using the single value \ensuremath{\lambda}=14 nm, we have been able to calculate these diameters in a-Si and a-Ge in reasonable agreement with experimental track diameters, taking into account the large differences between the macroscopic thermodynamic parameters of both materials. This \ensuremath{\lambda} value is less than that for the crystalline state. In the case of a-${\mathrm{Fe}}_{85}$${\mathrm{B}}_{15}$, the diameters calculated with use of \ensuremath{\lambda}=19 nm are in agreement with the ones determined recently by electrical-resistivity change.

\ensuremath{\alpha}-quartz has been irradiated with heavy ions: $^{19}\mathrm{F}$, $^{32}\mathrm{S}$, and $^{63}\mathrm{Cu}$ at an energy of about 1 MeV/amu in order to cover a range of electronic stopping powers dE/dx between 2.4 and 9 keV/nm and $^{58}\mathrm{Ni}$, $^{86}\mathrm{Kr}$, $^{128}\mathrm{Te}$, $^{129}\mathrm{Xe}$, $^{181}\mathrm{Ta}$, and $^{208}\mathrm{Pb}$ between 1 and 5.8 MeV/amu for dE/dx>7 keV/nm. The extent of the induced damage is determined using Rutherford backscattering ion channeling with a 2-MeV $^{4}\mathrm{He}$ beam. The damage cross section A is obtained using a Poisson law ${\mathit{F}}_{\mathit{d}}$=1-exp(-A\ensuremath{\varphi}t), where \ensuremath{\varphi} is the flux and t the irradiation time. This damage cross section is linked to the effective radius ${\mathit{R}}_{\mathit{e}}$ through the relation A=\ensuremath{\pi}${\mathit{R}}_{\mathit{e}}^{2}$, where ${\mathit{R}}_{\mathit{e}}$ is the radius of an equivalent cylinder of damage. Using high-resolution electron microscopy, cylinders of amorphous matter have been observed, whose radius corresponds to ${\mathit{R}}_{\mathit{e}}$ when the track is continuous (i.e., for A\ensuremath{\ge}1.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ ${\mathrm{cm}}^{2}$; ${\mathit{R}}_{\mathit{e}}$\ensuremath{\ge}2 nm). A thermal-spike model is applied to calculate the radii of the observed tracks assuming that the observed amorphous cylinders correspond to a rapid quench of a molten liquid phase along the ion path. The model is applied only when the latent track is continuous and cylindrical. A good agreement is obtained taking into account that the initial spatial energy deposition on the electrons depends on the ion velocity.

The damage in ferrimagnetic yttrium iron garnet, ${\mathrm{Y}}_{3}$${\mathrm{Fe}}_{5}$${\mathrm{O}}_{12}$ or YIG, induced by energetic heavy-ion bombardment in the electronic stopping-power regime has been studied in the low-velocity range (for a beam energy E\ensuremath{\le}3.6 MeV/amu). Epitaxial thin films of YIG on [111]-${\mathrm{Gd}}_{3}$${\mathrm{Ga}}_{5}$${\mathrm{O}}_{12}$ substrates were thus irradiated at room temperature with 15-MeV $^{19}\mathrm{F}$, 50-MeV $^{32}\mathrm{S}$, 650-MeV $^{181}\mathrm{Ta}$, 750-MeV $^{208}\mathrm{Pb}$, and 666-MeV $^{238}\mathrm{U}$. The damage-cross-section A is extracted from channeling-Rutherford-backscattering spectroscopy and compared to previous works. All the experimental results show that at one given value of dE/dx, the damage cross section is higher for low-velocity ions than for high-velocity ions over a large range of dE/dx. At constant dE/dx, the larger the difference between the ion velocities is, the larger the difference between the damage cross sections. Such a deviation might be explained by the effect of the energy deposition being more localized for the low-velocity ions than for the high-velocity ions. This work clearly indicates that the electronic stopping power is not the only key parameter in the creation of ion tracks, and that the damage cross section depends on the lateral distribution of the energy deposition.

We report on the observation of a fine structure in ion tracks in amorphous SiO2 using small angle x-ray scattering measurements. Tracks were generated by high energy ion irradiation with Au and Xe between 27 MeV and 1.43 GeV. In agreement with molecular dynamics simulations, the tracks consist of a core characterized by a significant density deficit compared to unirradiated material, surrounded by a high density shell. The structure is consistent with a frozen-in pressure wave originating from the center of the ion track as a result of a thermal spike.

The serine/threonine kinase Akt is a downstream target of dopamine receptor signaling that is inhibited/dephosphorylated in response to direct and indirect dopamine receptor agonists. Although pharmacological studies uncovered the involvement of D2-class dopamine receptors in Akt regulation, they did not identify the role of individual receptor subtypes in this process. Here we used knock-out mice lacking the D1, D2, D2 long, or D3 dopamine receptors as well as a D4 receptor-selective antagonist to address the function of each of these receptors in the regulation of Akt in vivo. Under basal conditions, D2, D2 long, and D3 knock-out mice display enhanced striatal Akt activation, whereas D1 knock-out mice and mice treated with the D4 receptor antagonist L745870 (3-[[4-(4-chlorophenyl)piperazin-1-yl]methyl]-1H-pyrrolo[2,3-b]pyridine trihydrochloride) have phospho-Akt levels comparable with those of normal control animals. Furthermore, both amphetamine and apomorphine lose their ability to inhibit Akt in D2 knock-out mice but retain their normal effect on this signaling molecule in D1 knock-out animals. Finally, D3 knock-out mice show a reduced sensitivity of Akt-mediated signaling to dopaminergic drugs but retain the action of these drugs on Akt at high dose regimens. These results indicate that D2 receptors are essential for the inhibition of Akt by dopamine and that D3 receptors also participate in this signaling potentially by enhancing D2 receptor response. Identification of the functions of individual dopamine receptor subtypes in Akt regulation may help the development of new pharmaceutical approaches for mental disorders related to abnormal dopamine transmission such as bipolar disorder and schizophrenia.

Cross sections for electron emission in the energy range from $2--300\mathrm{eV}$ were measured for $60\mathrm{MeV}/u$ ${\mathrm{Kr}}^{34+}$ ions impacting on ${\mathrm{H}}_{2}$. Model calculations are introduced to guide the search for interference effects in the electron spectra produced by the coherent emission of electrons from the two H atoms in analogy with Young's two-slit experiment. Experimentally, a full sinusoidal-like oscillation was observed in the energy range up to $250\mathrm{eV}$ in good agreement with the calculations. The oscillatory structure is found to be similar for the observation angles $20\ifmmode^\circ\else\textdegree\fi{}$, $30\ifmmode^\circ\else\textdegree\fi{}$, $150\ifmmode^\circ\else\textdegree\fi{}$, and $160\ifmmode^\circ\else\textdegree\fi{}$.

We report a study of resistive switching in a silicon-based memristor/resistive RAM (RRAM) device in which the active layer is silicon-rich silica. The resistive switching phenomenon is an intrinsic property of the silicon-rich oxide layer and does not depend on the diffusion of metallic ions to form conductive paths. In contrast to other work in the literature, switching occurs in ambient conditions, and is not limited to the surface of the active material. We propose a switching mechanism driven by competing field-driven formation and current-driven destruction of filamentary conductive pathways. We demonstrate that conduction is dominated by trap assisted tunneling through noncontinuous conduction paths consisting of silicon nanoinclusions in a highly nonstoichiometric suboxide phase. We hypothesize that such nanoinclusions nucleate preferentially at internal grain boundaries in nanostructured films. Switching exhibits the pinched hysteresis I/V loop characteristic of memristive systems, and on/off resistance ratios of 104:1 or higher can be easily achieved. Scanning tunneling microscopy suggests that switchable conductive pathways are 10 nm in diameter or smaller. Programming currents can be as low as 2 μA, and transition times are on the nanosecond scale.

The spectroscopic properties and laser operation of a new neodymium-doped vanadate crystal, Nd:LuVO4, grown by the flux technique are reported. Polarized absorption and emission spectra were recorded at low and room temperatures, excited-state absorption was measured near 1060 and 1340 nm, and laser emission at 1066 nm was obtained after pumping near 809 and 880 nm.

The coordinating role of nuclear factor erythroid-2-related factor 2 (Nrf2) in cellular function is undeniable. Evidence indicates that this transcription factor exerts massive regulatory functions in multiple signaling pathways concerning redox homeostasis and xenobiotics, macromolecules, and iron metabolism. Being the master regulator of antioxidant system, Nrf2 controls cellular fate, influencing cell proliferation, differentiation, apoptosis, resistance to therapy, and senescence processes, as well as infection disease success. Because Nrf2 is the key coordinator of cell defence mechanisms, dysregulation of its signaling has been associated with carcinogenic phenomena and infectious and age-related diseases. Deregulation of this cytoprotective system may also interfere with immune response. Oxidative burst, one of the main microbicidal mechanisms, could be impaired during the initial phagocytosis of pathogens, which could lead to the successful establishment of infection and promote susceptibility to infectious diseases. There is still a knowledge gap to fill regarding the molecular mechanisms by which Nrf2 orchestrates such complex networks involving multiple pathways. This review describes the role of Nrf2 in non-pathogenic and pathogenic cells.

We report what is believed to be the first demonstration of a high-power passively mode-locked diode-pumped femtosecond laser based on an Yb3+:CaF2 single crystal, directly pumped by a 15-W fiber-coupled laser diode. With a 5-at. % Yb3+ -doped sample and prisms for dispersion compensation we obtained pulses as short as 150 fs, with 880 mW of average power and up to 1.4-W average output power, with a pulse duration of 220 fs, centered at 1049 nm. The laser wavelength could be tuned from 1040 to 1053 nm in the femtosecond regime. Using chirped mirrors for dispersion compensation, the oscillator provided up to 1.74 W of average power, with a pulse duration of 230 fs, corresponding to a pulse energy of 20 nJ and a peak power of 85 kW.

Resistive switching offers a promising route to universal electronic memory, potentially replacing current technologies that are approaching their fundamental limits. In many cases switching originates from the reversible formation and dissolution of nanometre-scale conductive filaments, which constrain the motion of electrons, leading to the quantisation of device conductance into multiples of the fundamental unit of conductance, G0. Such quantum effects appear when the constriction diameter approaches the Fermi wavelength of the electron in the medium - typically several nanometres. Here we find that the conductance of silicon-rich silica (SiOx) resistive switches is quantised in half-integer multiples of G0. In contrast to other resistive switching systems this quantisation is intrinsic to SiOx, and is not due to drift of metallic ions. Half-integer quantisation is explained in terms of the filament structure and formation mechanism, which allows us to distinguish between systems that exhibit integer and half-integer quantisation.

Helium single ionization by 3.6 me V/u ${\mathrm{Ni}}^{24+}$ impact was explored in a kinematically complete experiment by combining a high-resolution recoil-ion momentum spectrometer with a novel $4\ensuremath{\pi}$ low-energy electron analyzer. More than 90% of the "soft electrons" (${E}_{e}\ensuremath{\lesssim}50$ eV) are ejected in the forward direction in agreement with classical-trajectory Monte Carlo predictions. The electron longitudinal momentum is not balanced by the longitudinal momentum change of the projectile but mainly by the backwards recoiling ${\mathrm{He}}^{1+}$ ion. Energy losses of the 0.2 GeV projectiles as small as $\frac{\ensuremath{\Delta}{E}_{P}}{{E}_{P}}=3.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ are observable.

For the first time, latent tracks induced by swift Kr ions have been directly observed in mica. These tracks are imaged by atomic-force microscopy as hollows which are associated with softer areas in the mica surface. The track core is formed by disordered mica. The mean diameter of the observed hollows increases with the electronic stopping power of the ions. The track shape along the ion path is deduced from the analysis of both the number of the tracks per unit area and their diameter distribution. These observations are the first images of nanometric changes of elastic properties.

Atom probe tomography allows for three-dimensional reconstruction of the elemental distribution in materials at the nanoscale. However, the measurement of the chemical composition of compound semiconductors may exhibit strong biases depending on the experimental parameters used. This article reports on a systematic analysis of the composition measurement of III–N binary (AlN, GaN) and ternary compounds (InGaN, InAlN), MgO, and ZnO by laser-assisted tomographic atom probe as a function of laser power and applied DC bias. We performed separate series of measurements at constant bias, constant laser pulse energy, and constant detection rate and a spatial analysis of the surface field through detector hitmap ratios of elemental charge states. As a result, (i) we can determine the separate roles of laser energy and surface field—the latter being the dominant factor under standard conditions of analysis; (ii) we compare the behavior of different samples and (iii) different materials; and (iv) we critically discuss the reliability of the measurement of InxGa1–xN and InxAl1–xN alloy fractions and of the Tb concentration in rare-earth-doped ZnO.

Structural modification of vitreous SiO${}_{2}$ by Au ion irradiation is investigated over an energy regime (\ensuremath{\sim}0.3--15 MeV) in which the decrease of the nuclear energy loss with increasing energy is compensated by the increase of the electronic energy loss, leading to a nearly constant total energy loss of \ensuremath{\sim}4 keV/nm. The radii of damaged zones resulting from the ion impact, deduced from changes in infrared bands as a function of ion fluence, decrease from 4.9 nm at 0.3 MeV to 2.5 and 2.6 nm at 9.8 and 14.8 MeV, respectively. Based on previous data where vitreous SiO${}_{2}$ was irradiated with much higher energy Au ions, the damage zone radius increases from 2.4 nm at 22.7 MeV to 5.4 nm at 168 MeV, and a U-shaped dependence on energy is observed is observed in the energy region from 0.3 to 168 MeV. The current results demonstrate that large damage radii at low and high ion energy can be explained by the elastic or inelastic thermal spike model, respectively. In the transition regime where both nuclear and electronic energy loss are significant, a unified thermal spike model consisting of a coherent synergy of the elastic collision spike model with the inelastic thermal spike model is suggested to interpret and describe the radius evolution from the nuclear to the electronic energy regime.

Abstract It has long been proposed that water incorporation in olivine has dramatic effects on the upper mantle properties, affecting large‐scale geodynamics, and triggering high electrical conductivity. But the laboratory‐based laws of olivine electrical conductivity predict contrasting effects of water, precluding the interpretation of geophysical data in term of mantle hydration. We review the experimental measurements of hydrous olivine conductivity and conclude that most of data are consistent when errors in samples water contents are considered. We report a new law calibrated on the largest database of measurements on hydrous olivine oriented single crystals and polycrystals. It fits most of measurements within uncertainties, and is compatible with most of geophysical data within petrological constraints on mantle olivine hydration. The conductivity anisotropy of hydrous olivine might be higher than dry olivine, but preferential orientation should produce moderate anisotropy (∼0–0.8 log unit). In the oceanic mantle, the enhancement of olivine conductivity is limited to ∼1 log unit in the maximum range of mantle olivine water concentrations (0–500 wt ppm). Strongest enhancements are expected in colder regions, like cratonic lithospheres and subduction settings. High conductivities in melt‐free mantle require great depths and high water concentrations in olivine (>0.1 S/m at >250 km and >200 wt ppm). Thus, the hydration of olivine appears unlikely to produce the highest conductivities of the upper mantle.

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We present evidence for the diffraction of light keV atoms and molecules grazingly scattered on LiF(001) and NaCl(001) surfaces. At such energies, the de Broglie wavelength is 2 orders of magnitude smaller that the mean thermal atomic displacement in the crystal. Thus, no coherent scattering was expected and interaction of keV atoms with surfaces is routinely treated with classical mechanics. We show here that well-defined diffraction patterns can be observed indicating that, for grazing scattering, the pertinent wavelength is that associated with the slow motion perpendicular to the surface. The experimental data are well reproduced by an ab initio calculation.

We present a theoretical and experimental investigation of an interferometric technique for converting a linearly polarized Gaussian beam into a radially polarized doughnut beam. The experimental setup accomplishes the coherent summation of two orthogonally polarized TEM01 and TEM10 beams that are obtained from the transformation of a TEM00 beam by use of a simple binary diffractive optical element. We have shown that the degree of radial polarization is maximum at a given distance from the interferometer output port that depends on the diameter of the incident beam at the interferometer input port.

Classical Rayleigh theory predicts an instability of a surface charged liquid sphere, when the Coulomb energy ${E}_{C}$ exceeds twice the surface energy ${E}_{S}$. Previously, electrified liquid droplets have been found to disintegrate at a fissility $X={E}_{C}/2{E}_{S}$ well below one, however. We determine the stability of charged droplets in an electrodynamic levitator by observing the amplitude and phase of their quadrupolar shape oscillations as a function of the fissility. With this novel approach, which does not rely on an independent determination of the charge and surface tension of the droplets, we are able to confirm for the first time the Rayleigh limit of stability at $X=1$ for micrometer sized droplets of ethylene glycol.