Electric field induced avalanche breakdown and non-volatile resistive switching in the Mott Insulators AM4Q8

The Mott insulator compounds AM₄Q₈ exhibit a new type of volatile and non volatile resistive switchings that are of interest for RRAM application. We found that above a threshold electric field E _TH of the order of a few kV/cm these compounds undergo a volatile resistive switching based on an avalanche process. For electric field much higher than the threshold avalanche breakdown field, the resistive switching turns non volatile. Our EDXS and STEM analyses show that the non volatile resistive switching originating from the avalanche breakdown can neither be ascribed to local chemical modifications nor to a local phase change with symmetry breaking at a resolution better than a few nanometer. This is in strong contrast with non volatile resistive switching reported so far that are all based on chemical or structural changes. Conversely, our results suggest that the avalanche breakdown induce the collapse of the Mott insulating state at the local scale and the formation of a granular conductive filament formed by compressed metallic domains and expanded “superinsulating” domains.     

Efficient and Magnetically Recoverable “Click” PEGylated γ‐Fe2O3–Pd Nanoparticle Catalysts for Suzuki–Miyaura,Sonogashira, and Heck Reactions with Positive Dendritic Effects

The engineering of novel catalytic nanomaterials that are highly active for crucial carbon–carbon bond formations, easily recoverable many times, and biocompatible is highly desirable in terms of sustainable and green chemistry. To this end, catalysts comprising dendritic “click” ligands that are immobilized on a magnetic nanoparticle (MNP) core, terminated by triethylene glycol (TEG) groups, and incorporate Pd nanoparticles (PdNPs) have been prepared. These nanomaterials are characterized by transmission electron microscopy (TEM), high‐resolution TEM, inductively coupled plasma analysis, Fourier transform infrared spectroscopy, X‐ray photoelectron spectra and energy‐dispersive X‐ray spectroscopy. They are shown to be highly active, dispersible, and magnetically recoverable many times in Suzuki, Sonogashira, and Heck reactions. In addition, a series of pharmacologically relevant or natural products were successfully synthesized using these magnetic PdNPs as catalyst. For comparison, related PdNP catalysts deposited on MNPs bearing linear “click” PEGylated ligands are also prepared. Strong positive dendritic effects concerning ligand loading, catalyst loading, catalytic activity, and recyclability are observed, that is, the dendritic catalysts are much more efficient than non‐dendritic analogues.     

Synthesis, structure, and magnetic behavior of a series of trinuclear Schiff base complexes of 5f (UIV, ThIV) and 3d (CuII, ZnII) ions

The reaction of [M(H(2)L(i))] (M = Cu, Zn) and U(acac)(4) in refluxing pyridine produced the trinuclear complexes [[ML(i)(py)(x)](2)U] [L(i) = N,N'-bis(3-hydroxysalicylidene)-R, R = 1,2-ethanediamine (i = 1), 2-methyl-1,2-propanediamine (i = 2), 1,2-cyclohexanediamine (i = 3), 1,2-phenylenediamine (i = 4), 4,5-dimethyl-1,2-phenylenediamine (i = 5), 1,3-propanediamine (i = 6), 2,2-dimethyl-1,3-propanediamine (i = 7), 2-amino-benzylamine (i = 8), or 1,4-butanediamine (i = 9); x = 0 or 1]. The crystal structures show that the central U(IV) ion adopts the same dodecahedral configuration in all of these compounds, while the Cu(II) ion coordination geometry and the Cu...U distance vary with the length of the diimino chain of the Schiff base ligand L(i). These geometrical parameters have a major influence on the magnetic properties of the complexes. For the smallest Cu...U distances (i = 1-5), the Cu-U coupling is antiferromagnetic and weak antiferromagnetic interactions are present between the Cu(II) ions, while for the largest Cu...U distances (i = 6-9), the Cu-U coupling is ferromagnetic and no interaction is observed between the Cu(II) ions. The magnetic behavior of the [[CuL(i)](2)Th] compounds (i = 1, 2), in which the Th(IV) ion is diamagnetic, confirms the presence of weak intramolecular antiferromagnetic coupling between the Cu(II) ions.     

Laser filaments generated and transmitted in highly turbulent air

The initiation and propagation of a filament generated by ultrashort laser pulses in turbulent air is investigated experimentally. A filament can be generated and propagated even after the beam has propagated through strongly turbulent regions, with structure parameters C(n)2 as many as 5 orders of magnitude larger than those encountered in the usual atmospheric conditions. Moreover, the filament's position within the beam is not affected by the interaction with a turbulent region. This remarkable stability is allowed by the strong Kerr refractive-index gradients generated within the filament, which exceed the turbulence-induced refractive-index gradients by 2 orders of magnitude.     

Multiple filamentation of non-uniformly focused ultrashort laser pulses

We propose the impingement of non-uniform wavefront curvature as a simple way to improve the longitudinal homogeneity of the plasma density along filaments generated by ultrashort laser pulses. We characterize multiple filamentation of a multiterawatt beam with different wavefront curvatures applied to specific regions in the transverse beam profile. In adequate conditions, the filamenting region is more homogeneously ionized, in the longitudinal direction, than in the case of uniform focusing. Moreover, the ionization maximum is located between the middle and the two thirds of the filaments in all investigated chirps and focus configurations.     

Nonlocal effects in flows of wormlike micellar solutions

The flow curve of wormlike micelles usually exhibits a stress plateau sigma* separating high and low viscosity branches, leading to shear-banded flows. We study the flow of semidilute wormlike micellar systems in a confined geometry: a straight microchannel. We characterize their local rheology thanks to particle image velocimetry. We show that flow curves cannot be described by a simple constitutive equation linking the local shear stress to the local shear rate. We demonstrate the existence of nonlocal effects in the flow of wormlike micellar systems and make use of a theoretical framework allowing the measurement of correlation lengths.     

Generator-coordinate study of elastic 16O + 40Ca scattering

Elastic ¹⁶O + ⁴⁰Ca phase shifts are calculated using a generator-coordinate method. A rotational band of molecular resonances is found with a rotational constant of about 30 keV. The width of these resonances should make them observable in the energy range 30 to 50 MeV. These features can be approximately reproduced by a shallow local potential. Properties of the imaginary part of the optical potential are discussed qualitatively. The optical potential should be transparent in a window of J-values comprised between about 16 and 25. Odd-even effects are shown to be negligible in the real part of the potential but might be important in its imaginary part. A possible confirmation of these predictions is found in an experimental excitation function.     

Retrospective analysis of air samples in the U.K. 1957–1974

The results from the retrospective analysis of a library of filters used for the collection of air samples at Chilton, Oxfordshire during the period 1957–74 are presented.     

Proprietes optiques de la phase Na3La1−x−yCexTby(PO4)2

Dans la phase Na₃La_1?x?yCe_xTb_y(PO₄)₂, le terbium présente une émission très intense sous excitation U.V. en raison d'un transfert efficace de l'excitation du cérium au terbium, le recouvrement des spectres d'émission de Ce³⁺ et d'excitation de Tb³⁺ étant particulièrement bon. L'ordre cationique éloigne les ions activateurs les uns des autres, de sorte que le rendement quantique de l'émission reste élevé aux fortes concentrations en terbium.