Resonance and absorption band in the classical magnetoactive semiconductor-insulator superlattice

Electromagnetic wave propagation in the classical semiconductor-dielectric superlattice is investigated in the limit of wavelengths, exceeding the lattice spatial period. The resonance and absorption band have been experimentally discovered in a frequency range below the semiconductor plasma frequency in the presence of the dc magnetic field normal to the wave vector and the lattice periodicity direction.     

Models with rising cross sections and the inverse amplitude sum rules

Models for pion-nucleon scattering with rising cross sections are analyzed using dispersive sum rules which are valid for the inverse of the scattering amplitude. The validity of these sum rules is intimately connected with the positivity of Im T(s, t = 0). A numerical analysis shows that some of the models have difficulties when analytically continued into the complex energy plane.     

Estimating the "steric clash" at cis peptide bonds

To account for the scarcity of cis peptide bonds in proteins, especially in nonproline (or secondary amide) cases, a steric-clash argument is often put forward, in a scheme where the R lateral chains are facing parallel one another, and the backbone is kept in an "all- trans"-like arrangement. Although such a steric conflict can be partly relieved through proper adjustment of the backbone dihedral angles, one can try to estimate its associated energy cost. To this end, quantum-chemistry approaches using a differential-torsion protocol and bond-separation-energy analyses are applied to N-ethyl propionamide CH3-CH2-CO-NH-CH2-CH3, regarded as a model capable of exhibiting C beta...C beta interaction as in alanine succession. The calculations provide an increment of 9 kcal/mol, quite close to that obtained in the nearly isostere (gsg) rotamer of n-hexane (10 kcal/mol), suggesting the local effects induced by methyl-methyl contact are similar in both cases. Analogous treatments on larger radicals as encountered in leucine or phenylalanine dimers do not change this increment much, which therefore defines the basic reference per-plaque quota to be overcome along all- cis chains. Explicit modeling indicated it can be reduced by up to a factor of 4.     

Interaction mechanisms between uranium(VI) and rutile titanium dioxide: from single crystal to powder

This paper is devoted to the study of the mechanisms of interaction between uranyl ion and rutile TiO2. Among the radionuclides of interest, U(VI) can be considered as a model of the radionuclides oxo-cations. The substrate under study here is the rutile titanium dioxide (TiO2) which is an interesting candidate as a methodological solid since it can be easily found as powder and as manufactured single crystals. This material presents also a wide domain of stability as a function of pH. Then, it allows the study of the retention processes on well-defined crystallographic planes, which can lead to a better understanding of the surface reaction mechanisms. Moreover, it is well-established that the (110) crystallographic orientation is dominating the surface chemistry of the rutile powder. Therefore, the spectroscopic results obtained for the U(VI)/rutile (110) system and other relevant crystallographic orientations were used to have some insight on the nature of the uranium surface complexes formed on rutile powder. This goal was achieved by using time-resolved laser-induced fluorescence spectroscopy (TRLFS) which allows the investigation, at a molecular scale, of the nature of the reactive surface sites as well as the surface species. For rutile surfaces, oxygen atoms can be 3-fold, 2-fold (bridging oxygens), or single-fold (top oxygens) coordinated to titanium atoms. However, among these three types of surface oxygen atoms, the 3-fold coordinated ones are not reactive toward water molecules or aqueous metallic cations. This study led to conclude on the presence of two uranium(VI) surface complexes: the first one corresponds to the sorption of aquo UO22+ ion sorbed on two bridging oxygen atoms, while the second one, which is favored at higher surface coverages, corresponds to the retention of UO22+ by one bridging and one top oxygen atom. Thus, the approach presented in this paper allows the establishment of experimental constraints that have to be taken into account in the modeling of the sorption mechanisms.     

Continuous‐wave vertically emitting photonic crystal terahertz laser

Compact semiconductor light sources with high performance continuous‐wave (CW) and single mode operation are highly demanded for many applications in the terahertz (THz) frequency range. Distributed feedback (DFB) and photonic crystal (PhC) quantum cascade (QC) lasers are amongst the leading candidates in this field. Absorbing boundary condition is a commonly used method to control the optical performance of a laser in double‐metal confinement. However, this approach increases the total loss in the device and results in a large threshold current density, limiting the CW maximum output power and operating temperature. In this letter, a robust surface emitting continuous‐wave terahertz QC laser is realized in a two‐dimensional PhC structure by a second order Bragg grating extractor that simultaneously provides the boundary condition necessary for mode selection. This results in a 3.12 THz single mode CW operation with a 3 mW output power and a maximum operation temperature (T_max) of 100 K. Also, a highly collimated far‐field pattern is demonstrated, which is an important step towards real world applications.     

Combining coordination chemistry and catalysis to tie a knot by an active-metal template strategy

Collar and tie men: the smallest trefoil knot reported to date has been prepared by an active metal template synthesis. Copper(I) ions are able to constrain the well-designed structure so that it can form the loops by complexing to the bipyridine moieties in the core of the thread and the two ends of the entangled lace on opposite faces of the loop, before acting as a catalyst to close the lace.     

Magnetism of actinide compounds

We review the physical properties of actinide compounds, with particular emphasis on their magnetic characteristics. Because of the complexity of these materials, mainly the simplest compounds are treated, some of which have properties that can be theoretically explained.     

Isomerisation and phase transitions in small sodium clusters

Using a distance-dependent tight-binding hamiltonian, we have studied the influence of the temperature on the geometries of small alkali clusters (Na₄, Na₈, and Na₂₀). We have applied a Monte-Carlo thermodynamical method which consists in performing canonical samplings for various temperatures, these samplings being reexpressed in the microcanonical ensemble. This method provides thermodynamical values such as the entropy and the specific heat. Their behaviour shows one phase transition in the case of Na₄ and Na₈, and two phase transitions for Na₂₀. As concerns Na₄ and Na₈, the transition occurs at 200 K, between a solid-like phase and a phase for which the geometry of these clusters oscillates between numerous shapes. In the case of Na₂₀, the two observed phase transitions can be described as a melting of the surface atoms (at 200 K) preliminarily to the fluctuation of an inner icosahedron seed (at 300 K).