Artificial pinning in thick YBCO films: Pinning potential and c-axis correlation

We have introduced artificial pinning centres in thick (>1 μm) YBCO films grown by Pulsed Laser Deposition using substrate decoration, quasi-multilayers, and target doping approaches. We have found that the frequency dependence of critical current density is consistent with a logarithmic dependence of pinning potential on current density. For most of materials used as nano-dots, artificially-induced pinning centres have a larger potential than natural ones. From angle-dependent in-field transport measurements and from Transmission Electron Microscopy we have found evidence of c-axis correlated pinning centres.     

Controlled trapping of single particle states on a periodic substrate by deterministic stubbing

A periodic array of atomic sites, described within a tight binding formalism is shown to be capable of trapping electronic states as it grows in size and gets stubbed by an ‘atom’ or an ‘atomic’ clusters from a side in a deterministic way. We prescribe a method based on a real space renormalization group method, that unravels a subtle correlation between the positions of the side coupled atoms and the energy eigenvalues for which the incoming particle finally gets trapped. We discuss how, in such conditions, the periodic backbone gets transformed into an array of infinite quantum wells in the thermodynamic limit. We present a case here, where the wells have a hierarchically distribution of widths, hosting standing wave solutions in the thermodynamic limit.     

DFT calculations as a powerful tool for ESR spin trapping experiments

A combined electron spin resonance (ESR) and density functional theory study has been carried out to obtain deeper insight in the reaction mechanism of the photorearrangement of 3‐hydroxyindolic nitrones. Also, in this case, the combination of these techniques constitutes a powerful tool when discriminating between different reaction pathways. In particular, density functional theory calculations played an important role for unraveling the overlapping of signals coming from different species in the ESR spin trapping experiments. In the present study, the computed energies and the corresponding ESR parameters of each possible isomer hypothesized have been considered, taking also into account their different possibilities of intramolecular H‐bond formation. However, the results obtained indicate that the possibility of intramolecular H‐bonding did not play a determinant role in this case. Copyright © 2016 John Wiley & Sons, Ltd.     

Birkhoff Centre of a Poset

In this paper, it is proved that the Boolean centre of a semigroup S with sufficiently many commuting idempotents is isomorphic to the inverse limit of the directed family of Birkhoff centres (or Boolean centres) of a class of bounded semigroups. The Birkhoff centre is defined for any poset and proved that it is a relatively complemented distributive lattice whenever it is nonempty. It is observed that for a semilattice S, the Birkhoff centres as a semigroup and as a poset coincide. Also it is observed that for a Lattice (L, , ), the Birkhoff centres of the semilattices (L, ) and (L, ) coincide with the Birkhoff centre of L. Finally it is proved that for a lattice (L, , ), the Boolean centres of the semilattices (L, ) and (L, ) coincide with the Boolean centre of L.AMS Subject classification (1991): 06A12, 20M15     

Membrane properties of cationic liposomes composed of dipalmitoylphosphatidylcholine and dipalmityldimethylammonium bromide

Cationic liposomes composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmityldimethylammmonium bromide (DPAB) were prepared by the Bangham method and the effect of DPAB on the membrane properties was examined in terms of liposomal shape, particle size, trapping efficiency, surface potential and dispersibility. The dispersibility of the mixed DPPC/DPAB liposomes (the mole fraction of DPAB (X_DPAB)  0.05) was excellent and the dispersibility was maintained for 6 months, since the zeta-potential of the mixed liposomes was approximately +40 mV. The trapping efficiency of the mixed DPPC/DPAB liposomes (X_DPAB = 0.05) was 10 times greater than that of the DPPC liposomes, and the value was largest among the mixed liposomes (X_DPAB = 0–1.0). Freeze-fracture electron micrographs indicated that the shape of the mixed DPPC/DPAB liposomes (X_DPAB = 0.05) was that of large unilamellar vesicles (LUVs) with a diameter of approximately 2 μm, while the shape of the DPPC liposomes was that of multilamellar vesicles (MLVs). The mixed liposomes had, therefore, a high trapping efficiency. Furthermore, the shape of the mixed DPPC/DPAB liposomes (X_DPAB = 0.75) was also that of LUVs with a diameter of approximately 2 μm and these had a high trapping efficiency. Whereas, the particle size (500 nm) of the mixed DPPC/DPAB liposomes (X_DPAB = 0.25) was smaller than that of the former and had the minimum trapping efficiency. The phase transition temperature of the liposomal bilayer membranes indicated a maximum value at 0.25–0.30 mole fractions of DPAB. These facts were considered to be due to the fact that DPPC and DPAB, whose molar ratio was 7.5:2.5, were tightly packed in the liposomal bilayer membranes and that the curvature of the liposomal particle was resultantly large. Nevertheless, LUVs having a high trapping efficiency were easily obtained by mixing a small amount of DPAB with the DPPC.     

Laser-active colour centres with functional periodic structures in LiF fabricated by two interfering femtosecond laser pulses

We report a new technique to fabricate both laser-active F₂ and F₃⁺ colour centres in lithium fluoride and permanent periodic gratings with fringe spacings as fine as sub-micron size simultaneously by two interfering infrared femtosecond (fs) laser pulses. In particular, the optical properties of such colour centres produced by a single fs laser pulse are compared with those created by damage from radiation such as X-rays. Moreover, the present technique is applied to the first production of three-dimensional active channel waveguide and a pulsed distributed-feedback (DFB) laser at around 700 nm in LiF containing F₂ colour centres with fine-pitched micro-grating structures.     

Effects of Trapped Electrons on Off-axis Lower Hybrid Current Drive

The effects of trapped electrons on off-axis lower hybrid current drive （LHCD） in tokamaks are studied. The influence of the resonance regime on the current drive efficiency as well as the influence of trapped particle fraction on the current drive efficiency are emphasized.     

Synthesis, photoluminescence, thermoluminescence and electron spin resonance investigations of CaAl12O19:Eu phosphor

The present paper describes the synthesis of europium-doped calcium aluminate phosphor using the combustion method. An efficient blue emission phosphor can be prepared at reaction temperatures as low as 500 °C in a few minutes by this method. Characterization of the powder was done by X-ray diffraction, transmission electron microscopy, scanning electron microscope analysis and the optical properties were studied by photoluminescence spectra. Thermoluminescence (TL) studies also have been carried out on CaAl₁₂O₁₉:Eu²⁺ phosphor. The TL glow curve shows peaks at 174 and 240 °C. Defect centres formed in irradiated phosphor have been studied using the technique of electron spin resonance. Step annealing measurements indicate that one of the annealing stages of a defect centre appear to correlate with the release of carriers resulting in TL peak at 174 °C. The centre is characterized by an isotropic g-value of 2.0046 and is assigned to a F⁺ centre.     

Surface electroluminescence phenomena correlated with trapping parameters of insulating polymers

Electroluminescence (EL) phenomena are closely linked to the space charge and degradation in insulating polymers, and dominated by the luminescence and trap centers. EL emission has been promising in defining the onset of electrical aging and in the investigation of dissipation mechanisms. Generally, polymeric degradation reveals the increment of the density of luminescence and trap centers, so a fundamental study is proposed to correlate the EL emission of insulating polymers and their trapping parameters. A sensitive photon counting system is constructed to detect the weak EL. The time- and phase-resolved EL characteristics from different polymers (LDPE, PP and PTFE) are investigated with a planar electrode configuration under stepped ac voltage in vacuum. In succession, each sample is charged with exposing to multi-needle corona discharge, and then its surface potential decay is continuously recorded at a constant temperature. Based on the isothermal relaxation current theory, the energy level and density of both electron and hole trap distribution in the surface layer of each polymer is obtained. It is preliminarily concluded that EL phenomena are strongly affected by the trap properties, and for different polymers, its EL intensity is in direct contrast to its surface trap density, and this can be qualitatively explained by the trapping and detrapping sequence of charge carriers in trap centers with different energy level.