National Institute Of Materials Physics - Romania

A new description of the switching phenomenon is given. The switching is regarded as due to the formation and breaking of the links between the dendrites of crystalline nuclei in bulk materials, as a consequence of the energy pumped by an electrical field. This mechanism explains the very short switching time (<20?ns), the possibility to get smart memories based on multisteps of resistivity and the high number of cycles supported by the cell (1016). A cellular automaton mechanism was created on the basis of this model for switching. Multistage memory in chalcogenide materials has also been explained by this mechanism.

Effect of an antibacterial drug, sulfacetamide, IUPAC name N-[(4-aminophenyl) sulfonyl] acetamide (APSA), on the corrosion products formed on carbon steel surface in 1.0 mol L-1 HCl solution has been investigated using mass loss, X-ray photoelectron spectroscopy (XPS), and simultaneous thermal and differential scanning calorimetry/differential thermal analysis (TG/DSC/DTA). Mass loss measurements reveal that the corrosion rate of carbon steel is retarded by APSA and that the inhibition efficiency of this inhibitor increases with increasing the concentration. XPS analysis shows that, at this stage, the main product of corrosion is a non-stoichiometric Fe3+ oxyhydroxide, consisting of a mixture of FeO(OH) in anhydrous or hydrated forms, containing Cl- inclusions and adsorbed APSA molecules. The mechanism of inhibition was discussed in light of the chemical structure of the investigated inhibitor. The corrosion products were analyzed using TG/DSC/DTA technique.

High-temperature superconducting objects of Bi2Sr2CaCu2O8 and YBa2Cu3O7 highly curved in the ab-plane, such as curved/ kinked whiskers, bows and ring-like structures, were obtained within a solid-liquid-solid (SLS) grass-like growth mechanism. As-grown objects are crystals with three-dimensional epitaxy similar to conventional single crystals: they can be viewed as crystal parts 'cut' from a conventional rectangular crystal. Between our curved objects and conventional crystals, whiskers or thin films there are some differences in the superconducting properties induced only by the shape factors and no new physics is observed. Some details of the growth mechanism are discussed, emphasizing curved-line formation.

We used preceramic polysiloxane polymers to fabricate superconducting MgB2 composites that are doped with carbon and nanosized inclusions to improve the pinning properties. The polysiloxanes were prepared by atom transfer radical polymerization and the composites were fabricated by the short time spark plasma sintering method. We found that the superconducting critical temperatures were higher than expected from the carbon content found from the X-ray diffraction analysis of the (110) peak of MgB2. To explain this finding we propose that the grains are unevenly doped, with a core-shell distribution. We also found that both, the upper critical fields and the critical current densities are higher in the preceramic-doped samples than in pure MgB2, in agreement with the carbon doping level. When ferrocene-grafted polysiloxane is used, the upper critical field is the largest, while the critical current density is the lowest. We attribute this fact to the fact that the polymer pyrolysis results in iron-based nanostructures which have a pair breaking effect. (C) 2012 Elsevier B. V. All rights reserved.

Mesoporous carbon frameworks were synthesized using the soft-template method. Ca(BH4)(2) was incorporated into activated mesoporous carbon by the incipient wetness method. The activation of mesoporous carbon was necessary to optimize the surface area and pore size. Thermal programmed absorption measurements showed that the confinement of this borohydride into carbon nanoscaffolds improved its reversible capacity (relative to the reactive portion) and performance of hydrogen storage compared to unsupported borohydride. Hydrogen release from the supported hydride started at a temperature as low as 100 degrees C and the dehydrogenation rate was fast compared to the bulk borohydride. In addition, the hydrogen pressure necessary to regenerate the borohydride from the dehydrogenation products was reduced.

The low-temperature states of bosonic fluids exhibit fundamental quantum effects at the macroscopic scale: the best-known examples are Bose-Einstein condensation and superfluidity, which have been tested experimentally in a variety of different systems. When bosons interact, disorder can destroy condensation, leading to a 'Bose glass'. This phase has been very elusive in experiments owing to the absence of any broken symmetry and to the simultaneous absence of a finite energy gap in the spectrum. Here we report the observation of a Bose glass of field-induced magnetic quasiparticles in a doped quantum magnet (bromine-doped dichloro-tetrakis-thiourea-nickel, DTN). The physics of DTN in a magnetic field is equivalent to that of a lattice gas of bosons in the grand canonical ensemble; bromine doping introduces disorder into the hopping and interaction strength of the bosons, leading to their localization into a Bose glass down to zero field, where it becomes an incompressible Mott glass. The transition from the Bose glass (corresponding to a gapless spin liquid) to the Bose-Einstein condensate (corresponding to a magnetically ordered phase) is marked by a universal exponent that governs the scaling of the critical temperature with the applied field, in excellent agreement with theoretical predictions. Our study represents a quantitative experimental account of the universal features of disordered bosons in the grand canonical ensemble.

We present excitation-pulse-width-and pump-power-dependent microelectroluminescence and photon statistics measurements on electrically driven single-photon devices based on InP/AlGaInP quantum dots (QDs). For an excitation regime far below QD saturation, the results show a characteristic decrease of the purity of the single-photon emission [g((2))(0) value] with increasing excitation pulse width. For stronger excitation pulses close to QD saturation, strong antibunching is maintained for a much larger pulse width. In this case the ground-state exciton emission, which is used for the single-photon source, is inhibited during the pump pulse due to the presence of higher excited states. This prevents multiple-ground-state emission and reexcitation during long pump pulses and delays the single-photon emission to the end of the pulse, as predicted by theory and confirmed experimentally.

Large-scale microstructures were imprinted on the surface of silicon with dimensions of 1 mm x 1 mm by femtosecond laser line-by-line scanning irradiation. The scanning was made under air and under chlor/hydrogen based liquid layers. Scanning electron microscope investigations evidenced homogeneous surface microstructures, such as: ripples with sub-wavelengths dimensions, Si pillars and directional oriented bacilliform structures. The dependence of the surface morphology on laser energy, scanning speed and irradiation media was analyzed. In air, the microstructure changes from directional-arranged bacilliform structures to well-known ripple structures with a width of about 525 nm. When using the liquid media, we observe ripple structures with a width of about 370 nm and an overlapping of those that evolve in certain regions into Si pillars. The surfaces show interesting gradient topography behaviour which could be used as model scaffolds for the systematic exploration of the role of 3D micro/nano morphology on cell adhesion and growth. By using chlor and hydrogen based liquids we were able to explore the microstructuring of the silicon by line-by-line irradiation process using the femtosecond laser. (C) 2011 Elsevier B. V. All rights reserved.

Sm3+-doped Sc2O3 translucent polycrystalline ceramics were fabricated by solid-state reaction method in order to evaluate its potential for visible emission. The optical spectra in the UV-IR range of Sm3+ in these samples, at different temperatures (10-300 K), were performed. A series of data on Sm: Sc2O3 system not investigated previously were obtained from the analysis of the absorption and visible emission spectra as well as the emission kinetics: an extended energy level scheme, absorption cross sections for different bands, lifetimes, etc. Additional spectroscopic parameters were evaluated in the frame of the Judd-Ofelt (J-O) theory: J-O intensity parameters, oscillator strengths, radiative transitions probabilities, radiative lifetimes, branching ratios, and the cross sections (by Fuchtbauer-Ladenburg) of the main three visible emissions: (4)G(5/2) -> H-6(5/2) (yellow), (4)G(5/2) -> H-6(7/2) (orange) and (4)G(5/2) -> H-6(9/2) (red). (C) 2012 Elsevier B.V. All rights reserved.

Single molecule magnets are of great interest due to a multitude of potential applications for some of which thin films are required. Traditional physical vapor deposition techniques are not suitable for the deposition of these fragile materials with low decomposition temperatures. Matrix Assisted Pulsed Laser Evaporation technique has been employed for the growth of thin films of the single molecule magnet Mn-12(Propionate) on Si and glass substrates. In this paper we report on the appropriate growth conditions and also the morphology, chemical composition and magnetic behavior of the films. Continuous Mn-12(Propionate) films with properties similar to bulk materials have been obtained. (c) 2011 Elsevier B.V. All rights reserved.