National Institute Of Materials Physics - Romania

We present an investigation of the field dependence of the pinning force of dense MgB2-based superconducting composites as obtained by spark plasma sintering with different ingredients which are designated to enhance the pinning force F. Generally, the latter quantity, scaled to the maximum pinning force F-max, obeys a scaling law as a function of the scaled field h when the irreversibility field B-irr is considered as scaling field. The scaled function is described in terms of a generalized scaled function h(p)(1-h)(q). However, in our samples the scaling is absent. In addition, the peak of the scaled function shifts to higher reduced fields h when the temperature increases. Depending on the level of doping and the nature of the nanosized particles used to built the superconducting composites, we found that the scaled force can be depicted either with a combination of generalized scaled functions with different exponents or with a single function but multiplied with an envelope, usually an exponential factor. The former dependence is present in the samples with a high amount of nanoparticles and mirrors two types of pinning of close weights. The presence of the exponential factor in the latter dependence is attributed to the combined effect of the intrinsic anisotropy of MgB2 and the random orientation of the grains relative to the magnetic field.

In this work we design some atomic scale simulation methods as investigative tools in the study of the formation of compounds for the reversible storage of hydrogen in bulk materials. It was verified that the reaction between the LiBH4 and MgH2 is energetically favored for temperatures above 280 K and that this system can be used in the hydrogen storage and the fuel cell application. To identify the reaction mechanism at the interface of LiBH4, MgH2 and carbon layers we performed some Molecular Dynamics simulations and QM/MM calculations. The results show that the layers of ions formed at the interface with the graphite may assure the right arrangement of the atoms to start the formation of the crystals. Moreover, the presence of the hexagonal layers of graphite may play a role as a pattern template for the layers of boron atoms in the MgB2 lattice.

This chapter presents a theoretical framework for the description of the light-matter interaction of a single semiconductor quantum dot in a microcavity. A particular focus is the semiconductor properties of the device, the contributions of various electronic configurations, and the embedded nature of the quantum dot due to its coupling to delocalized states. Stimulated emission from the excitonic ground state and from higher excited states is discussed together with the photon statistics and cavity emission spectra. The role of dephasing for transitions between different configurations is explained. We exploit a direct solution of the von Neumann equation for the statistical operator including the full light-matter interaction in terms of the Jaynes-Cummings interaction Hamiltonian, as well as the Coulomb interaction of the quantum-dot carriers. Various dissipative processes are systematically considered using Lindblad terms.

The article reports on the tilted growth of textured aluminium nitride thin films obtained by radio-frequency magnetron sputtering onto 50 mm diameter Si (111) wafers, in reactive atmosphere, in a planar sputtering system without tilting the substrate and with no additional sputtering geometry alterations. The films were investigated using, X-ray diffraction, spectroscopic ellipsometry and scanning electronic microscopy, done by local measurements on the wafer surface, at different distances from the centre. A progressive increase of the tilt angle when moving away from the sample centre has been found. The maximum tilt angle of the columnar AlN crystallites, obtained near the edges of the wafer, is about 7 degrees. The results showed also that tilting is associated with smaller thickness and larger dispersion of the c axis orientation. Synthesizing inclined c axis AlN films should allow the fabrication of surface acoustic wave devices based on shear waves for liquid sensor applications.

A high performance piezoceramic material of PZT type, doped with 6 atomic % W, was prepared in order to be used as sensing element for a vibration transducer used to equilibrate the motor-fan group of auto vehicles. The piezoelectric parameters of material showed high values which recommended it for the vibration transducer. The transducer consists of a ring shaped active element sandwiched between two steel blocks. The transducer works in quasistatic regime, at frequencies much lower than the resonance frequency, being tightly screwed in the measurement system by a metallic cover attached at the back end.

We report on the synthesis of electrode layers by Pulsed Laser Deposition technique. We selected Nickel Oxide-8Yttria-Stabilized Zirconia as anode, La0.6Sr0.4Co0.2Fe0.8O3-delta perovskite material as cathode, and Yttria doped (Ceria-alpha Alumina) as electrolyte. We studied electrode structure (average grain size, porosity, and grain shape), the deposition uniformity and electrode-electrolyte interfaces. The electrodes were investigated in terms of morphology (Scanning Electron Microscopy) and composition (Energy Dispersive X-ray Spectroscopy). The initial powder and the synthesized cathodes have been investigated superficially and in bulk by Mossbauer spectroscopy, in order to detect and analyse local properties. We identified the conditions for electrode layer synthesis in respect with porosity, composition, and mean grain size.

The functions of bio-molecules depend on their structural fluctuations, which are thought to be 'slaved' (i.e. coupled) to those of nearby bulk water. Slaving was suggested to explain the universal pattern of propagation of mechanical excitations through concentrated water-containing solutions of polymers, such as the cytoplasm. To obtain a simplified model of this phenomenon, we propose to use a cellular automaton of water-based solutions. During iterations, the model computes the compactness of both the solvent and the solute (equivalent of their density), as the average number of neighbors of each class of particles. As a surrogate of slaving, we studied the temporal co-variation of these variables, using the Pearson correlation coefficient (S-av). We found that S-av depends in a biochemically-meaningful manner on the concentration of solute, on its hydrophatic character and on molecular flexibility. The simulations also show that S-av is robust to mild hypothermia. In conclusion, our cellular automaton is capable to generate a slaving-like behavior of solutes in water, as an emergent phenomenon occurring in dissolved molecular systems.

In this paper, we present recent results obtained on single-walled carbon nanotubes (SWNTs) and carbon nanotube/conjugated polymer composites by using resonant Raman scattering and Surface Enhanced Raman Scattering (SERS). Besides the characterization of these materials, we report on peculiar properties observed in the anti-Stokes Raman branch of the Raman spectra. They consist in an abnormal anti-Stokes Raman emission which is explained by a mechanism reminiscent of a Coherent anti-Stokes Raman Scattering (CARS) emission. It results from a wave mixing process between the incident laser light and Stokes Raman light, generated by the SERS mechanism. In a parallel way, we have investigated in details the resonance effects which also induce anomalies in the anti-Stokes/Stokes intensity ratios, as a function of several parameters including the observation temperature, the environmental conditions, the dilution in solvents, etc. Studies extended to composites based on carbon nanotubes and conjugated polymers reveal also interesting properties. In the case of poly(bithiophene) (PBTh), one observes a strong amplification of the 1450 cm(-1) Raman line in the anti-Stokes branch, generated by the plasmon excitation of metallic tubes. This phenomenon occurs in several other conjugated polymers such as PEDOT and PPV for modes located around 1500 cm(-1). The role of metallic SWNTs is discussed. Finally, an anti-Stokes luminescence excited in the low energy tail of the absorption band of PPV and PPV/SWNTs composites has even been observed for the first time, explained through a phonon-energy up-conversion mechanism.

The study of the electrodeposition and anodic oxidation processes of ZnMn alloy submicron wires is reported; ZnMn wire arrays were electrodeposited using a pulse current technique and an aqueous solution containing 0.2 mol dm(-3) Zn(II)-EDTA + 0.2 mol dm(-3) MnSO4 + 0.6 mol dm(-3) sodium citrate. The obtained alloy was anodized at a current constant density, in a methoxyethanol solution containing sodium hydroxide and in an aqueous potassium zincate solution. Photoluminescence spectra and SEM images showed the presence of the oxide layer on the surface of the anodized ZnMn wires.

Epitaxial Pb(Zr0.52Ti0.48)O-3 (PZT) thin film has been successfully integrated on Si(001) substrate by sol-gel method. SrTiO3 (STO) layer deposited on Si by Molecular Beam Epitaxy (MBE) acts as a template layer in this study to avoid the formation of amorphous SiO2, and allows the chemical compatibility for further epitaxial growth. For bottom electrode, SrRuO3 (SRO) layer grown by Pulsed Laser Deposition (PLD) on STO/Si was used. Epitaxial single crystalline growth of PZT film after Rapid Thermal Annealing (RTA) at 650 C was evidenced by X-Ray Diffraction (XRD). The following relationship in the heterostructure was deduced: [110] PZT (001) // [110] SRO (001) // [110] STO (001) // [100] Si (001). A clear piezoelectric response of the film was observed by Piezoresponse Force Microscope (PFM). Moreover, the structural STO quality was proved to have a major impact on the electrical properties of PZT films.