National Institute Of Materials Physics - Romania

The electronic and optical properties of self-assembled InN/GaN quantum dots (QDs) are investigated by means of a tight-binding model combined with configuration-interaction calculations. Tight-binding single-particle wave functions are used as a basis for computing Coulomb and dipole matrix elements. Within this framework, we analyze multiexciton emission spectra for two different sizes of a lens-shaped InN/GaN QD with wurtzite crystal structure. The impact of the symmetry of the involved electron and hole one-particle states on the optical spectra is discussed in detail. Furthermore we show how the characteristic features of the spectra can be interpreted using a simplified Hamiltonian which provides analytical results for the interacting multiexciton complexes. We predict a vanishing exciton and biexciton ground-state emission for small lens-shaped InN/GaN QDs. For larger systems we report a bright ground-state emission but with drastically reduced oscillator strengths caused by the quantum confined Stark effect.

High density (more than 90% of the theoretical density) bulk samples with relatively large size (2.0 cm in diameter) of MgB2, pristine and doped with 5 mol% SiC and B4C were obtained by the field-assisted sintering technique (FAST) from commercial MgB2 powder. The superconducting properties in the magnetic field of the doped samples show depressed upper critical field H-c2 and irreversibility field H-irr when compared with pristine MgB2. The reason for better H-c2 and H-irr in the undoped sample is not clear. Most probable is that this is due to the limited diffusion time which restricts the substitution of carbon for boron in this short-time processing method as well as due to the way grain boundaries are formed in the FAST process. However, all our samples have shown higher values of H-c2 and H-irr than the values reported previously for FAST-MgB2 samples. The data were further investigated through scaling analysis. Data suggest that both additions of SiC and B4C stabilize two well defined pinning regimes in FAST-processed MgB2. At low temperatures, the pinning occurs at the grain border whereas at high temperatures, T >= 24 K, there is mixed pinning (H <= 5 T).

Current-voltage characteristics of samples containing silicon nanodots embedded in an amorphous silicon dioxide matrix were investigated. A percolation mechanism was evidenced by the dependence on the concentration of the initial differential conductance and by the appearance of several thresholds in the I-V characteristics.

Thin films of nonstoichiometric Heusler alloys Co2MnSbxSn1-x (x = 0.2; 0.4; 0.6; 0.8) have been grown by pulsed laser deposition (double-target/double beam configuration) on Si (10 0) substrates using a KrF excimer laser (lambda = 248 nm, tau = 20 ns). The substrate temperature was held at 300 K in all experiments to prevent interface interdiffusion of the species. A comparison between the compositions of films and corresponding targets has been done through energy dispersive X-ray spectroscopy (EDS) analysis showing a very satisfactory match. Scanning electron microscopy (SEM) imaging served to investigate the morphology of the films in order to determine the size and density of droplets which may influence the optical data. Optical conductivity derived from reflectivity measurements shows absorption onsets close to 1eV, which corresponds to the onset of valence-to-conduction transitions in the minority spin bands theoretically predicted. The values of the saturation magnetisation measured at 300 K on the quaternary alloys are very close to those of ternary ones for which either half-metallic properties or high spin polarisation were theoretically predicted. (c) 2007 Elsevier B.V. All rights reserved.

We report the synthesis of well crystallized stoichiometric Er:YAG thin films by pulsed laser deposition from Er:YAG targets in low pressure oxygen, followed by a post-deposition treatment at 1400 degrees C in environmental air for 19 h. The structure and morphology of the films were examined by transmission electron microscopy and selected area electron diffraction. We demonstrated that the subsequent application of pulsed laser deposition and heat treatment resulted in the formation of well crystallized single phase cubic Er:YAG nanostructured coatings. (c) 2007 Elsevier B.V. All rights reserved.

Barium strontium titanate (BST) bulk ceramic with composition Ba0.5Sr0.5TiO3 was produced by solid-state reaction and was used as target for PLD thin film deposition. A Nd-YAG laser, working at 5-10 Hz and different wavelengths has been used. Thin films of stoichiometric BST were deposited on alumina substrate with the thickness between 400 and 500 nm. An aditional annealing was carried out at 800 degrees C for 6 h. XRD and SEM were used for sample characterization. Capacity measurements were performed versus temperature at 100 kHz in a large temperature range. A diffuse ferroelectric-paraelectric phase transition with T-C = -72 degrees C for the BST thin film was determined. A 5% tunability was measured in the transition region. (c) 2007 Elsevier B.V. All rights reserved.

We report an optimized kinetic regime to grow in situ YBa2Cu3O7-delta (YBCO) films on LaAlO3 substrates, by pulsed laser deposition (PLD), which is technically more advantageous than the usual one. Atomic force microscope (AFM) pictures show that the in situ grown YBCO films have much smoother surfaces than the post-deposition annealed YBCO films. The influence of the substrate's surface quality and that of the pulsed laser fluence used in the deposition process on the surface quality and on the critical temperature (T-C) of the in situ YBCO films are described. For measurements we utilized the atomic force microscopy and X-ray diffraction (XRD) techniques. (c) 2007 Elsevier B.V. All rights reserved.

Fe-doped titania films were deposited by RF sputtering onto different substrates (glass and ITO/glass) in the same deposition run. The rutile nanocrystalline structure of Fe-doped thin films deposited on glass substrates and anatase nanocrystalline structure of Fe-doped thin films deposited on ITO/glass substrates were evidenced by XRD. SEM investigations showed a smooth surface with a dense nanostructure. XPS study evidenced an almost stoichiometric composition with different iron contents. EPR and XPS studies evidenced that iron entered into TiO2 lattice by substitution, as isolated and dimer species. In Fe-doped thin films deposited on ITO/glass substrates the iron content is ten times higher than in Fe-doped thin films deposited on glass substrates and that a part of them entered as Fe2+. (C) 2007 Elsevier B.V. All rights reserved.

A barium titanate precursor with barium: titanium ratio of 2: 9 was prepared by coprecipitation of mixed barium and titanium chlorides with ammonium oxalate aqueous solution at pH 7-7.5. Single-phase of Ba2Ti9O20 crystallised in the triclinic system and dense ceramic are obtained by sintering at 1375 degrees C for 4-8.5 h. The grain size of the sintered ceramics is 3 to 5 mu m. Dielectric properties of the sintered ceramics in the microwave frequency region were measured using the TE0(11) mode. Good microwave characteristics for Ba2Ti9O20 Were found. At 5 GHz, the dielectric constant and the dielectric loss (tg sigma) were 36 respectively, 0.6x10(-3). The quality factor due to dielectric loss at 5 GHz was similar to 1700.