National Institute Of Materials Physics - Romania

The use of semiconductors as detectors for particle and astroparticle physics imposes the understanding of the processes produced in material, their effects on the electronic properties, in their microscopic structure and at the device level. Calculations of LET and NIEL for different ions including hypothetical exotic particles put into evidence the peculiarities of these interactions.

In this paper a novel configuration of microwave planar filters, with multiple cross-couplings and with a number of poles of attenuation NZ equal to the order N of the filter, is investigated The position of the poles on the frequency axis can be controlled, allowing the design of band-pass filters with improved selectivity with respect to the adjacent channels. The new configurations were designed and verified by em-field simulation. The responses of the designed filters are in good agreement with the specification, confirming the possibilities of designing microwave band-pass filters of a relatively low order with moderate losses and with improved performances.

Room temperature mechanical treatment consisting of multiple torsion loadings (named pre-torsion) was applied to Nb3Sn composite wires with different architecture to reduce thermal residual compressive strain experienced by the superconducting Nb3Sn filaments. Due to this effect all investigated wires have shown enhancement of the critical current density, I-c up to 56% at 15T and 4.2K. Enhancement of I-c was larger for the reinforced wires than for the wires without reinforcement suggesting that reinforcement is useful in strain relaxation during pre-torsion. The best results were obtained when the position of the reinforcement was located in the outer region of the wire. Pre-torsion is similar or more efficient than cycles of bending loadings (named pre-bending). For pre-torsion important parameter is rotation angle per length of the wire and optimum conditions for short wires are valid for long wires. Uniformity of Ic along the length of a 99cm wire after pre-torsion was within +/-2.1A.

Some aspects of microwave propagation through left-handed materials are investigated in this paper. The strong distortion of a Gaussian TEM pulse through a material with negative refraction index is studied in the time domain. Moreover, a method of extracting the constitutive parameters of a material by using the scattering parameters of a sample two-port is developed This method is then applied to some left-handed microstrip structures.

Thermoluminescence (TL) curve recorded after gamma-irradiation at room temperature of Mn2+-doped MgF2 phosphor shows TL peaks at 91, 147, 200, 235 and 295 degrees C. The 147 degrees C peak was tentatively attributed to the F centres recombination with hole centres trapped in the neighbourhood of the Mn2+ impurity. This peak shows a good stability (low fading) and has been used to characterise the TL phosphor dosimetric properties. TL dose dependence has shown a good linearity in the range from 0.2 to 10 mGy, which makes it suitable for personal and medical dosimetry. The TL curves recorded after gamma rays or particles irradiation are similar, which suggests the possibility to use the phosphor also for particle detection. (C) 2007 Elsevier Ltd. All rights reserved.

Using surface enhanced Raman scattering (SERS) and FTIR spectroscopy, a covalent functionalization of single wall carbon nanotubes (SWNTs) with polystyrene (PST) is demonstrated. For this, two types PST/SWNTs composites were used: one resulting from the radical polymerization reaction of styrene achieved at 90 degrees C in the presence of benzene and benzoyl peroxide, latter being the initiator and another obtained by mixing the two constituents. These compounds reveal different SERS and photoluminescence spectra. The main experimental facts supporting the covalent functionalization of SWNTs with PST are provided by FTIR spectroscopy. In this frame significant is the increase of the intensities of absorption bands at 1270 and 1721 cm(-1), which are associated with the tangential C-H bending+C-O stretching and C=O stretching vibration modes, respectively. The presence of these absorption bands in the FTIR spectra of the PST/C-60 composites proves the formation of a compound with a similar molecular structure. in the case of polystyrene functionalized SWNTs, the appearance of a new FTIR band at ca. 1635 cm(-1) attributed to the C-C in cyclic hydrocarbons ring di-substituted vibration mode indicates the formation of an additional reaction product. (C) 2008 Elsevier B.V. All rights reserved.

Recent models for the occurrence of high-temperature superconductivity predict that the superconducting transition of underdoped cuprates is determined by the phase fluctuations of the order parameter, while the modulus of the wave function remains finite up to the pseudogap temperature T*. Here we show that the mean-field transition temperature cannot be identified with T*.

ac susceptibility measurements were performed on pure and SiC-doped MgB2 prepared by field assisted sintering technique in the temperature range between 5 and 50 K. The activation energy for flux-creep U for both samples was determined at dc-fields of 0, 0.5, and 5 T, respectively, from the frequency dependence of the imaginary part of the ac-susceptibility chi '' data measured in the range 17-9997 Hz. It was found that at 0 and 0.5 T, the activation energy of the SiC-doped sample is almost half the energy of the pristine sample whereas at 5 T the activation energies of the two samples become almost equal. A similar behaviour show the H-p(T) lines as obtained from susceptibility data in the range 0-9 T. For fields lower than 7.5 T, the H-p field of the doped sample is lower than for the undoped MgB2. At similar to 7.5 T, the H-p(T) for SiC doped MgB2 crosses over the H-p(T) of pure MgB2 and for higher fields the doped samples exhibits a higher irreversibility H-p. This field dependence was extracted from the evolution of the. chi(1)''(T) data. This behaviour is a consequence of the morphology of the sample with a large spread in grain size.

In order to determine the effect of sample shape, compacting pressure and green density gradients on the final shape and density of sintered bodies a study was made on a PZT type powder, pressed within a large range of shapes from long cylinders up to thin discs and at pressures ranging between 40 and 180 MPa. Important shape distorsion were found for both long cylinders and thin discs but surprisingly the final density was not essentially changed even if the green compacted density showed important changes with in the samples. Differences in the shrink age of diameter and thickness brought about by sintering for samples with different shapes were found.

A composite prepared front aerosil A380 and the liquid crystal (LC) 4-hexyl-4'-cyanophenyl benzoate (CP6B) was investigated by broadband dielectric spectroscopy in a large temperature range. The selected high silica density (ca. 7 g aerosil/1 g of CP6B) allows the observation of a thin layer (two-monolayer structure) adsorbed on the surface of the silica particles. For the composite one relaxation process is observed at frequencies much lower than that of the processes found for bulk CP6B. It is assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its relaxation rates obeys the Vogel-Fulcher-Tammann law, which is characteristic for glass-forming liquids. The quasi 2D character of the observed glassy dynamics in the surface layer is discussed. The temperature dependence of the CP6B relaxation in the composite is compared with that of related hexylcyanobiphenyl molecules in the surface layer of aerosil composite with a similar concentration.