National Institute Of Materials Physics - Romania

The importance of vertex corrections in the description of the absorption spectra in semiconductors is discussed. These corrections are responsible for the excitonic correlation of the phonon-scattered carriers and it is shown that these correlations are essential for the description of excitonic sidebands in the case of intermediate coupling. Different approximation schemes are compared in numerical examples.

The fundamental and third-harmonic susceptibility response of YBa2Cu3O7-delta (YBCO) and TlBa2Ca2Cu3O10-y (Tl:1223) high-critical temperature (T-c) superconductors with preferentially oriented crystallites has been measured in various magnetic fields up to 14 T, and with very low ac field amplitude, to probe the melting transition of the vortex matter. This method does not require large single crystals, being therefore suitable for high-T-c superconductors with small crystallites, prepared with high-pressure synthesis techniques. We determine experimentally the melting line of vortices and the anisotropy factor in high-pressure-grown Tl:1223 with a high T-c of 133.5 K. Results from similar measurements on YBCO with preferentially oriented grains are consistent with both commonly accepted models of the melting line and values of the anisotropy factor, supporting the validity of our approach. (C) 2003 American Institute of Physics.

A model for trapping phenomena in nanocrystalline silicon investigated by the optical charging spectroscopy method is proposed. The model takes into account all the possible contributions to the discharge current. The results previously obtained on fresh and passivated porous silicon samples are interpreted within the framework of the model, which provides a good fit to the experimental data. The role played by the different kinds of trapping centers (donors and acceptors, surface and bulk) and the different trap parameters is analyzed. (C) 2003 American Institute of Physics.

Mossbauer spectrometry and magnetic measurements are employed to experimentally investigate the magnetic behavior of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 ribbons obtained by appropriate annealing of the amorphous precursor. A detailed analysis of the correlation between the microstructure of annealed samples and their magnetic properties is provided. Thermomagnetic data allow the Curie temperatures of both amorphous residual matrix and nanocrystalline phase to be estimated. The differences between Curie temperatures of amorphous residual matrix and amorphous precursor are investigated and explained in terms of magnetic polarization of the matrix by exchange fields arising from the nanocrystalline grains. Theoretical systems of spins consisting of a single ferromagnetic nanocrystalline grain immersed in weakly ferromagnetic environment, quite similar to our real samples, are considered and their magnetic behavior is investigated by Monte Carlo simulation of low temperature spin ordering, with emphasize on the matrix-nanocrystalline grain interface which is shown to exhibit peculiar magnetic behavior. The magnetic features of the matrix-nanocrystalline grain interface are studied, as depending on matrix-nanocrystalline grain exchange coupling as well as crystalline fraction of the nanocrystalline systems.

PbS films were chemically deposited on glass substrates from chemical reducing bath doped with Bi. The growth and properties of PbS layers are investigated. The effect of the reducer and Bi3+ ions on the deposition process was studied with a quartz-crystal microbalance technique. The reducer introduces a new deposition mechanism of electrochemical nature, which allows for thicker films. On the other hand, Bi3+ ions introduce nucleation centers in the solution (Bi(OH)(3)), which accelerate the homogeneous precipitation and diminish the film thickness. The PbS grain size within the layer increases with increasing the Bi content in the deposition bath. A considerable enhancement of the photoconductive signal was found in case of PbS films deposited from Bi doped bath. The signal has nonlinear behavior with the Bi content in the chemical bath. There is an optimum quantity of Bi for which the photoconductive characteristic reaches maximum. (C) 2003 Elsevier Science B.V. All rights reserved.

The abnormal polarization offset observed in case of graded ferroelectric structures is explained assuming the presence of a nonreversible part of polarization due to the imposed polarization gradient. It is shown that an upper limit for the offset should exist, fixed by the remnant polarization of the component materials considered as independent layers. This is valid for the structures in which polarization increases or decreases in steps from one layer to the other, but should also be valid for the structures with continuous gradient. However, large values of the charge offset could be achieved in the last case. The electric field dependence of the polarization offset is predicted, together with the possibility of obtaining large nonconventional pyroelectric coefficients. The simulation developed in the case of a bilayer ferroelectric structure and using data from the lead-lanthanum-zirconate-titanate system describes well the observed features of the graded ferroelectric structures. (C) 2003 American Institute of Physics.

Composites containing 4-n-octyl-4'-cyanobiphenyl (8CB) either confined to nanopores of molecular sieves with very large pores or coating silica nanoparticles of aerosil type at high silica-to-8CB ratios are investigated by IR spectroscopy. Band shape analysis was performed in wavenumber regions in which the peaks due to CN stretching, CH stretching and CH out-of-plane vibrations appear. Some of molecules confined to molecular sieves show spectroscopic features characteristic to a bulk-like 8CB matter located in the centre of the pores or in the inter-grain space. Other features of the IR spectra are due to 8CB molecules located in the surface layer, mostly forming hydrogen bonds between their CN groups and surface OH groups. Another part of the 8CB molecules in the surface layer may also interact by pi electrons of the aromatic rings. Hydrogen bonding is less hindered for the molecules of the surface layers onto aerosil particles than inside pores of the molecular sieves. Comparison is also made with the case of composites based on molecular sieves with small pores. (C) 2003 Elsevier Science B.V. All rights reserved.

IR photoconductive properties of PbS films can be controlled using two different methods: (1) chemical, controlling the film morphology and, thus, the physical film properties by adding small amounts of impurities in the deposition bath, (2) physical, using field effect in complex PbS/SiO2/Si pseudo-MOS structures to control the photoconductivity of PbS film. (C) 2003 Elsevier Science B.V. All rights reserved.

An iterative method is presented for the characterization of lossy piezoelectric materials in the thickness resonant mode, which allows to calculate the frequencies at which the electrical impedance is to be measured. This new method considerably reduces the measurement time by separating the programs for data acquisition and for processing thus simplifying the measurement procedure. The accuracy of the method was tested for several piezoceramic materials, covering a wide range of values of the thickness coupling factor and mechanical quality factor. The new method proved to be as accurate as other iterative methods, but it is easier to apply. (C) 2002 Elsevier Science Ltd. All rights reserved.

We investigated the superconducting critical temperature, the intra- and intergranular critical current density, and the thermopower properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)(y) samples. All these properties have been compared with those of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)(y) specimens. It was found that the critical temperature determined from resistive and AC complex susceptibility measurements show a maximum and the transition width shows a minimum for the intermediate values of y. Powder X-ray diffraction studies and the AC complex susceptibility measurements reveal that in our samples the amount of Bi2Sr2Ca2Cu3O10+delta high-temperature superconducting phase is maximum for y approximate to 0: 02: The amount of LiCl in Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)(y) changes the superconducting properties of the grains as well as of the intergrain matrix. The splitting of the peak in the temperature dependence of the imaginary part of the complex susceptibility, corresponding to the dissipation inside the grains, was also observed.