National Institute Of Materials Physics - Romania

The properties of the electron paramagnetic resonance spectra of Gd3+ ions in PbWO4 single crystals have been investigated in the X-band microwave frequency region, in the 1.5 to 290 K temperature range. The observed S-4 symmetry of the local crystal field at the Gd3+ impurity ions strongly suggests that the Gd3+ ions substitute for the Pb2+ lattice cations, with charge compensation at a distance. The spin Hamiltonian parameters are comparable with those of the Gd3+ ions in other isomorphous tungstates. The temperature variation of the fine structure parameters B-2(0) and B-4(0) points to an energy transfer between the impurity ions and the host lattice, which takes place mainly through a local vibrational mode of frequency omega = 3.1 x 1013 rad s(-1).

Detectors processed on epitaxial silicon are a promising solution for the extreme radiation levels in the innermost tracking layers at future particle physics experiments as in the upgraded (S-LHC). In order to systematically investigate their radiation tolerance.. sets of 25 and 50-mu m-thick diodes had been irradiated with 24GeV/c protons up to fluences Of Phi(eq) = 10(16) cm(-2). The full depletion voltage was measured during full isothermal annealing cycles at both 60 and 80 degrees C. A voltage of only 290 V was measured for the 50 mu m diodes after Phi(eq) = 1016cm(-2). In contrast to likewise thin float zone (FZ) diodes, the depletion voltage in epi-devices decreases with annealing during foreseen operational and storage periods. Low-temperature storage during beam-off periods as inevitable for FZ devices could be avoided. The reverse annealing components reveal an overall similar functional time dependence as observed in FZ diodes. However, the most important stable component is drastically different. In contrast to the well-known negative space charge build up observed in FZ diodes, the behavior in epi-diodes can only be explained by an additional formation of positive space charge. The immense improvement of the radiation tolerance of epi-detectors is thus attributed to a compensation effect between acceptor generation and donor creation unique for the epi-material. Charge collection efficiency drops only to about 80% after Phi(eq) = 6 x 10(15)cm(-2) proving that epi-detectors are indeed adequate candidates for the tracking area at extremely large radiation levels. (c) 2005 Elsevier B.V. All rights reserved.

Four different kinds of silicon materials (standard float zone-FZ, oxygen enriched FZ-DOFZ, Czoehralski-Cz and thin epitaxial layers grown on Cz substrates-Epi/Cz) have been investigated in this work. They have been irradiated with (CO)-C-60-gamma- radiation and 24 GeV/c protons. The differences in the changes observed in the effective doping concentration (N-eff) after proton irradiation of Cz and Epi silicon can be explained by the balance between the formation of two types of defects-a deep acceptor (the I center, referred to as It, in this paper in order to avoid any confusion with interstitials) and a shallow donor (the bistable donor (BD) complex). While the formation of the I-P center depends on the concentration of interstitial oxygen, the BD is generated in materials with high concentration of oxygen dimers. Following the generation of the IO2i defect-detected after low irradiation fluences only in Epi and Cz material-the average dimer concentration in 50 put Epi/Cz was estimated to be of only 2.7 times lower than in Cz diodes. A value of similar to 2.7 x 10(-12) cm(2) for the electron capture cross-section of the Thermal Double Donors (TDDs), present in the Cz material, was also measured for the first time. The positive space charge introduced by ionization of the BD centers in Epi diodes was directly determined from TSC experiments for two irradiation fluences. The determined values show a linear fluence dependence for the formation of BD centers. By taking into account the BD and the I-p center generation as well as the donor removal, the change of the effective doping concentration N-eff at 20 degrees C in Epi/Cz and Cz diodes after an annealing time of 120 min at 60 degrees C can be explained up to a negative space charge introduction rate of similar to 2.2 x 10(-2) cm(-1) thought to be determined by negatively charged clusters. Long-time annealing experiments at 80 degrees C have shown that the generation of BDs represents a stable damage. (c) 2005 Elsevier B.V. All rights reserved.

We have studied a portion of H-T phase diagram of the anisotropic Bi2223 bulk system using multi-harmonic ac susceptibility measurements (chi(n)). Based on the behavior of the third harmonic modulus,vertical bar chi(3)vertical bar, the irreversibility line (IL) was obtained from the onset of vertical bar chi(3)vertical bar, and an anomalous peak effect (PE) was observed. It has been shown that the anomalous peak is due to the crossover between a three-dimensional (3D) and a quasi-two dimensional (quasi-2D) peculiarity of vortex dynamics, the crossover magnetic field being H-CR approximate to 0.1T. The obtained portion of B-irr(T) is well described by the melting line of the flux lattice. The 3D flux fluctuation part (low field and high temperature) is described by the nearly parabolic temperature dependence B=B-o(Tc/T - 1)" with n = 1.58 (sample 1) and n = 1.48 (sample 2). The other region (high field and low temperature) is well described by the temperature dependence of the quasi-2D flux fluctuations, T-m(H) = T-m(2D)[1 + b/(In B/B-cr )(1/v)] corresponding to the weak interaction between pancake vortices from adjacent planes. The 2D limit temperature was determined as T-m(2D) = 36.7K (sample 1) and T-m(2D) = 27.3K (sample 2). (c) 2005 Elsevier B.V. All rights reserved.

A microscopic theory for the interaction of carriers with LO phonons is used to study the ultrafast carrier dynamics in nitride-based semiconductor quantum dots. It is shown that the efficiency of scattering processes is directly linked to quasi-particle renormalizations. The electronic states of the interacting system are strongly modified by the combined influence of quantum confinement and polar coupling. Inherent electrostaticfields, typical for InGaN/GaN quantum dots, do not limit the fast scattering channels.

We show the role of the Fano interference on the magnetoresistance properties of a T-shaped double-dot placed between ferromagnetic electrodes. In the linear voltage regime, the magnetoresistance is a non-monotonic function of the gate voltage. We find high values of the magnetoresistance at the Fano dip and far from it. For resonant transmission, the magnetoresistance is lower and it can be negative for asymmetric couplings. Particularities of the system in nonequilibrium transport are also analysed. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

An unconventional synthesis method, implying ultrasonic field, was tested in order to improve the properties of CaCo3 precipitated via liquid-liquid route. Working parameters for calcium carbonate precipitation in the absence of ultrasonic field so that the solid material displays aragonite morphology were established. The evolution of the precipitation reaction in ultrasonic field provided a solid phase with aragonite morphology slightly, impurified with vaterite. Grain size distribution was signficantly improved. The solid phase presents an important fraction of nanometer size particles.

A combined sintering-sublimation method has been used to grow CdS or CdSeS photo-conducting polycrystalline thin films. By a fast firing technique, the films are grown on Cr2O3-x ceramic pellet sintered substrate. The film growing and the fast firing sintering processes take place simultaneously. The sintering-sublimation technique has been designed to increase the reliability of photoconducting devices, mainly by decreasing the donors' concentration which determines a higher sensitivity. This process becomes more effective when pellets containing raw CdS, respectively CdSeS, and a less reactive component - such as nonstoichiometric chromium oxide, are used. This oxide provides surfaces where thin CdS or CdSeS layers are crystallizing. At the same time, the nonstoichiometric Cr2O3-x can react with the oxygen allowing a more efficient control of the properties of CdS and CdSeS thin layer which is generated as a result of the above-mentioned sublimation-condensation process.

EPR and TEM investigations on sublimated solid Alq(3), [tris(8-hydroxyquinoline)aluminum(Ill)] are reported. Different paramagnetic centers in microcrystalline Alq3 fractions as synthesized and annealed are observed. Their nature, thermal treatment dependence, as well as correlation with the crystal structure are analyzed. The existence of randomly oriented triplets and quartets in annealed Alq3 fractions is evidenced from the temperature dependence of X- and Q-band EPR spectra. The spin Hamiltonian parameters are given and proposals for the model structures are made.

High quality superconducting thin films of HTS have been grown by MOCVD on substrates with artificial steps of predefined height and width. The surface of the films grown on the steps having width equal to the 'double of the migration length' of the atomic species depositing on the substrate is totally free of precipitates: precipitates are gathered at the step edges where the free energy is lowest. The method has several advantages: it is simple, universal (it is independent of the materials, substrates, deposition technique or application) and allows control of precipitates segregates so that the quality and growth conditions of the films are the same as for the films grown on conventional substrates. The method is expected to result in new opportunities for the device fabrication, design and performance.