National Institute Of Materials Physics - Romania

The influence of the direct pumping into the F-4(3/2) emitting level on the output characteristics of continuous-wave (CW) pumped, passively or actively (acoustooptic, AO) Q-switched Nd lasers is discussed. In case of passive Q-switching by Cr4+: YAG saturable absorber (SA) crystal, the change of pumping wavelength from 0.81 mu m into the highly-absorbing F-4(5/2) level to 0.88 mu m into the F-4(3/2) level of Nd does not modify the energy of the Q-switch pulse, but increases the pulse repetition rate and the laser average power for the same absorbed pump power. This is demonstrated with 0.81 and 0.88 gm CW laser diode-pumped Nd:YAG and Nd-vanadate lasers with average output power in the watt-level range at 1.06 mu m. The effect is explained by the control of passive Q-switching by the intracavity photon flux that is influenced by the pump wavelength and by the initial transmission of the SA crystal. On the other hand, it is discussed and experimentally proved that due to the possibility to control externally the frequency of switching, in case of the AO Q-switched Nd laser the change of the pump wavelength from 0.81 to 0.88 pm increases the pulse energy for a fixed frequency, leading to a corresponding increase of the average laser power. (C) 2009 Elsevier B.V. All rights reserved.

A new fluorine-free chemical solution deposition route has been developed for YBa2Cu3O7-x (YBCO) thin films. The precursor was formed by dissolving metal nitrates and citric acid in ethylene glycol. Polyethyleneimine was used to modify the solution. The preparation of YBCO thin films consisted of repeated dip coating, drying, heat-treating in air and annealing in flowing argon with a small addition of oxygen and with final treatment in pure oxygen. The films were characterized by XRD and SEM and show dominating and well-textured YBCO phase. The AC susceptibility and magnetization measurements reveal a critical temperature about 90 K and a high critical current density, comparable with that of pulse laser deposited films. The simplicity and high performance of the method may be attractive for large-scale superconducting applications.

We have investigated by DC magnetization measurements and frequency-dependent AC susceptibility the critical current density (J (c)), pinning force (F (p)) and pinning potential in thick (1.3-1.6 mu m) YBa2Cu3O7-delta (YBCO) films grown by Pulsed Laser Deposition on SrTiO3 substrates decorated with LaNiO3 nanodots deposited by a few (5-15) laser pulses, in comparison with those of a 1 mu m thick YBCO reference sample. Experiments show that the highest improvement of superconducting properties was achieved for films grown on substrates decorated with 10 laser pulses on the LaNiO3 target, which have, at 77.3 K, a J (c) of 40-125% higher than in pure YBCO in fields between 1 and 2 T, and F (p) increased by 40%. These results could be important for further improvement of current-carrying capability of coated conductors for in-field power applications.

We report measurements of critical current in YBa2Cu3Ox films deposited on SrTiO3 substrates decorated with silver and gold nanodots. An increase in critical current in these films, in comparison with the films deposited on non-decorated substrates, has been achieved. We argue that this increase comes from the c-axis correlated extended defects formed in the films and originated from the nanodots. Additionally to creating extended defects, the nanodots pin them and prevent their exit from the sample during the film growth, thus keeping a high density of defects and providing a lower rate of decrease of the critical current with the thickness of the films. The best pinning is achieved in the samples with silver nanodots by optimising their deposition temperature. The nanodots grown at a temperature of a few hundred degrees C have a small diameter of a few nanometres and a high surface density of 10(11)-10(12) particles/cm(2). We give evidence of c-axis correlated extended defects in YBa2Cu3Ox films by planar and cross-sectional atomic force microscopy, transmission electron microscopy and angle-dependent transport measurements of critical current. (C) 2009 Elsevier B.V. All rights reserved.

Samples of LaNi5-xFex were prepared by arc melting and subsequent melt spinning procedure. Structural, magnetic and Mossbauer investigations were performed for physical characterization while the hydrogen storage properties were carefully analyzed. Detailed discussions concerning the sample's quality, the site assignment, the effect of temperature and applied fields on hyperfine parameters and the related Debye temperatures were provided. (C) 2008 Elsevier B.V. All rights reserved.

Six tetranuclear complexes [Fe(III)(3)Ln(mu(3)-O)(2)(CCl3COO)(8)(H2O)(THF)(3)]center dot THF center dot C7H16 [Ln=Gd(III) (1), Tb(III) (2), Dy(III) (3), Ho(III) (4), Y(III) (5), and Lu(III) (6)] have been studied by magnetic susceptibility and Moumlssbauer spectroscopy. These isostructural molecules have a "butterfly" structure core consisting of two Fe(2)Ln(mu(3)-O) triangular "wings" which share a common Ln-Fe "body"; the dihedral angle between the wings is ca. 148 degrees. The coordination spheres of the iron ions are essentially distorted octahedral. The lanthanides are eight-coordinate with coordination polyhedra that may be described as distorted tetragonal bipyramids. Variable-temperature solid-state magnetic susceptibility in the temperature range 1.8-300 K and magnetization at 1.8 K for compounds 1-6 were measured. The spin state of Fe is S=5/2 in all cases. In compounds 5 and 6, where Ln(III) (Y and Lu, respectively) is diamagnetic, the three Fe atoms form an obtuse isosceles triangle with antiferromagnetic interactions J(Fe-Fe)=-50 K between the wing-tip Fe-w and body Fe-b atoms, and negligible interaction between the Fe-w's, resulting in a ground state of effective spin S=5/2 per cluster. In the complexes with paramagnetic lanthanide ions, the interaction between the Fe-3 triangle and the Ln(III) center is described by an effective exchange constant which is antiferromagnetic and 1 order of magnitude weaker. Besides, at 3 K incipient spin blocking, characteristic of single molecule magnets, was found to occur in the out-of-phase component of the ac susceptibility in {Fe3TbO2}, {Fe3DyO2}, and {Fe3HoO2}. The activation energy of a Debye process describing the magnetization reversal has been determined to be, E-a approximate to 8, 9, and 10 K for the Ln=Tb, Dy, and Ho, respectively, and the prefactor tau(0)approximate to 10(-7) s. The high spin states of the Fe(III) centers were confirmed by the Moumlssbauer spectra, in which two distinguishable Fe sites could be resolved above 80 K, corresponding to the Fe-w and Fe-b sites, respectively. Relatively larger values of the quadrupole splitting of the Moumlssbauer spectra were observed for the Fe-w pair as compared with that for the Fe-b, and both quadrupole splittings diminished with increasing temperature. At 3 K the Moumlssbauer spectra showed a state with blocked spins (sextets) for the {Fe3TbO2} and {Fe3DyO2} cases. From the E-a and tau(0), determined in the ac susceptibility, the relaxation time at 3 K is estimated as tau approximate to 10(-5)-10(-6) s much longer than the time window of Moumlssbauer spectroscopy and compatible with the single molecule magnet behavior. In the presence of a strong magnetic field the moments of the Ln(III) cation and the Fe-3 triangle are polarized. For some compounds at low temperature a magnetic pattern (sextet) for each of the three Fe sites appeared, and the antiferromagnetic coupling within the Fe-3 cluster was directly proved by the opposite trend of the field dependence of the two Fe-w sextets as compared with the Fe-b third one.

In order to study the mechanism of the small n-value for Bi2212, E - J characteristics were measured carefully for various kinds of Bi2212 tapes and round wires in high magnetic fields. Some of wires were heat-treated in a magnetic field of 5 T. We found that the n-values of all the samples increase by the in-field heat-treatment at 5 T. In addition, we analyzed the E - J properties by the combination of the percolation model based on the local J(c) distribution and the two directional J(c) distribution model. The improvement of the n-value by the in-field heat treatment can be understood by the change of the aspect-ratio of the grains. Therefore, the aspect-ratio of the grains plays an important role in the current transport influencing not only J(c) but also n-value.

We attempted to grow non-c axis heterostructures of substrate-IS type with S = BSCCO and I = (Sr, Ca)CuO(2). A careful selection of the films-substrate relationship is revealed as a key condition toward growth of layered heterostructures with different orientations. However this powerful principle, that is also a mandatory condition, is not sufficient in order to obtain high quality structures. The reasons are discussed looking for further solutions. Our comparative analysis is expanded to other heterostructures we reported in other works trying to reveal some of the more general problems of the non c-axis heterostructures growth.

For power applications of superconducting films, the critical current density (J(c)) and the thickness of the film (d) should be as high as possible. Since J(c) decreases with both thickness and magnetic field, artificial pinning centers in addition to natural ones are required to keep J(c) high. The earliest cost-effective method used for introducing artificial pinning centers was the so-called substrate decoration, i.e., growing nanoscale islands (nano-dots) of certain materials on the substrate prior to the deposition of the superconducting thin film. Later on other two approaches proved to be successful: building up a layered distribution of a second phase using a multilayer deposition (quasi-superlattices) and distributing a secondary phase in the film from a compositionally changed target. Several materials have been used for the creation of artificial pinning centers. Here we report on the artificial pinning centers induced in YBCO thick films by substrate decoration and quasi-superlattice approaches using nano-dots of Ag, Au, Pd or non-superconducting YBCO. The cross-sectional AFM images show evidence of c-axis correlated columnar defects. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films.

The laser pyrolysis became a useful tool, providing various ways, in production of nano materials. The iron Mossbauer spectroscopy is one very accurate method in evidencing the physical properties and related processes in the nano scale compounds. The effect of pressure, laser spot area and induced combustion, of gas mixture and laser power on the phase composition and inside particle distribution, grain size as well as the related phenomena were investigated by temperature dependent Mossbauer spectroscopy. A selection of most relevant properties is presented and discussed in details.