P. Paturi

Preparation of superconducting YBa2Cu3O7-δ nanopowder by deoxydation in Ar before final oxygenation

Abstract Superconducting YBa2Cu3O7−δ nanopowder is prepared by the citrate-gel modification of the sol–gel method. Most important advantages of this method are the homogeneity at atomic level and possibility to reduce the calcination temperature to limit thermal agglomeration of the particles. A preparation temperature ⩽800 °C is used with a deoxydation step in Ar before the final oxidation. The X-ray diffraction data confirm the orthorhombic structure of the nanopowder. Superconducting properties are investigated with a SQUID magnetometer showing that the global Tc=92 K. Single grains of the powder have average diameter of 60–80 nm, depending on the calcination process used. The grains are weakly connected stacks of flat particles having thickness of a few nanometer.

Irreversible metamagnetic transition and magnetic memory in small-bandwidth manganite Pr1−xCaxMnO3 (x = 0.0–0.5)

The present paper reports detailed structural and magnetic characterization of the low-bandwidth manganite Pr(1-x)Ca(x)MnO(3) (with x = 0.0-0.5) (PCMO) polycrystalline samples. With increasing Ca content, reduction of the unit cell volume and improvement in perovskite structure symmetry was observed at room temperature. Magnetic characterization shows the signature of coexisting AFM-FM ordering and spin-glass phase at the low doping range (x = 0.0-0.2) while increased hole doping (x = 0.3-0.5) leads to charge ordering, training effect and an irreversible metamagnetic phenomenon. The large irreversible metamagnetism in the CO phase of PCMO and the corresponding spin memory effect is a direct consequence of hysteretic first-order phase transition arising from the weakening of the CO state under the external magnetic field and trapping of the spins due to a strong pinning potential in the material.

Magnetic field protects plants against high light by slowing down production of singlet oxygen

Recombination of the primary radical pair of photosystem II (PSII) of photosynthesis may produce the triplet state of the primary donor of PSII. Triplet formation is potentially harmful because chlorophyll triplets can react with molecular oxygen to produce the reactive singlet oxygen (¹O₂). The yield of ¹O₂ is expected to be directly proportional to the triplet yield and the triplet yield of charge recombination can be lowered with a magnetic field of 100-300 mT. In this study, we illuminated intact pumpkin leaves with strong light in the presence and absence of a magnetic field and found that the magnetic field protects against photoinhibition of PSII. The result suggests that radical pair recombination is responsible for significant part of ¹O₂ production in the chloroplast. The magnetic field effect vanished if leaves were illuminated in the presence of lincomycin, an inhibitor of chloroplast protein synthesis, or if isolated thylakoid membranes were exposed to light. These data, in turn, indicate that ¹O₂ produced by the recombination of the primary charge pair is not directly involved in photoinactivation of PSII but instead damages PSII by inhibiting the repair of photoinhibited PSII. We also found that an Arabidopsis thaliana mutant lacking α-tocopherol, a scavenger of ¹O₂, is more sensitive to photoinhibition than the wild-type in the absence but not in the presence of lincomycin, confirming that the target of ¹O₂ is the repair mechanism.

Structural Properties of YBCO Thin Films Deposited From Different Kinds of Targets

The accommodation field of YBa2Cu3O6+infin (YBCO) a thin films depends on the microstructure of the films and specifically the amount of pinning sites, which can be controlled by the type of the target used in the laser deposition. Typical values for the accommodation field at 5 K are 40-100 mT, 160 mT and 500 mT for films deposited from a micrograined, nanograined and optimally BaZrO3 (BZO) doped targets, respectively. We present structural data on YBCO thin films prepared by laser deposition from a nanograined target, a micrograined target and nanograined targets with added BZO. The characterizations were done with XRD and TEM and it was found that the main difference in the undoped films (nano and micro) was the stiffer lattice of the nanofilms. The BZO inclusions were found to induce a large amount of dislocations to the YBCO lattice surrounding the BZO. This can explain the increase observed in the accommodation field.

Magnetic relaxation and flux pinning in YBCO films prepared by PLD from a nanocrystalline target

The magnetic flux pinning properties of YBCO films prepared by pulsed laser deposition from nano- (n) and microcrystalline (μ) targets are studied and compared by magnetic relaxation and hysteresis measurements. The most striking difference between the films is the higher critical current density of the n-films (Jc(0 T,77 K) = 1 × 107 A cm−2) compared to the μ-films (Jc(0 T,77 K) = 3 × 106 A cm−2). The accommodation field B* that determines the end of the low field plateau of Jc is three times higher in the n-film than in the μ-film. In higher fields Jc(B) follows a power-law behaviour with exponents α = −0.6 in the intermediate field region and −2 in the high fields. The results are discussed on the basis of the recent strong pinning theories and it is concluded that the observed stronger pinning in the n-film is due to higher density of extended defects.

Laser deposition from a nanostructured YBaCuO target: Analysis of the plume and growth kinetics of particles on SrTiO3

The plume generated by a pulsed XeCl laser from a novel nanostructured YBaCuO target (n target) is investigated by methods of optical emission spectroscopy and atomic force microscopy. While the spectral positions of the emission lines are the same, stronger line intensities, pertinent to higher kinetic energy of the particles, are observed in the plumes generated from the n target than from a target having micron size grains (m target). The size of small clusters captured on Si plates assembled inside the plume grows in directions perpendicular to the axis of the plume. As shown by x-ray photoelectron spectroscopy investigations, in the particles deposited on a SrTiO3 substrate at Ts=700 °C in oxygen the correct 1-2-3 composition is achieved. The average ratio of the heights of the particles deposited from the n target and from the m target is hn/hm=0.6, both in the plume and on SrTiO3. This can explain the smoothness of YBaCuO films prepared by laser deposition from the n targets.

Laser deposition of thin superconducting films from a nanocrystalline target

A target prepared from nanopowder is used for pulsed laser deposition of thin films on substrates. Investigations by atomic force microscopy show that in this way it is possible to prepare smooth films which are free from droplets. In optimized growth conditions the critical current density in zero field at 77 K, superconducting transition temperature K and transition width of K are obtained.

Crystalline orientation and twin formation in YBCO thin films laser ablated from a nanocrystalline target

Two sets of superconducting thin films were deposited on SrTiO3 by laser ablation, the first using a nanocrystalline target (n-films) and the second using a commercial microcrystalline target (μ-films). The effect of film thickness on the structure of the films was investigated using detailed x-ray diffraction analysis. It was found that for both types of film growth is a three-phase process, where at the beginning some grains with their a-axis perpendicular to the substrate surface are formed. In the second phase a purely c-axis oriented film is formed and in the last phase some a-axis oriented grains are again observed. The last phase was observed only for the μ-films. Both types of film were found to have equal numbers of c-axis oriented grains oriented with their a-axis to the = 0° and to the 90° direction. It was also found that the twin structure observed in both types of film develops much faster in the n-films, which might be the reason for the observed high critical current densities in n-films.

Growth and c-axis flux pinning of nanostructured YBCO/BZO multilayers

A systematic investigation of superconducting multilayered films composed of alternating nanostructured YBa2Cu3O6+x (YBCO) and secondary phase BaZrO3 (BZO) layers is carried out. Detailed growth mechanism studies of these structures as well as superconducting properties in a wide temperature and magnetic field range are investigated by varying the thicknesses of layers grown by pulsed laser deposition. Structural analysis shows that multilayering changes clearly the growth mechanism of YBCO. Alternating thin BZO layers smoothen the film surface, create in-plane mosaic spread structure of the YBCO crystals, produce stress in the BZO interface region and moreover improve the out-of-plane alignment of YBCO. Magnetic measurements of superconducting properties demonstrate that the critical temperature decreases slightly when the thickness of the BZO layers increases relative to the YBCO layers. This indicates a growing strain effect at the layer interfaces. At the same time, critical current densities of multilayers in the whole used temperature and magnetic field range are increased to the same level as in conventionally BZO-doped YBCO films when very thin alternating YBCO/BZO layer structures are deposited. This thickness dependent result is explained by the dense flux pinning centre network in layered superconductors, giving the opportunity to increase the overall thickness of a film and further enhance the current-carrying capability.

Modeling flux pinning in thin undoped and BaZrO3-doped YBCO films

A simple model based on distributions of twin boundaries, dislocations, and BaZrO3 nanorods is presented to describe the Jc properties of undoped and BaZrO3 (BZO)-doped YBa2Cu3Ox thin films. The model accurately describes the shape of Jc(B,T) curves of the films, when the pinning site distributions are taken from distributions of twin spacings and BZO nanorods from transmission electron microscope images. Thus, assuming that the model can be used for prediction of the Jc properties, we conclude that for enhancement of undoped films more crystalline defects are needed and for doped films a dopant that would create slightly larger rods would be optimal.