Transport properties of YBa 2 Cu 3 O 7, PrBa 2 Cu 3 O 7-superlattices

The understanding of the coupling between superconducting YBa2Cu3O7 (YBCO) layers decoupled by non superconducting PrBa2Cu3O7 (PBCO) layers in c-axis oriented superlattices was the aim of this thesis. For this purpose two conceptually different kind of transport experiments have been performed. rnrnIn the first type of transport experiments the current is flowing parallel to the layers. Here the coupling is probed indirectly using magnetic vortex lines, which are penetrating the superlattice. Movement of the vortex segments in neighbouring YBCO layers is more or less coherent depending on the thickness of both the superconducting and non superconducting layers. This in-plane transport was measured either by sending an external current through bridges patterned in the superlattice or by an induced internal current. rnThe vortex-creep activation energy U was determined by analysis of the in-plane resistive transition in an external magnetic field B oriented along the c-axis. The activation energies for two series of superlattices were investigated. In one series the thickness of the YBCO layers was constant (nY=4 unit cells) and the number of the PBCO unit cells was varied, while in the other the number of PBCO layers was constant (nP=4) and nY varied. The correlation length of the vortex system was determined to be 80 nm along the c-axis direction. It was found that even a single PBCO unit cell in a superlattice effectively cuts the flux lines into shorter weakly coupled segments, and the coupling of the vortex systems in neighbouring layers is negligible already for a thickness of four unit cells of the PBCO layers. A characteristic variation of the activation energy for the two series of superlattices was found, where U0 is proportional to the YBCO thickness. A change in the variation of U0 with the current I in the specimen was observed, which can be explained in terms of a crossover in the vortex creep process, generated by the transport current. At low I values the dislocations mediated (plastic) vortex creep leads to thermally assisted flux-flow behaviour, whereas at high current the dc transport measurements are dominated by elastic (collective) creep.rnThe analysis of standard dc magnetization relaxation data obtained for a series superlattices revealed the occurrence of a crossover from elastic (collective) vortex creep at low temperature to plastic vortex creep at high T. The crossover is generated by the T dependent macroscopic currents induced in the sample. The existence of this creep crossover suggests that, compared with the well known Maley technique, the use of the normalized vortex creep activation energy is a better solution for the determination of vortex creep parameters.rnrnThe second type of transport experiments was to measure directly a possible Josephson coupling between superconducting CuO2 double planes in the superlattices by investigation of the transport properties perpendicular to the superconducting planes. Here three different experiments have been performed. The first one was to pattern mesa structures photolithographically as in previous works. The second used three-dimensional nanostructures cut by a focused ion beam. For the these two experiments insufficient patterning capabilities prevented an observation of the Josephson effect in the current voltage curves. rnA third experiment used a-axis and (110) oriented YBCO films, where in-plane patterning can in principle be sufficient to measure transport perpendicular to the superconducting planes. Therefore the deposition of films with this unusual growth orientation was optimized and investigated. The structural and microstructural evolution of c-axis to a-axis orientation was monitored using x-ray diffraction, scanning electron microscopy and magnetization measurements. Films with full a-axis alignment parallel to the substrate normal could be achieved on (100)SrTiO3. Due to the symmetry of the substrate the c-axis direction in-plane is twofold. Transferring the deposition conditions to films grown on (110)SrTiO3 allowed the growth of (110) oriented YBCO films with a unique in-plane c-axis orientation. While these films were of high quality by crystallographic and macroscopic visual inspection, electron microscopy revealed a coherent crack pattern on a nanoscale. Therefore the actual current path in the sample was not determined by the macroscopic patterning which prohibited investigations of the in-plane anisotropy in this case.rn

Pulsed laser deposition of YBa2Cu3O7-[delta]/PrBa2Cu3O7-[delta] /

The process of depositing thin films by the use of pulsed laser deposition (PLD) has become a more widely used technique for the growth of substances in a thin film form. Pulsed laser deposition allows for the stoichiometric film growth of the target which is of great significance in the deposition of High Temperature Superconducting materials. We will describe a system designed using an excimer laser and vaccum chamber in which thin films and superlattices of YBa2Cuj07_i, PrBa2Cu307_i, and YBajCujOr-j/ PrBajCusOr-^ were deposited on SrTiOs. Results of resistivity measurements using the four probe technique will be shown.

Deposició de capes superconductores de YBa2Cu3O7 amb textura bidimensional a partir de solucions metal·lorgàniques

L’objectiu d’aquest projecte és l'obtenció de capes primes superconductores de YBa2Cu3O7 mitjançant el procés de descomposició metal·lorgànica, a partir de la combinació de sals precursores amb un contingut en fluor inferior a l’estàndard utilitzat pel grup de superconductivitat de l’ICMAB, així com l’estudi i caracterització d’aquestes capes mitjançant les tècniques utilitzades pel mateix grup. Per tal de poder analitzar els resultats, es realitzarà una comparació entre aquesta capa obtinguda a partir de la nova solució i una altra desenvolupada pel mètode estàndard. Sobre aquestes dues mostres es realitzaran tots els assaigs necessaris per tal d’obtenir les seves propietats físiques i químiques per tal d’analitzar la qualitat de la capa i avaluar la viabilitat de la nova solució.

Graphoepitaxy of CeO2 on MgO and its application to the fabrication of 45° grain boundary Josephson junctions of YBa2Cu3O7-x

We communicate a detailed study of the epitaxial growth of CeO2 on MgO. The key feature of the growth is the dependence of the in¿plane orientation of the CeO2 epitaxial layer on the MgO surface morphology. Atomic force microscopic (AFM) measurements, x¿ray analyses, as well as high¿resolution transmission electron microscopy (HRTEM) investigations reveal that on rough substrates a cube¿on¿cube growth of CeO2 on MgO occurs while on smooth substrates the CeO2 unit cell is rotated around the surface normal by 45° with respect to the MgO unit cell when the deposition rate is low (~0.3 Å/s) during the first stages of growth. This growth mechanism can be used for a defined fabrication of 45° grain boundaries in the CeO2 layer by controlling the surface roughness of the MgO substrate. This report demonstrates that these 45° grain boundaries may be used to fabricate YBa2Cu3O7¿x Josephson junctions.

Síntesi de pólvores de superconductors d’alta temperatura : YBa2Cu3O7 i GdBa2Cu3O7 per via de dissolució-polimerització

L’objectiu del projecte és obtenir pólvores dels superconductors d’alta temperatura YBa2Cu3O7 i GdBa2Cu3O7 per la via de dissolució-polimerització. Aquestes pólvores seran obtingudes a partir d’òxids (Itri o Gadolini i Coure) i carbonat de bari amb àcid nítric en una dissolució aquosa de Polietinenglicol (PEG), per tal de convertir aquests òxids en nitrats coordinats amb les cadenes de PEG. Aquest producte intermedi es sotmetrà a una piròlisis per tal d’obtenir òxids i carbonats, però amb una mida més reduïda respecte als productes inicials i un grau de barreja més íntim, permetent que reaccionin entre ells amb més facilitat. Finalment, per tal d’obtenir els superconductors YBa2Cu3O7 i GdBa2Cu3O7 es farà un últim tractament tèrmic a alta temperatura en el qual es produirà la reacció desitjada. Durant tot el procés es duran a terme anàlisis d’aquest per mitjà de tècniques de Termogravimetria (TGA), d’Anàlisi Tèrmica Diferencial (DTA), espectroscòpia Infraroja (IR) i la difracció de raig X (DRX) . També s’analitzaran els diferents productes que es van obtenint (intermedis i finals) amb l’ajuda de l’espectrometria IR i la difracció de pólvores. En base a aquests resultats, es variaran els paràmetres de la reacció (concentracions, temperatures, etc.) i dels tractaments tèrmics (atmosfera, temperatura, temps, etc.) per tal d’optimitzar el producte

Optical Emission Diagnostics Of Laser Produced Plasma From Graphite And Yba2cu3o7

This thesis is entitled “OPTICAL EMISSION DIAGNOSTICS OF LASER PRODUCED PLASMA FROM GRAPHITE AND YBa2Cu3O7. The work presented in this thesis covers the experimental results on the plasma produced with moderately high power laser with irradiance range in between 10 GW cm 2 to 100 GW cm -2. The characterization of laser produced plasma from solid targets viz. graphite and high temperature superconducting material like YBa2Cu3O7 have been carried out. The fundamental frequency from a Q - switched Nd: YAG laser with 9 ns pulse duration is used for the present studies. Various optical emission emission diagnostic techniques were employed for the the characterization of the LPP which include emission spectroscopy, time resolved studies, line broadening method etc. In order to understand the physical nature of the LPP like recombination, collisional excitation and the laser interaction with plasma, the time resolved studies offer the most logical approach

Temporal and Spatial Behavior of Electron Density and Temperature in a Laser-Produced Plasma from YBa2Cu3O7

Spectroscopic studies of laser -induced plasma from a high-temperature superconducting material, viz., YBa2Cu3O7 (YBCO), have been carried out. Electron temperature and electron density measurements were made from spectral data. The Stark broad ening of emission lines was used to determine the electron density, and the ratio of line in tensities was exploited for the determination of electron temperature. An initial electron temperature of 2.35 eV and electron density of 2.5 3 1017 cm2 3 were observed. The dependence on electron temperature and density on different experimental parameters such as distance from the target, delay time after the in itiation of the plasm a, and laser irradiance is also discussed in detail. Index Headings: Laser -plasma spectroscopy; Plasma diagnostics; Emission spectroscop y; YBa2Cu3O7.

Collective behavior of laser-produced plasma from a multicomponent YBa2Cu3O7 target in air

The dynamics of diffusion of electrons and ions from the laser-produced plasma from a multielement superconducting material, namely YBa2Cu3O7, using a Q-switched Nd:YAG laser is investigated by time-resolved emission-spectroscopic techniques at various laser irradiances. It is observed that beyond a laser irradiance of 2.6 \xC3\x97 1011 W cm-2, the ejected plume collectively drifts away from the target with a sharp increase in velocity to 1.25 \xC3\x97 106 cm s-1, which is twice its velocity observed at lower laser irradiances. This sudden drift apparently occurs as a result of the formation of a charged double layer at the external plume boundary. This diffusion is collective, that is, the electrons and ions inside the plume diffuse together simultaneously and hence it is similar to the ambipolar diffusion of charged particles in a discharge plasma

Time evolution of the electron density and temperature in laser-produced plasmas from YBa2Cu3O7

Laser radiation at 1.06 µm from a pulsed Nd:YAG laser was focused onto a multielement YBa2Cu3O7 target in vacuum and the plasma thus generated was studied using time-resolved spectroscopic techniques. Line broadening of the Ba I emission line at 553.5 nm was monitored as a function of time elapsed after the incidence of a laser pulse on the target. Measured line profiles of barium species were used to infer the electron density and temperature, and the time evolution of these important plasma parameters has been worked out.

Temporal and spatial evolution of laser ablated plasma from YBa2Cu3O7

YBa2Cu307 target was laser ablated, and the time-of-flight (TOF) distributions of Y, Y+., and YO in the resultant plasma were investigated as functions of distance from the target and laser energy density using emission spectroscopy. Up to a short distance from the target (-1.5 cm), TOF distributions show twin peaks for Y and YO, while only single-peak distribution is observed for Y+. At greater distances (>1.5 cm) all of them exhibit single-peak distribution. The twin peaks are assigned to species corresponding to those generated directly/m the vicinity of target surface and to those generated from collisional/recombination process.

Investigations on nanosecond laser produced plasma in air from the multi-component material YBa2Cu3O7

The laser produced plasma from the multi-component target YBa2CU3O7 was analyzed using Michelson interferometry and time resolved emission spectroscopy. The interaction of 10 ns pulses of 1.06 mum radiation from a Q-switched Nd:YAG laser at laser power densities ranging from 0.55 GW cm-2 to 1.5 GW cm-2 has been studied. Time resolved spectral measurements of the plasma evolution show distinct features at different points in its temporal history. For a time duration of less than 55 ns after the laser pulse (for a typical laser power density of 0.8 GW cm-2, the emission spectrum is dominated by black-body radiation. During cooling after 55 ns the spectral emission consists mainly of neutral and ionic species. Line averaged electron densities were deduced from interferometric line intensity measurements at various laser power densities. Plasma electron densities are of the order of 1017 cm-3 and the plasma temperature at the core region is about 1 eV. The measurement of plasma emission line intensities of various ions inside the plasma gave evidence of multiphoton ionization of the elements constituting the target at low laser power densities. At higher laser power densities the ionization mechanism is collision dominated. For elements such as nitrogen present outside the target, ionization is due to collisions only.

Spectral features of laser induced plasma from YBa2Cu3O7 and GdBa2Cu3O7 highTc superconductors

Laser induced plasma emission spectra from highT c superconducting samples of YBa2Cu3O7 and GdBa2Cu3O7 obtained with 1.06µm radiation from a Q switched Nd:YAG laser beam has been analysed. The results clearly show the presence of diatomic oxides in addition to ionized species of the constituent metals in the plasma thus produced.

Superconducting vortex fluctuations in YBa2Cu3O7-δ

Measurements of magnetization in YBa2Cu3O7-δ single crystals were performed for applied fields H parallel and perpendicular to the ab planes. The data show a temperature T = T* at which the magnetization M(T*) is independent of the applied field. This result is interpreted as due to vortex fluctuations of an anisotropic 3-D superconductor.

Caracterização de filmes finos de YBa2Cu3O7-x e La0.7Ca0.3MnO3 produzidos via síntese química

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)