Prasenjit Guptasarma

Synthesis of polycrystalline YBa2Cu4O8 without the need of high oxygen pressure

Abstract Conventional methods of synthesis of the oxide ceramic superconductor YBa2Cu4O8 (1–2–4) require high oxygen pressure during synthesis and long heating schedules, making the process tedious and limited to a few groups. We report here the possibility of synthesis of 1–2–4 with a single firing in air. The technique involves formation, in solution, of a fine-particle-complex based precursor. The 1–2–4 phase then forms in air with a single firing of 30 minutes at 900°C and a Tc of 79.6K. The presence of 1–2–4 is confirmed by structural studies and then by thermogravimetric analysis.

Size effects in microparticle high temperature superconducting ceramics YBa2Cu3O7-δ and Bi2CaSr2Cu2O8+y

Abstract Single phase, microcrystalline YBa 2 Cu 3 O 7-δ and Bi 2 CaSr 2 Cu 2 O 8+ y were synthesized and carefully characterised in the lowest size range achieved so far, 75–300 nm. Size reduction effects are investigated and discussed in relation to phonon-mediated BCS coupling, magnetic field penetration, oxygen doping, structural changes and T c . We observe a marked deviation from empirical laws in the temperature dependence of the London penetration depth and this deviation is found to increase with decreasing particle size. This could arise as an effect of limiting long range order in these systems and suggests that the critical exponents in this regime need further investigation.

Thin and ultra-thin epitaxial films of YBa2Cu3O7−δ deposited on LiNbO3 substrates by pulsed excimer laser ablation

Abstract We present here detailed transport, magnetic, structural and morphological studies on thin and ultra-thin films of superconducting YBa 2 Cu 3 O 7−δ (1-2-3) deposited on single crystalline substrates of LiNbO 3 . The films were deposited using the technique of excimer laser ablation and have been found to have their c -axis oriented perpendicular to the plane of the film. We report results for films of thickness 500 A, 1000 A, 1600 A and 6000 A. Superconducting transition temperatures ( T c ) have been studied by DC-resistivity, AC- and DC-magnetization. Critical current densities ( J c ) from transport measurements are found to be in excess of 10 5 A/cm 2 at 77 K. The temperature dependence of J c is also examined and the flux pinning energy is estimated within the flux creep picture. High field hysteresis loop studies are in agreement with the above results and yield meaningful indications of critical fields H c and irreversibility fields H r . A study of the evolution of film morphology with increase of film thickness is also reported and it reflects on the growth mechanism of 1-2-3 on LiNbO 3 .

Influence of magnetic impurities on current transport in epitaxial thin films of Y1Ba2Cu3O7−δ

Abstract The critical current density (J c ) and the nature of its temperature dependence is examined for the first time for the case of Fe doped epitaxial thin films of Y 1 Ba 2 Cu 3 O 7− δ . These J c systematics are analysed in terms of flux pinning at Twin boundary (TB) planes, tunneling across such planes and influence of magnetic impurities on superconducting gap parameter via Cooper pair breaking.

K4C60 is a zero-gap semiconductor - a resonant Raman study

Abstract We study electronic Raman scattering of K 4 C 60 exciting at 1.16 eV, which is resonant with the t 1u −t 1g transition and permits us to observe interband scattering between the non-degenerate t 1u and t 1u ∗ states. We clarify that the band structure of K 4 C 60 is characterised by small a gap of 50 meV between t 1u and t 1u ∗ bands.

Synthesis of YBa2Cu4O8 in air through complex precursor formation and fine-particle techniques

Abstract We report the successful synthesis in air of the superconductor YBa2Cu4O8 (1-2-4) without the need of high oxygen pressure or an alkali-carbonate based catalyst. The technique involves formation, in solution, of a fine-particle-complex based precursor. The 1-2-4 phase then forms in air with a single firing of 30 min at 900°C and a Tcnear 80 K. We also report some results of attempts to prepare 1-2-4 using precursors from sol-gel and spray drying.

Epitaxial thin films of YBa2Cu4O8 superconductor deposited by laser ablation of solution derived complex targets

Abstract We report here the successful fabrication of single phase epitaxial thin films of the superconductor YBa2Cu4O8 (1-2-4) deposited on monocrystalline (100) substrates of yttriastabilized ZrO2 using the technique of pulsed-laser ablation. The targets of 1-2-4 were fabricated through a novel low temperature synthesis technique whereby it was possible to synthesize reacted 1-2-4 pellets without the application of high oxygen pressure. In this report, the target material was composed of an unreacted solution derived Y-Ba-Cu (1-2-4)-complex. The films have superconducting transition temperatures varying from 70–80K and critical current densities higher than 5 x 10 6 A cm 2 below 60K. X-ray diffractograms indicate that the films are oriented with their c-axis perpendicular to the surface of the substrate.

Limiting long-range-ordered solids to finite sizes in condensed-matter physics

Abstract We have examined some of the consequences of finite size effects in solids which exhibit cooperative behaviour. These are materials with long-range interactions based on ferromagnetism, ferroelectricity, and superconductivity. Limiting the range of interaction is achieved by creating nanocrystals, nanodomains, disorder, high-concentration solid solutions and submicron microstructure in sintered compacts. The attempt is to elucidate underlying principles essential at a microscopic level for each of the three phenomena. Not much work has been reported in this area earlier and most of the results are from our own studies. Some of the work is reported for the first time. The experimental techniques used are Mossbauer, Raman and infrared spectroscopies, high-temperature x-ray diffraction, electron energy loss spectroscopy, dielectric and piezoelectric response, ferromagnetic resonance and spinwave dispersion, 4-probe resistivity, DC & AC magnetic susceptibilities and differential scanning calorimetry.

Non-isovalent substitution of Ba2+ in superconducting YBa2Cu3O7-y

We report the structural, magnetic and superconducting properties of YBa 2−x M x Cu 3 O 7 (M=La 3+ or K + ). Single-phase compounds form over a wide range of x in YBa 2−x M x Cu 3 O 7 and up to x = 0.2 in YBa 2−x M x Cu 3 O 7 . A substituent concentration of 15% triggers an orthorhombic to tetragonal phase15 transition in both systems. In spite of the associated disorder in the Cu(1)-0(1) chains, such a transition does not affect T c drastically. T c gets systematically depressed with increasing x. A temperature-dependent paramagnetic response is observed in YBa 2−x M x Cu 3 O 7 between 90 and 300K.

High temperature superconductivity in the Bi-Ca-Sr-Cu-O system with several variations in stoichiometry

The authors have made four-probe conductivity, AC susceptibility and X-ray diffraction measurements on the following compounds: nine compositions of BiCa1-xSrxCu2Oy with 0.1<or=x<or=1, four samples of BiCaSrCu2O2 with variations in the ambient and in heat treatment, and the samples with the starting compositions BiCa1.25Sr0.75Cu2Op, Bi2CaSrCu2Oq and Bi2Ca2.25Sr0.75Cu2Or. Many of these Bi-containing compounds exhibit superconductivity around 80 K. Sharp drops in resistivity near 120 K have also been observed in a few cases. The dependence of Tc on starting stoichiometry, synthesis conditions and structural parameters are discussed.