Bala Maheswaran

Bulk Superconductivity at 122 K in T1(Ba,Ca)2Ca3Cu4O10.5+8 with Four Consecutive Copper Layers

The observed increase of superconducting transition temperature (Tc) with the number of copper oxide planes continues in the four-[CuO2]-2 layer (single TI layer) oxide superconductor, which has been prepared with > 80% purity and was magnetically aligned for crystallographic identification. A master scaling curve is proposed, which ties together the Tcs of virtually all known Bi and Tl oxide superconductors, and shows that the Tl(Bi) layers play an essential role in the superconductivity. publication 350 of the Barnett Institute.

Critical current enhancement in field‐oriented YBa2Cu3O7−δ

The crystalline anisotropy of YBa2 Cu3 O7−δ is found to have a significant effect in degrading critical current Jc . Pressed polycrystalline pellets of field‐oriented grains have significantly higher Jc values (>5 times larger) than unoriented samples of the same material. Effects of annealing and metal doping are also discussed.

Flux-lattice melting and flux creep in a high-Tc superconductor

The authors report measurements of the magnetisation-derived critical current jc in TlBa2Ca3Cu4Ox a 122 K superconductor. Analysis of the field H*c2, at which jc scales to zero, naturally falls into two regimes: a low-temperature regime, which can be described by flux-lattice melting, and a high-temperature regime dominated by flux creep. In the latter regime, the scaling of both H*c2 and the sample resistivity with temperature are in good agreement with a model proposed by Tinkham, when anisotropy effects are taken into account. From the former regime, they infer Hc2//(T=0) approximately=430 T.

Scaling of critical current density in field‐aligned grains of (Tl0.75Bi0.25)1.33Sr1.33Ca1.33Cu2 O6.67+δ, the ‘‘1212’’ phase

We have measured intragrain critical currents in field‐oriented samples of a rare‐earth‐free high‐Tc superconductor (Tl0.75Bi0.25)1.33Sr1.33Ca1.33Cu2 O6.67+δ, the ‘‘1212’’ phase. The critical current Jc is anisotropic. For fields oriented along the conducting planes, a simple scaling of Jc with field is found. The critical current scales to zero at a field HBc2 (T) which is in good agreement with the form predicted by the flux creep theory.

Biaxial alignment of high- T c superconductor polycrystals VIII: Phi-circle scan analysis of EuBa 2 Cu 3 O 7−δ (Eu-123)

To reduce grain boundary weak links due to large-angle grain orientation mismatch, bulk (thick film) EuBa 2 Cu 3 O 7−δ high- T c superconductor specimens were biaxially aligned by a mechanical force such that their [001] axes were perpendicular to the specimen surfaces and by a magnetic force so as to orient their [010] axes to lie parallel to the magnetic field direction in the specimen plane. We describe here the characterization of such EuBa 2 Cu 3 O 7−δ thick films by φ-circle XRD scans of (013) reflections, demonstrating a high degree (33%) of c -axis alignment with FWHM of a , b -axis alignment with a FWHM of 47°. The formation of “granular single crystals”, toward which this work is a further advance, can be seen as establishing a new paradigm in ceramics science.

Biaxial alignment of high-Tc superconductor polycrystals VII: Demonstration of biaxial alignment of EuBa2Cu3Ox (Eu-123) by pole figure measurement

Abstract Bulk EuBa 2 Cu 3 O x high- T c superconductor thick film specimens of 350 μ thickness were fabricated using a combination of magnetic field and mechanical force (acting normal to each other) to form a “granular single crystal” to reduce the grain boundary weak links due to large-angle grain orientation mismatch. The biaxially aligned (and hence, triaxially aligned) specimens exhibited textures in which the grains of the superconductor ceramics were aligned by a mechanical force such that their [001] axes were perpendicular to the surfaces of the thick film specimens, whereas their [010] axes were oriented by a magnetic force to lie parallel to the magnetic field direction in the specimen plane. The top and bottom surfaces of the double aligned thick films have been characterized by reflection XRD (2θ-scan and χ-circle scan) and XRD pole figure determination. We describe here the biaxial alignment process for EuBa 2 Cu 3 O c , as currently practiced by this group and report the first semiquantitative results of an X-ray pole figure analysis of material prepared by this method.

Effect of excess BaCuO2 on grain boundaries of melt-regrown EuBa2Cu3O7−δ (Eu-123)☆

Abstract BaCuO2 is found to segregate to the grain surface in Eu-123, where it melts at a temperature below the bulk melting point of Eu-123, coating the grain surfaces, enhancing grain growth, but reducing critical currents. This happens in stoichiometric Eu-123, but can be greatly enhanced by excess BaO. The variation of the ‘irreversibility line’ with BaO content is studied.

Shubnikov-de Haas effect in a field-induced two-dimensional hole-gas

Abstract In the susceptibility of stage-two bromine-graphite intercalation compounds, sharp negative spikes of 4πχ signal the existence of a gap between Landau levels above a critical magnetic field. We report measurements of both ac and dc resistivity of these materials in fields up to 15 Tesla, which are consistent with this picture of a field-induced two-dimensional hole gas.

Terrace growth in partially melt-regrown EuBa{sub 2}Cu{sub 3}O{sub 7-{sigma}} high Tc superconductors

An SEM and STM study was made of terrace formation in partial melt-regrown EuBa2Cu3O7−δ. Terraces form above 920°C, well below the bulk melting temperature (1075±10°C). The terrace formation is accompanied by significant grain growth, possibly triggered by a “surface-melting” transition. Melting occurs in a layer a few monolayers thick, and the highly mobile layers reconstruct into terraced form due to a faceting transition. The terrace growth can be reproduced by a theoretical faceting model. The STM also reveals a finer-scale sinusoidal modulation of the surface, of unknown origin.

YBa2Cu3O7−δ: Enhancing j c by Field Orientation

Transport properties of dense, pressed pellets of field-oriented grains of YBa2Cu3O7−δ have been studied in the superconducting phase (T=77K) as a function of current, annealing time, and magnetic field. These pellets have strikingly different annealing properties from similarly prepared but unoriented samples. The oriented samples show significant enhancements in density and degree of orientation and transport j c with length of annealing time. The best samples have less than 2% of the x-ray intensity corresponding to misoriented grains, and a density so high that excess low-T oxygen annealing is required for the best j c (currently, j c (77K) > 1500A/cm2). These values are limited by the magnetic field generated by the current itself.