B.L. Ramakrishna

Magnetic properties of semiconducting Y2BaCuO5: DC susceptibility and electron paramagnetic resonance study

Abstract The semiconducting, green Y2BaCuO5 compound, which is structurally related to the 90K superconductor YBa2Cu3O7−δ, was found to behave like a S= 1 2 Heisenberg quadratic-layer antiferromagnet with an exchange constant of 2J=33.3 ± 0.5 K. There is a peak in the susceptibility at 30 ± 0.5 K, above which it exhibits Curie-Weiss behavior with the Weiss constant estimated at +55.4 K. The local symmetry around the copper (II) ions was found to be rhombic. The orbital reduction factor for Cu2+ in this compound was estimated to be k=0.52, indicating appreciable covalency in the Cu-O bonds.

Magnetic properties of Y2Cu2O5

Abstract DC magnetic susceptibility of Y2Cu2O5 measured between 2 K S = 1 2 Heisenberg chain with ferromagnetic intrachain exchange constant J F k B = 36 ± 4 K . The chains are weakly coupled antiferromagnetically with an interchain exchange constant of 2z AF J AF k B = -6.5 ± 1 K . The magnetization behavior (M vs H) exhibits a change in slope at about 17 k Gauss. Its electron paramagnetic resonance signal consisted of a single broad line at both X- and Q-bands with gav = 2.108. The linewidth, ΔHpp, at X-band was 640 Gauss and temperature independent.

Microstructure and superconductivity in a new superconductor series: (Tl,Pb)1Sr2Can−1CunO2n+3−δ

The synthesis procedure and superconducting properties of the first two members of the new series of superconductors of a general formula: (Tl, Pb)1 Sr2 Can−1 Cun O2n+3−δ , are reported. The detailed atomic level microstructure of the compounds have been determined and computer simulated with structural models. Tc increases with n, the number of Cu‐O2 layers per unit cell, as in the single Tl‐layer Tl‐Ba‐Ca‐Cu‐O compounds, but the new materials have higher Tc ’s for the same value of n.

EPR in YBa2Cu3O7−δ and YBa2Cu3O6+x

Abstract Electron paramagnetic resonance (EPR) measurements on polycrystalline YBa 2 Cu 3 O 7−δ prepared via the citrates and the usual ceramic route of firing the oxides and BaCO 3 are reported. The single phase samples of orthorthombic YBa 2 Bu 3 O 6.9 and tetragonal YBa 2 Cu 3 O 6 obtained from a citrate precursor do not exhibit an EPR signal. However, a sample of the tetragonal and non-superconducting YBa 2 Cu 3 O 6.15 prepared from a master sample of citrate-route synthesized YBa 2 Cu 3 O 6.9 displays an EPR signal whose intensity peaks near 460 K in agreement with neutron diffraction data that show an antiferromagnetic transition in the 400 to 500 K temperature range. A probable explanation for these results is discussed.

Synthesis microstructure and superconducting properties of Ce-Doped Ti-Sr-Ca-Cu-O

Abstract A new Tl 1− x Ce x Sr 2 CaCu 2 O 7 compound with x =0.25, which forms a primitive tetragonal structure with a=b=3.816 A and c=12.112 A , has been synthesized in nearly pure form. The structure has been confirmed for the heavy atoms at the microstructural level by high resolution lattice imaging techniques. Selected area electron diffraction did not indicate the presence of structural modulations. The presence of Ce has been confirmed by analytical electron microscopic methods and by X-ray photoemission spectroscopy; the latter also indicates that cerium is present as Ce +4 ions. SQUID magnetometry, AC susceptibility and four-probe resistance measurements indicate superconductivity with a T c between 62 and 68 K. Meissner fractions greater than 10% of ideal diamagnetism signal the presence of bulk superconductivity in the samples. The nature of the substitution process, and the reasons for a 20 K drop in T c from a value of near 85 K for the isostructural Tl 0.5 Pb 0.5 Sr 2 CaCu 2 O 7 superconductor, are discussed.

Shock‐wave synthesis of a thallium‐based superconductor with a novel defect microstructure

We report the shock‐wave synthesis at a yield ≳80% by volume of the single copper layer thallium superconductor of composition Tl2Ba2CuO6. The as‐synthesized material displays zero resistance near 55 K and a diamagnetic onset to bulk superconductivity at 70 K. Lattice imaging indicates that the superconducting microcrystals consist of a novel defect microstructure involving an intergrowth of two copper‐oxygen layers probably interleaved by partial thallium and barium occupancy.

Structure and bulk superconductivity of BaPb0.5Bi0.25Tl0.25O3-δ

Abstract Bulk superconductivity near 9 K has been observed in BaPb0.5Bi0.25Tl0.25O3−δ. Superconductivity is observed in samples that have been prepared by a three-step process, using sequential anneals in Ar, O2 and 5% O2-95% Ar (at 775°C, 475°C and 650°C, respectively). Samples annealed at 600°C after the third step in pure Ar are not superconducting, but remain superconducting after pure O2 annealing. Structural studies have been carried out by Rietveld refinement of neutron powder data for samples in both the superconducting regions and the pure Ar annealed nonsuperconducting region. All three systems have essentially identical tetragonal structures at 305 K, with space group I4/mcm. The (0, 0, 1 4 ) oxygen site is displaced to the four-fold (x, 0, 1 4 ) s ite and probably fully occupied; the (x, x+ 1 2 , 0) site appears approximately 6% deficient for the Ar annealed sample and 4% deficient for the other two samples. The title compound represents the first thallium substituted non-copper oxide superconductor and hence some of its characteristics are briefly compared with that of the isomorphous BaPb0.75Bi0.25O3 (Tc=13 K) and BaPb0.75Sb0.25O3 (Tc=3.5 K) superconductors.

Microstructure and superconductivity of shock processed high TC superconductors

Abstract Explosive shock compaction of furnace synthesized 1 micron size powders of YBa 2 Cu 3 O 7 and Bi 2 Sr 2 CaCu 2 O 8 have been carried out both in the planar and cylindrical geometries. In both cases interesting microstructures and near theoretical packing densities are obtained. However bulk superconductivity is degraded but restored with some interesting changes on reannealing. The tetragonal and orthorhombic Tl 2 Ba 2 CuO 6 phases were successfully shock-synthesized in the time domain of microseconds at 100kbar. The products obtained ranged in phase purity from 85 to nearly 100%. The shock-synthesized tetragonal phase is a bulk superconductor with T c (zero) of 55K. High resolution electron microscopy shows double copper layer defects in the shock synthesized tetragonal Tl 2 Ba 2 CuO 6 . The importance of shock induced defects and microstructures in the magnetic flux pinning mechanisms are briefly discussed.

Effect of explosive-shock processing on the structure and superconducting properties of YBa2Cu3O7−δ

Abstract We have carried out explosive shock compaction of YBa 2 Cu 3 O 7−δ of average grain sizes of 1μm (prepared using BaCO 3 ) and 10μm (prepared using BaO 2 ) at dynamic pressures of 60 GPa. Characterization of the material was carried out in the post-shocked phase using a variety of techniques.

Low-Field Microwave Absorption in High-T C Superconducting Oxides

Low-field non-resonant microwave absorption in an EPR spectrometer has been observed in the superconducting state of different samples of the new high-T oxides, YBa 2 Cu 3 O 7-δ and La 1.8 Sr 0.2 CuO 4 . It has been attributed to flux slippage occurring at the Josephson junctions existing in the granular composite material. The dependence on particle size, temperature, microwave power, DC modulation amplitude and magnetic field has been studied.