I.K. Gopalakrishnan

Zero resistance at 120 K in Bi(Pb)-Ca-Sr-Cu oxide

Abstract Bulk Bi 1.6 Pb 0.4 Ca 2 Sr 2 Cu 3 O y samples synthesised by a precursor matrix reaction method show a zero resistance state at 104 K after long-term sintering at 1140 K. Samples subjected to an additional heating close to the melting point, for short durations, attain a record zero-resistance temperature of 120 K. Superconducting behaviour of these samples is stable and does not show any degradation upon thermal cycling.

Magnetic and electrical properties of La0.67Ca0.33MnO3 as influenced by substitution of Cr

Abstract The compounds La 0.67 Ca 0.33 Mn 1− x Cr x O 3 ( x =0.00, 0.05, 0.10, 0.15, 0.20, 0.35, 0.50 and 0.60) have been studied by X-ray diffraction (XRD), AC and DC magnetization, and DC electrical resistivity measurement techniques. Ferromagnetism is observed up to as high as 60% of Cr substitution. Electrical resistivity behavior of the compounds as a function of temperature could be explained on the basis of adiabatic small polaron model.

On the preparation and structure of the 81 K single phase superconductir in the BiCaSrCuO system

Abstract A new and elegant method to synthesize a single phase 2122 compound in the Biue5f8Caue5f8Srue5f8Cuue5f8O system is described. Electrical resistivity measurements show the superconducting transition temperature ( R = 0) at 81 K. X-ray and neutron diffraction analysis indicate that the single phase compound has cell parameters a = 5.362(6) A , b = 5.443(6) A and c = 30.77(2) A and space group Fmmm. Results of neutron diffraction profile analysis show that the orthorhombic distortion is significantly larger than hitherto reported and also indicate possible substitution of Bi for Sr/Ca.

Stabilization of superconductivity in TlBa2CuO5−δ at 9.5 K and its enhancement to 43 K in TlBaSrCuO5−δ

Abstract It is shown that the superconducting state can be stabilized ( T c =9.5 K) by slow-cooling TlBa 2 CuO 5 in a nitrogen environment. Interestingly, partial substitution (50%) of Ba 2+ by Sr + 2 yields a zero-resistance state at 33 K in TlBaSrCuO 5 , even for air-quenched samples. T c could be further enhanced to 43 K ( R =0) by a slow cooling treatment in flowing nitrogen. Contraction of the lattice for TlBaSrCuO 5−δ in comparison with TlBa 2 CuO 5 , supports the substitution of Sr 2+ -ions at Ba-sites.

Evidence for K-substitution in the Tl-sites of superconducting Tl2CaBa2Cu2Oχ: Neutron diffraction studies

Abstract A comparison of the Rietveld profile refinement of neutron diffraction data of the superconducting compounds Tl-2122 (i.e. Tl 2 CaBa 2 Cu 2 O χ ) and Tl 1 K 1 Ca 1 Ba 2 Cu 2 O y provides evidence that nearly half of the Tl-sites belonging to the pure Tl-2122 structure get substituted by K-ions in Tl 1 K 1 Ca 1 Ba 2 Cu 2 O y . The consequence of this is reflected in a substantial movement of Ba-ions towards the plane of the O(2)-ions in the K-substituted compound.

Magnetic Behavior of Manganese-Doped ZnSe Quantum Dots

Magnetic properties of manganese-doped ZnSe quantum dots with the size of approximately 3.6 nm are investigated. The amount of Mn in the ZnSe quantum dots has been varied from 0.10% to 1.33%. The doping level in the quantum dots is much less than that used in the precursor. The co-ordination of Mn in the ZnSe lattice has been determined by electron paramagnetic resonance (EPR). Two different hyperfine couplings 67.3×10−4 and 60.9×10−4u2009cm−1 observed in the EPR spectrum imply that Mn atoms occupy two distinct sites; one uncoordinated (near the surface) and other having a cubic symmetric environment (nanocrystal core), respectively. Photoluminescence measurements also confirm the incorporation of Mn in ZnSe quantum dots. From the Curie-Weiss behavior of the susceptibility, the effective Mn-Mn antiferromagnetic exchange constant (J1) has been evaluated. The spin-glass behavior is observed in 1.33% Mn-doped ZnSe quantum dots, at low temperature. Magnetic behavior at a low temperature is discussed.

X-ray diffraction coupled thermogravimetric investigations of YBa2Cu3O7−x

Abstract Isobaric thermogravimetry(TG) in air and pure oxygen (pO 2 = 1 atm) was carried out on YBa 2 Cu 3 O 7−xo at the heating and cooling rates ranging between 5 and 50 /min. The data obtained was complemented by the x-ray diffraction analysis of the samples of varying oxygen stoichiometry obtained by heating the above compound to different temperatures in argon, followed by cooling to room temperature. The observations made in these experiments indicated that YBa 2 Cu 3 O 7 exhibits a homogeneity range of composition upto x = 0.5. For oxygen content less than 6.5 atoms per formula weight of the compound, existence of biphasic region could be established. The two coexisting phases were orthorhombic and tetragonal, respectively. The tetragonal phase formed from low temperature orthorhombic structure by the loss of oxygen, also exhibited a wide homogeneity range and did not break to constituent oxides upto the composition YBa 2 Cu 3 O 5.92 . The dependence of T c on oxygen content of phase reported in the recent investigations could be rationalized in the light of observations made in the present study.

Synthesis and properties of a 125 K superconductor in the Tl-Ca-Ba-Cu-O system

A method for the synthesis of a 125 K superconducting phase having a nominal composition TlCaBaCu2 Ox is described. It is shown that by sintering at 1200 K in flowing oxygen containing a controlled amount of water vapor, a zero electrical resistance state (Tcf) at temperatures as high as 125 K can be obtained. A transport critical current density (Jc ) of ∼240 A/cm2 at 77 K was obtained in this 125 K material.

Effect of argon-annealing and subsequent oxygen-annealing on the superconductivity and structure of Tl2CaBa2Cu2O8-x

Abstract Annealing the single-phase Tl 2 CaBa 2 Cu 2 O 8 superconductor ( T c =108 K ) in argon environment at 950 K for 24 h brings the T c down to ≅80 K. Re-annealing the samples in oxygen under similar conditions restores the T c to 105±5 K. Rietveld profile refinement of the neutron diffraction spectra of the argon-annealed samples indicates that some Tl-ions, presumably Tl 1+ , move from the Tl double-layers to occupy Ca-sites. Upon subsequent annealing in oxygen, however, these Tl-ions escape from the lattice. Implications of this and other structural features are described.

Ambient pressure synthesis and neutron structure analysis of YBa2Cu4O8

Abstract The high- T c superconductor YBa 2 Cu 4 O 8 has been synthesized by the solid state reaction of Y 2 O 3 with a precursor matrix under ambient oxygen pressure. The final product is single phase 124 which shows zero resistance at 78.5 K ( ΔT c =2.0 K ). DC magnetic susceptibility studies confirm the presence of a single bulk superconducting phase ( T c ≅82.5 K ) and absence of 123 or 247 phases as impurities. Neutron profile refinement studies of the structure of the sample indicate vacancies at 9% of the linear chain oxygen sites, O(1); increased puckering of Cu(1)-0(1) layers, and a much reduced short-contact distance between neighbouring 0(1)-anions. Other notable features of the structure are vacancies (14%) at Ba-sites and unusually short bridging bond-length Cu(1)-0(4) and Cu(2)-0(4).