G.M. Phatak

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.

Low-field microwave absorption in Gd2CuO4: similarity and contrast with high-temperature superconducting materials

Abstract A low-field microwave absorption signal observed in Gd 2 CuO 4 below 260 K in EPR spectrometric measurements has features strickingly similar to those reported for high-temperature superconductors below T c . It has opposite phase compared to that of EPR absorption of Gd 3+ and is found to exhibit hysteresis effects on magnetic field cycling. Furthermore, the difference between zero-field cooled and field cooled samples is found to be similar to that expected for magnetization in spin glasses. The only contrasting feature, as compared to superconducting samples, is in the relative position of the signal during field increasing and decreasing cycles and this feature is shown to be most important in distinguishing the genuine signature of superconductivity. The possible consequences of the present observation on the mechanism of superconductivity are discussed.

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.

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.

Superconducting transition temperature of single-phase Tl-2223: Crucial role of Ca-vacancies and Tl-content

Abstract Detailed investigations on the synthesis and phase-indentification of a number of Tl-Ca-Ba-Cu oxides show that single-phase Tl-2223 superconductor with the best transition temperature ( R = 0 at 130 K) can be prepared from a thallium-deficient starting composition, namely Tl ≈1.0 Ca 2 Ba 2 Cu 3 O x . A number of other starting compositions containing Tl: Ca T c -values between 115 K and 130 K. It is argued that the existence of vacancies at Ca-sites is crucial to the production of pure 2223-phase and the highest T c (130 K) is obtained when the number of such vacancies reaches an optimum. Starting compositions deficient in Tl lead to such a possibility by aiding the migration of some Ca-ions to vacant Tl-sites. Under standardised preparation conditions single-phase Tl-2223 structure is obtained for a combined nominal stoichiometry 3 ⩽ (Tl + Ca) ⩽ 4.

Microwave absorption studies in new high-Tc superconducting oxide systems: Tl-Ba-Ca-Cu-O and Bi-Ca-Sr-Cu-O

Microwave absorption studies of the new high-Tc superconducting systems Tl-Ba-Ca-Cu-O (Tc( rho =0)=118 K) and Bi-Ca-Sr-Cu-O (Tc( rho =0)=84 K) were conducted using an X-band EPR spectrometer. The main features observed were (i) a sharp change in the leakage current at Tc; (ii) intense low-field (<50 G) absorption below Tc; and (iii) a largely field-independent absorption, the intensity of which was found to increase with increasing external magnetic field strength up to 10 kG. The dependence of the intensity on magnetic field is explained in terms of magnetic-field-induced decoupling of grains and field, and temperature-dependent lambda MW/ lambda , where lambda MW is the microwave penetration depth and lambda is the London penetration depth.

Separation of rare earths by ion exchange

Separation of rare earths from monazite source by ion-exchange process is discussed. Optimum conditions for obtaining kilogram quantities of individual rare earths of high purity are reported.

Phonon density of states in Tl2CaBa2Cu2O8

Abstract The neutron-weighted phonon density of states in the high-temperature superconductor Tl 2 CaBa 2 Cu 2 O 8 ( T c =107 K) is obtained from coherent inelastic neutron scattering measurements at the Dhruva reactor at 300 K. The phonon spectrum is qualitatively similar to that in the 90 K superconductor YBa 2 Cu 3 O 7 , and compares well with an independent lattice dynamical calculation.

Nature of the superconducting transition in high Tc Tl-Ca-Ba-Cu-O compounds: Positron annihilation studies

Abstract Doppler broadened annihilation radiation (DBAR) spectra have been recorded as a function of temperature for three Tlue5f8Caue5f8Baue5f8Cuue5f8O superconducting samples, namely A: mixed phase (2223 + 2122 with T c = 125 K, B: single phase (2122) with T c = 108 K, C: argon annealed analogue of B with T c = 95 K. The two component analyses of these spectra bring out certain interesting features. For all the samples the temperature dependent parameters T B (broad component width) and I N (narrow component intensity) were found to show a sharp maximum at respective T c s, indicating an increase in the oxygen valence electron momentum and increased electron density at oxygen vacancy/defect sites. On cooling below T c these parameters were found to decrease monotonically except for the T B of the mixed phase sample A which shows another maximum at 100 K - a manifestation of the presence of the 2122 phase. The observation of higher I N values for sample A compared to sample B and C is probably due to additional Cuue5f8O, Caue5f8O planes present in the former providing a higher number of oxygen vacancies/defect sites. The results are discussed.

Insensitivity of Tc to high K substitution in the Tl-Ba-Ca-Cu-O system

Abstract We report that substitution of potassium for thallium upto as high as 70% in the Tl-Ba-Ca-Cu-O superconducting system does not adversely affect the T c(zero) which can be as high as 123 K for 50% K and 118 K for 70% K containing compounds. The major lines in the X-ray diffraction patterns of the 50% K substituted compounds are quitte similar to those in the pure Tl containing compound. Possibility of complete substitution of Tl by K was also explored and the results discussed.