Effect of praseodymium substitution on the growth and properties of Y1−x Pr x Ba2Cu3O7−δ (o <x < 0·4) single crystals

In the present paper, a modified self-flux technique has been successfully employed for the growth of pure and praseodymium substituted (partially) large single crystals of high temperature superconducting Y_1−x Pr _x Ba₂Cu₃O_7−δ (x = 0·0,0·2,0·4). Typical sizes of the platy and bulky crystals of pure YBCO(123) material are ≈ 2 × 2 × 0·1 mm³ and 4 × 1 × 1 mm³, respectively. In case of Pr-substitution, the typical sizes of platy and bulky crystals of Y_0·8Pr_0·2Ba₂Cu₃O_7−δ and Y_0·6Pr_0·4Ba₂Cu₃O_7−δ materials are ≈ 2 × 3 × 0·1 mm³ and 5 × 1 × 1 mm³ and ≈ 1 × 1·5 × 0·1 mm³ and 7 × 0·2 × 0·1 mm³, respectively. The morphology and growth habit of the as-grown single crystals and the critical transition temperature (T _c) of the oxygenated crystals were found to depend on the Pr-content. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Enhancement of Pinning Role by Nonstoichiometry in YBa2Cu3O7?y Superconductors

Considerably improved flux pinning and critical current density J c values have been achieved in Y-deficient Y-123 superconductors by directional solidification in air. In comparison with the regular Y-123 composition, Y-deficient one also has an orthorhombic structure and Y-123 main crystal phase remains in it. Whereas with the shortage of Y, Y_1–x Ba₂Cu₃O_7–y can be regarded as (YBa₂Cu₃)_1–x O_7–t(Ba₂Cu₃) _x O _t–y or (YBa₂Cu₃O_7–z )( _x ^YO _z–y ), so there may develop several kinds of microstructure defects as pinning sites in the system, such as highly dense, fine-scale, and faultlike defects, as well as localized superstructure, which are able to induce the increasing in flux pinning and J c values in higher external magnetic fields. This kind of simple nonstoichiometric route could lead to a commercial technique for flux-pinning enhancement in Y-123 bulk materials.     

High-capacity superconducting current leads of Y1Ba2Cu3O7?x

We have fabricated and measured a high-capacity superconducting current lead composed of a Y₁Ba₂Cu₃O_7–x cylinder, 20 cm long and 0.9 cm² cross section. A steady-state, d.c., critical current of 225 A at a temperature of 77 K was measured in this sample, using a voltage criterion of 2×10^–7 V/cm (p = 8×10^–10 ohm-cm). This current was limited by the currentinduced, self magnetic field. To our knowledge this is the largest d.c. critical current so far reported in a Y₁Ba₂Cu₃O_7–x sample and demonstrates the possibility of using hightemperature superconducting HTS materials for current leads to low-temperature superconducting LTS magnets or in power distribution systems.     

Prediction ofT c for YBa2Cu3Oz doped with Ca using neural network

In order to predict the superconducting transition temperatureT _c of YBa₂Cu₃O_z doped with Ca, we have constructed a neural network NN which is composed of three layers; we considered 28 constituent elements and their concentration in the input layer to provideT _c in the output layer. The NN was trained by the error-back-propagation method using data from the database “SUPERCON” for high-T_c superconducting materials. The effect of Ca doping onT _c of (Y_1-x-Ca_x)Ba₂Cu₃O_z and Y(Ba_2-xCa_x)Cu₃O_z was investigated using this method. It turned out that a higherT _c is expected for the sample withx = 0.3 andz = 6.5 in the former system and that the oxygen concentrationz can be varied for different Ca contents in the latter system.     

Phononic and spin excitations in Y1?xPrxBa2Cu3O6.9 crystals

The Y_1–x Pr _x Ba₂Cu₃O_6.9 system has been investigated by quantitative-Raman measurements. The magnetic response of the antiferromagnetic ordered Cu spins in the CuO₂ plane has been observed via the two-magnon-Raman scattering. The in-plane exchange energy is determined to 780 cm^–1 in the system withx=1. Phonon behavior as well as magnetic scattering indicate that the band structure of the CuO₂ planes in PrBa₂Cu₃O_6.9 is similar to that in insulating YBa₂Cu₃O₆.     

Effect of Cd and Cd-Ca Substitution on the Superconductivity in Y1−xPrxBa2Cu3O7−δ Superconductor

The structural and superconducting properties of Y_1−v−η Pr_NCd{η}¨zCa _z Ba₂Cu₃O_7−δ system are investigated using X-ray diffraction, ac susceptibility, dc resistivity, and oxygen content measurements. The effect of increasing Cd concentration substituting the Y-site in Y_0.8Pr_0.2Ba₂Cu₃O_7-δ suppresses the superconducting transition temperature and lowers the hole concentration, unlike that of Ca substitution in Y_0.8Pr_0.2Ba₂Cu₃O_7-δ which increasesT _c due to hole doping by Ca. The suppression ofT _c due to Cd substitution can be counterbalanced by simultaneous hole doping by Ca which increases the T_c with increasing Ca concentration. In spite of similarity in the ionic radii and valency, the role played by Cd and Ca substitution at the Y-site in the Y_0.8Pr_0.2Ba₂Cu₃O_7-δ system is opposite in nature as Cd doping helps in T_c suppression due to the Pr effect, suggesting that Cd does not provide the necessary holes like Ca substitution which helps to increase the T_c by the hole doping mechanism.     

Low-field magnetic properties of high-TcGdxY1?xBa2Cu3Oy superconductor

The magnetization of Gd_0.85Y_0.15Ba₂Cu₃O _y high-T _c superconductor was measured at low fields (<100 Oe) and at a constant temperature of 77 K. The magnetization curves deviate from linearity at 25 Oe, causing the destruction of the weak couplings of the superconducting grains. The hysteresis due to granular superconductivity was also observed.     

Y1?xPrxBa2Cu3O7: Charge redistribution between planes and chains?

A qualitative discussion is given of the electronic structure and properties of the system Y_1–x Pr _x Ba₂Cu₃O₇ in which there is a transition from a high-T _c superconductor to a magnetic insulator. To reconcile the apparently contradicting experimental data, it is suggested that the progressive hole redistribution between CuO₂ planes and chains occurs with increasingx so that for large Pr content all the holes are in chains and none remain in planes. In this picture one can naturally explain the main properties of this system; notably, one gets an insulating state for pure PrBa₂Cu₃O₇, the valence of Pr being still Pr³⁺. Many other properties of Y_1–x Pr _x Ba₂Cu₃O_7– are naturally explained as well, and several predictions are made.