The superconducting and normal state transport properties of ceramic samples with composition Bi 2 Sr 2− x K x CaCu 2 O y ( x  = 0, 0.1, 0.2, 0.3, 0.4) have been investigated by measuring thermoelectric power. The samples were prepared by the solid state route, and single phase Bi-2212 was found for all compositions. The T c (0) and thermoelectric power data were collected from 50 to 300 K. The thermoelectric power changes slightly with increasing doping level and increases linearly with decreasing temperature reaching a maximum value at T c (0). The T c (0), hole concentration and thermoelectric data were correlated. The results obtained are analysed in terms of phenomenological theoretical models that makes it possible to determine the band spectrum parameters in the normal state and to trace their changes with varying composition. Following models applied such as (a) Hubbard model, (b) two-band model, (c) Fermi-liquid behaviour. Applicability of these models to the present HTSC system is discussed and the parameters related to the models have been evaluated.

Thermoelectric power in Bi2Sr2−xKxCaCu2Oy

A series of sodium-doped Bi-2212 single-phase high- superconductors have been prepared by the solid state reaction method. After the usual characterization by XRD, SEM, bulk density porosity etc, ultrasonic longitudinal velocity studies were undertaken over the temperature range 80 - 300 K using the pulse transmission technique. In the temperature range 300 - 230 K, in contrast to normal solids, the velocity of Bi - Na(0) and Bi - Na(3) samples is found to decrease with decreasing temperature (elastic softening) followed by a velocity maximum at around 150 K for all the four samples, signifying the presence of lattice instabilities. Plausible explanations are given for both the elastic softening and the velocity maxima on the basis of microstructure and ordering readjustments of the oxygen atoms among the Cu - O planes of the samples.

http://iopscience.iop.org/article/10.1088/0953-2048/9/1/004/pdf

Elastic anomalies in sodium-doped Bi-2212 single-phase HTSC materials

Samples with the nominal composition Bi 2 Sr 2 Ca 1−x Li x Cu 2 O y (x = 0.2, 0.4, and 0.6) and Bi 2 Sr 2 CaCu 2−x Li x O z (x = 0.4, 0.6, and 1.0) were prepared by the solid state reaction method. The role of Li at both Ca-and Cu sites in the Bi-2212 composition were studied. From the X-ray diffraction data it was found that the Li-doped at the Ca site increases the c-axis and that doped at the Cu site decreases the c-axis. From the D.C. four-probe resistivity data it was found that Li-doped at the Ca site reduces the T c and the Li-doped at the Cu site gives T c (0) above the liquid nitrogen temperature. It was observed that the Li doping reduces the melting point of Bi-2212 composition. The presence of Lithium was confirmed by inductive the coupled plasma method

Effect of lithium doping in the Bi-2212 superconducting system

Samples with the stoichiometric composition with x = 0.1 and y = 0.1, 0.2, 0.3 and 0.4 were prepared by the solid state reaction method. The role of Na and K doped at the Ca and Sr sites simultaneously in the Bi-2212 composition was studied. From the x-ray diffraction data it was found that the structure is single-phase Bi-2212. From the resistivity data the varies from 63 to 67 K for different dopant levels. The diamagnetic nature of the samples was confirmed by ac susceptibility measurements. The oxygen content was determined by the two-step iodometric titration technique. The present results on the effect of double doping are compared with those of single dopants published earlier by our group. The microstructures of the samples were examined by scanning electron microscopy.

Superconductivity in the Bi - Sr/K - Ca/Na - Cu - O system

Samples with the nominal composition Bi2Sr2−xKxCaCu2Oy ( with x = 0.1, 0.2, 0.3, 0.4 and 0.5 ) were prepared by the solid state reaction method. The role of potassium at the Sr-site and the effect of various heat treatments in the 2212 composition were studied. From the X-ray diffraction data it is found that the samples annealed for longer have shown a single phase 2212 up to x = 0.4 dopant level. From d.c. four probe electrical resistance data, it was found that the T c (0) varies from 76 to 93 K for different dopant levels. The diamagnetic nature of the samples was confirmed by a.c. susceptibility measurements. The phase formation of 2212 and the presence of potassium in the doped samples was confirmed by electron microprobe analysis.

Potassium substitution in the Bi2Sr2CaCu2Oy superconductor

Samples with the nominal composition Bi 2 Sr 2 Ca 1− x Na x Cu 2 O y (if with x = 0.05, 0.2, 0.4 and 0.5) were prepared by the solid state reaction method. The role of Na and the effect of various heat treatments in the 2212 composition were studied. From the X-ray diffraction data it is found that the samples annealed for longer duration have shown a single phase 2212. From the DC four-probe resistivity data, it was found that the T c (zero) varies from 79 to 89 K. It is believed that doping with an alkali element would reduce the oxygen content due to monovalency and gives an effect similar to that during heat treatment under low oxygen pressure or quenching. The presence of sodium in the samples was confirmed by electron probe microanalysis.

R.D.K. Misra

Papers

Effect of sodium substitution on superconductivity in Bi-based (2212) system