Variation Of Hole Concentration Research Articles

Tuning Pb content in High-Tc Bi2Sr2CaCu2Oy superconductors studied by X-ray absorption near-edge structure spectroscopy

The variation of hole concentration within the CuO 2 planes via tuning Pb content in (Bi 2-x Pb x )Sr 2 CaCu 2 O y (0.2 ≤ x < 0.6) has been investigated by X-ray absorption near-edge structure (XANES) spectroscopy using synchrotron radiation. The profile of XANES spectra indicated that Pb can be incorporated into Bi-2212 phase under nitrogen atmosphere around x = 0.6. However, after annealing the nitrogen-sintered samples in oxygen, Pb solubility is found to be decreased only up to x = 0.2 in (Bi 2-x Pb x )ST 2 CaCu 2 O y , showing that compounds with high Pb content are more stable under oxygen-free armosphere.

Thermal and phase characterization of Bi-2223 superconductors

Advances in processing and fabrication of high critical current density ( J c) long length conductors, silver sheathed superconducting magnets and coils from silver sheathed Bi-2223 tapes, pancake coil magnets from 1 to 10 m length and mono-multifilament Bi-2223/2212 high critical temperature superconductors by powder in tube technique continue to bring these materials closer to commercial applications. In our laboratory Bi-2223 conductors doped with Sm, Nb, Ag and Gd were prepared by the heat treatment of rapidly quenched glass precursors. Activation energies and frequency factors, employing different models were evaluated. It was observed that both peritectic transition and reaction rates were dependent on ambient atmosphere. The resistivity measurements revealed that the critical temperature decreases with increasing Gd, Sm, Nb, and Ag concentration. The critical current density J c measured at 77 K from I–V data shows an increase with silver addition. The critical current densities of silver sheathed tapes using the powder in tube were found to be in the range 7×10 8 A/m 2. X-ray diffraction results showed that the volume fraction of the high- T c (2223) phase decreases and that of low- T c (2212) phase increases with increase of these rare-earth ion contents. These results are explained on the basis of possible variation of hole concentration with trivalent rare-earth ion substitution and also by considering the magnetic nature of the substituted ion in the composition.

Variation of hole concentration and structure induced by element substitution at Y and Cu sites in the YSr 2Cu 3O 7-δ system

We have achieved the preparation of the (Y 1-xCa x)Sr 2(Cu 3-yFe y)O 7-δ(0≤x≤0.2, 0.4≤y≤0.6) compounds. The transition temperature increases linearly with increasing Cu(1)-O(1) distance and decreasing Cu(2)-O(1) distance. It was also observed that Tc increases with the carrier concentration on the CuO plane(P sheet).

Superconducting and magnetic properties of Gd substituted 2:2:2:3 Bismuth cuprates (Bi1.7Pb0.3Sr2Ca2–xGdxCu3Oy)

The Bi1.7Pb0.3Sr2Ca2−xGdxCu3Oy system is investigated for different concentrations of Gd. X-ray diffraction, electrical resistance, ac susceptibility, and dc magnetization studies are performed to study the changes in the superconducting and magnetic properties. Substitution of dilute quantities of Gd3+ for Ca2+ is found to change drastically the superconducting properties of the system. X-ray diffraction results reveal that the volume fraction of the high-Tc 2223 phase that is present in samples without Gd3+ (x = 0) vanishes even for x = 0.01. The Tc onset obtained from χ'ac versus T decreases from 109 to 74 K for x = 0 to 0.1, whereas the loss peak from χ″ac versus T decreases from 108 to 56 K for x = 0 to 0.05. The same behaviour is noticed from resistance measurements. The dc susceptibility studies show that except for the x = 0 sample all other samples follow a Curie-Weiss behaviour in the normal state with Curie-Weiss temperature θ of the order of 20 K. The data obtained are compared with our earlier results on Sm3+ substituted Bi-2223 superconductors. The results are explained on the basis of a possible variation of hole concentration with trivalent rare earth ion substitution and also on the basis of the magnetic nature of the substituted ions.

Formation, characterization and electronic structure of thallium-based strontium analogues superconductors

XRD and EDS results suggested that three intermediates including 1201 ∗, 1212 ∗ and 1201 occurred in Y-sub. 1212 phase. XPS studies in Th-sub. ones indicated the variation of hole concentration may occur at Tl-0 layers as the Th content changed.

VARIATION OF HOLE CONCENTRATION INDUCED BY ELEMENT SUBSTITUTION AT Y AND Cu(I) SITES IN YBa2Cu3Oy SYSTEM

The origin and the regularity of the variation of hole concentration as well as the crystal structure and superconducting critical temperature versus the doping contents of Co for Cu (I) and Ca for Y in YBa2Cu3Oy are studied in this paper. The results reveal that the pattern in which the hole concentration in Cu-O sheets, Psh, is influenced greatly by the substitution depends upon the different substituting elements as well as the different atomic sites, e.g. the decrease of Psh in system substituted by Co for Cu (I) is from filling and localizing of holes, but the increase of Psh in system of Ca replacing for Y is due to the delocalization effect of the holes. When Y and Cu (1) are substituted simutaneously with Ca and Co respectively, the generation instead of the localization effect cf the holes is important for the change of Psh. In addition, the relationship between Psh and Tc is discussed.