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Journal ArticleDOI

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

01 Sep 1996-Superconductor Science and Technology (IOP Publishing)-Vol. 9, Iss: 9, pp 756-760

Abstract: 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.
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Abstract: 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.

18 citations

Journal ArticleDOI
Muhammad Javed Iqbal1, Rashid Mehmood1Institutions (1)
Abstract: Doping of a combination of Tl 0.3− x Sb x is found to promote the formation of high-Tc (Bi-2223) superconducting phase in Bi 1.7 Pb 0.4 Sr 2 Ca 2 Cu 3 O y compound which consequently results in enhancement of the critical temperatures, i.e. Tc (0) and Tc (onset) . The structural information about the samples is retrieved through powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) whereas the critical temperature are determined through measurement of electrical resistivity and magnetic susceptibility. Various proportions of both the high-Tc (Bi-2223) and the low-Tc (Bi-2212) phases are present in the synthesized samples. However, the sample containing a dopant content x = 0.06 exhibits comparatively the lowest value of the Δ T = 5 K and the highest content (82.5%) of the high-Tc phase and zero resistivity critical temperature Tc (0) = 109 K.

14 citations

Journal ArticleDOI
Abstract: Samples of stoichiometric composition Bi1.8Pb0.4Sr2−xKxCa2−yNdyCu3Oz with x = 0, 0.01, 0.02, and y = 0, 0.01, 0.02 were prepared by the solid state reaction method. Post-annealing experiments were carried out at 500 °C for 30 h in order to improve the oxygen homogeneity. They were characterized by the x-ray diffraction (XRD) technique, field emission scanning electronic microscopy (FE-SEM), resistivity, magnetization and Raman measurements. The superconducting transition temperature Tczero of the samples varies from 99 to 110 K for different doping. The superconductivity and the critical current density (Jc) of the sample single doped with K are improved compared with the undoped one. We attribute the change of Tc to the variation of carrier concentration. Additionally, the observed change of the normal state resistivity is related not only to carrier concentration but also to the nature of grain boundary links. These analyses are confirmed by Raman and SEM measurements.

13 citations

Journal ArticleDOI
Nawazish A. Khan1, Safeer Hussain1Institutions (1)
Abstract: A systematic study of the weak link behaviour for (Cu 0.5 Tl 0.5− x K x )Ba 2 Ca 2 Cu 3 O 10− δ ( x = 0, 0.25) superconductors samples has been carried out using electrical resistivity and AC-susceptibility techniques. The K-doped (Cu 0.5 Tl 0.5− x K x )Ba 2 Ca 2 Cu 3 O 10− δ samples were synthesized by solid-state reaction method. In magnetic susceptibility measurements, the real ( χ ′) and imaginary ( χ ″) parts of the magnetic susceptibility of (Cu 0.5 Tl 0.5− x K x )Ba 2 Ca 2 Cu 3 O 10− δ ( x = 0, 0.25) samples were measured as a function of temperature under various DC-magnetic fields up to 172 Oe. It is observed from these studies that the magnitude of the diamagnetism is substantially enhanced by K-doping. The possible reasons for the enhanced magnitude of diamagnetism have been investigated. It is observed from in-field magnetic measurements that the inter-grain coupling is improved with the K-doping. It is concluded from these studies that potassium atoms appearing at the crystal boundaries enhance inter-grain coupling and pinning mechanism in K-doped (Cu 0.5 Tl 0.25 K 0.25 )Ba 2 Ca 2 Cu 3 O 10− δ superconductors.

6 citations

Journal ArticleDOI
Nawazish A. Khan1, Shahreyar1, Najmul Hassan1Institutions (1)
Abstract: In order to study the higher carrier’s doping efficiency of (Cu 0.5 Tl 0.5 )Ba 2 O 4− δ charge reservoir layer to the conducting CuO 2 planes we have incorporated Bi in (Cu 0.5 Tl 0.5 )Ba 2 Ca 2 Cu 3 O 10− δ superconductors; the presence of Bi in the charge reservoir layer promotes highest anisotropy to these compounds. For such studies, we have successfully synthesized (Cu 0.25 Tl 0.5 Bi 0.25 )Ba 2 Ca 2 Cu 3 O 10− δ , (Bi 0.5 Tl 0.5 )Ba 2 Ca 2 Cu 3 O 10− δ , (Cu 0.25 Tl 0.25 Bi 0.25 Li 0.25 )Ba 2 Ca 2 Cu 3 O 10− δ and (Cu 0.25 Tl 0.25 Bi 0.25 Li 0.25 )Ba 2 Ca 2− y Be y Cu 3 O 10− δ superconductors at normal pressure and studied their superconducting properties. These studies have shown that incorporation of Bi at the charge reservoir layer promoted a decrease in the density of carriers in the conducting CuO 2 planes which in turn suppressed the magnitude of diamagnetism of final compound. The decreased density of the carriers in the CuO 2 planes, which most likely promotes inferior superconducting properties, was replenished by doping of Li in the charge reservoir layer and optimizing their number by post-annealing in oxygen atmosphere. It is observed that Bi doping in (Cu 0.5 Tl 0.5 )Ba 2 O 4− δ charge reservoir layer promotes an increase in the c -axis length which in turn increase the anisotropy and decrease the Fermi-vector [ k F = (3 π 2 N / V ) 1/3 ] and Fermi-velocity [ v F = ( π e c Δ / ℏ ) ] of the carriers. We have decreased the c -axis length of Bi doped compounds by doping Be at the Ca sites in (Cu 0.25 Tl 0.25 Bi 0.25 Li 0.25 )Ba 2 Ca 2− y Be y Cu 3 O 10− δ superconductors. It is observed from these experiments that Beryllium doping of y = 0.5 has produced excellent superconducting properties in terms of enhancement of T c ( R = 0) and magnitude of diamagnetism in this compound. A maximum decrease in the unit cell volume is observed with this Be doping concentration, which enhance k F , v F and superconducting order parameter of the carriers and hence promote the enhancement of superconducting properties. These studies have also shown that the role of density of the carriers in CuO 2 planes is pivotal in the mechanism of superconductivity in these compounds and Bi doping significantly affects them due to its presence in the charge reservoir layer.

4 citations

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Journal ArticleDOI
TL;DR: These data are interpreted to demonstrate that the (Cu-O) charge in the sheets largely determines T/sub c/ and that the varying oxygen content and the chains merely provide an insulating reservoir of charge.
Abstract: A series of new $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$-like materials are reported in which the oxygen content and the average [Cu-O] charge (or Cu valence) are separately varied over a wide range. We find a distinct boundary between samples showing anomalous insulating behavior and those with high ${T}_{c}$. These data are interpreted to demonstrate that the [Cu-O] charge in the sheets largely determines ${T}_{c}$ and that the variable oxygen content and the chains merely provide an insulating reservoir of charge.

467 citations

Journal ArticleDOI
Merrill W. Shafer1, T. Penney1, B. L. Olson1Institutions (1)
Abstract: The electron deficiency (hole concentration) in the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4\ensuremath{-}\ensuremath{\delta}}$ system is determined by a chemical method. Our results show a direct correlation between ${T}_{c}$ and hole concentration. The hole concentration equals the Sr concentration to about $x=0.15$. For $xg0.15$, the hole concentration decreases and oxygen vacancies are formed. These results strongly support an all-electronic mechanism for superconductivity in this system.

229 citations

Journal ArticleDOI
TL;DR: Un modele de structure de defaut du compose non stœchiometrique, combine a l'approximation des liaisons fortes, rend compte des transitions supraconductrice and magnetique du materiau, mettant ainsi en evidence l'importance des trous dans l'sapparition des phases suprachonductrice et antiferromagnetique.
Abstract: We show, using a proposed oxygen-defect structure for ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ and a tight-binding model based on density-functional calculations, that the distribution of holes between planes and (fragmented) chains causes jumps and plateaus for the hole counts in the planes as a function of the nonstoichiometry, $x$. These features can be correlated with the occurrence of the antiferromagnetic as well as the 60- and 90-K superconducting phases. Our findings are supportive of the view that the hole count is the controlling parameter for the superconductivity.

133 citations

Journal ArticleDOI
Abstract: The sensitivity of Tc to oxygen sorption and the Ca to Sr ratio has been studied for the homologous series Bi2.1(CaχSr1−χ)n+1CunO2n+4+δ for n=1, 2 and 3. The n=0 and n=∞ members have been produced as single phase samples but are found to be semiconducting. While single phase n=2 material has been prepared, the n=1 member always displays a small n=2 contribution to the X-ray diffraction pattern. n=3 is only found in multiphase samples. Single phase superconducting samples were equilibrated under a fixed partial pressure of oxygen at temperatures between 300 and 850°C and quenched in liquid nitrogen. It is shown that Tc can be controlled in a reversible and systematic way by varying the oxygen partial pressure and the annealing temperature and consequently the oxygen stoichiometry. The site energy per oxygen sorbed for n=2 is 3.24 eV, only a little less than that for YBa2Cu3O7. However, the volume change on sorption is ten times smaller than that for YBa2Cu3O7, and is manifested as a changing c-axis with the a-axis invariant. This can be interpreted as implying that oxygen sorption occurs within the Bi-O layers. Tc is found to be maximised for a Ca to Sr ratio of 1 to 2 for both n=1 and 2. Thermal expansion data for n=2 material is also reported to illustrate that oxygen sorption does effect the physical properties of these materials.

121 citations

Journal ArticleDOI
Yoji Koike1, Yoshihiro Iwabuchi1, Syoichi Hosoya1, Norio Kobayashi1  +1 moreInstitutions (1)
Abstract: Correlation between superconducting T c and hole concentration was investigated in the Bi 2 Sr 2 CaCu 2 O 8+δ system. The hole concentration was varied by substituting cations with different valences for Sr or Ca. In order to increase the hole concentration, some samples were annealed at 430°C in 250 bar of oxygen pressure. We find that T c increases, takes a maximum at the hole concentration corresponding to 0.2–0.3 holes per CuO 2 unit, and then decreases, with increasing hole concentration. This correlation is markedly similar to those in the La 2− x Sr x CuO 4 and YBa 2 Cu 3 O 7− δ systems.

97 citations