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Showing papers in "Physica C-superconductivity and Its Applications in 1991"


Journal ArticleDOI
TL;DR: In this paper, annealing results for several Tl-superconductors in the series Tlm(Ba, Sr)2Can−1CunO2n+m+2+δ, for both m=1 and 2, revealing two general results for this class of Bi or TI superconductors: (1) effects on Tc due to labile oxygen occur for all m=2 compounds but are absent in m = 1 compounds with Sr.
Abstract: We have previously shown that Tc for Bi2 (Sr, Ca)n+1CunO2n+4+δ (n=1, 2 and 3) varies with oxygen stoichiometry δ determined by annealing in a variety of oxygen partial pressures and temperatures. Annealing results are now also presented for several Tl-superconductors in the series Tlm(Ba, Sr)2Can−1CunO2n+m+2+δ, for both m=1 and 2, revealing two general results for this class of Bi or TI superconductors: (1) effects on Tc due to labile oxygen occur for all m=2 compounds but are absent in m = 1 compounds with Sr. m=1 compounds with Ba do have variable oxygen but the effects on the c-axis are the opposite to m=2 compounds. This suggests for the latter that the labile oxygen resides in interstitial sites within the Bi2O2 or Tl2O2 bi layers; (2) the hole concentration per Cu decreases in progressing from n = 1 to 2 to 3 in either class so that the parent n = 3 and n = 2 compounds lie respectively on the low- and high-hole concentration sides of the peak in Tc while the n = 1 compounds extend out into the non-superconducting domain at very high hole concentration.

712 citations


Journal ArticleDOI
TL;DR: Inelastic neutron scattering experiments have been carried out on YBa2Cu3O6+x single crystals in order to perform a systematic investigation of the spin dynamics in the various typical regimes as discussed by the authors.
Abstract: Inelastic neutron scattering experiments have been carried out on YBa2Cu3O6+x single crystals in order to perform a systematic investigation of the spin dynamics in the various typical regimes : the pure and doped AF-states (x = 0.15, 0.37), the weakly doped metallic and superconducting state (x= 0.45, 0.51) and the heavily doped metallic state with the Tc= 60 K (x = 0.69) and Tc = 90 K (x = 0.92) phases. The major results are the strong effect of hole doping on the AF-order, the persistence of dynamical AF-correlations in the metallic state and the observation of an energy gap in the spin excitation spectrum at low temperatures in superconducting samples.

443 citations


Journal ArticleDOI
TL;DR: In this paper, the authors show experimental evidence that flux motion results from a thermally activated crossover from 3D vortex lines to 2D independent pancake-like vortices in the Cu-O layers, which occurs when kBT exceeds the Josephson coupling energy of these layers.
Abstract: For H‖c-axis, the magnetic field induced broadening of the resistive transitions of high-Tc superconductors (HTS) is shown to depend strongly on the Cu-O layer spacing. For the highly anisotropic HTS, we show experimental evidence that flux motion results from a thermally activated crossover from three dimensional (3D) vortex lines to 2D independent pancake-like vortices in the Cu-O layers, which is intrinsic to the material and occurs when kBT exceeds the Josephson coupling energy of these layers. At low temperatures, however, thermally activated conventional depinning (which can be sample dependent) or melting in the uncoupled 2D Cu-O layers is also required for flux motion. For YBa2Cu3O7, this dimensional crossover does not occur belowHc2, presumably because the conducting Cu-O chains short-circuit the Josephson interlayer coupling, leading to better superconducting properties in a magnetic field. These results show that strong interlayer coupling is a key to finding good alternatives.

257 citations


Journal ArticleDOI
TL;DR: In this article, a highly effective additive for the melt growth processings attaining high critical currents in YBCO superconductor has been found; it is platinum and it behaves as an effective grain growth inhibitor for the Y 2 BaCuO 5 phase.
Abstract: Recently we have found a highly effective additive for the melt growth processings attaining high critical currents in YBCO superconductor. It is platinum and it behaves as an effective grain growth inhibitor for the Y 2 BaCuO 5 phase. Even with less than 1 wt.% doping. Y 2 BaCuO 5 particles become less than one micron in size and distribute themselves to become homogeneously embedded in the large grown YBa 2 Cu 3 O y grains. The sample shows large magnetic hysteresis and a typical J c value estimated by using Bean's model critical state model is 18 000 A/cm 2 at 77 K and 1 T. We found that rhodium has a similar remarkable effect.

222 citations


Journal ArticleDOI
TL;DR: The Luttinger liquid is a parameter-free theory of the normal state which explains all anomalies in ∼10 substances as discussed by the authors, and the T -dependent Hall effect as an example.
Abstract: I. The Luttinger liquid is a parameter-free theory of the normal state which explains all anomalies in ∼10 substances. II. The T -dependent Hall effect as an example. III. The basic physics behind the Luttinger liquid. IV. The superconducting mechanism and heuristics of the superconducting state.

185 citations


Journal ArticleDOI
TL;DR: In this paper, an explicit expression for the complex optical and AC conductivity of a homogeneous BCS superconductor with arbitrary electron mean free path is given, and a fast self-contained FORTAN program may be used to fit experimental data.
Abstract: An explicit expression for the complex optical and AC conductivity of a homogeneous BCS superconductor with arbitrary electron mean free path is given. This compact expression and a fast self-contained FORTAN program may be used to fit experimental data. For comparison, we give also the complex AC conductivity of high- T c superconductors containing an elastically pinned, viscously moving flux-line lattice with flux creep.

179 citations


Journal ArticleDOI
TL;DR: In this article, the Ba-Sr-Cu-O system was treated at 6 GPa and 1473 K, and it was shown that the system is free from any trivalent or quadrivalent counter cation in contrast with the known superconducting cupric oxides.
Abstract: Superconductivity with T c 's at 90 K and 60 K has been found in the Ba-Sr-Cu-O system treated at 6 GPa and 1473 K. It should be noticed that the system is free from any trivalent or quadrivalent counter cation in contrast with the known superconducting cupric oxides. It is known that use of high pressure for synthesis tends to stabilize a tetragonal structure of the Ca 0.84 Sr 0.16 CuO 2 type for a wide alkaline earth composition range including Sr 1- x Ba x CuO 2 ( x ⪅0.3), while transmission electron microscopic examinations of the present samples revealed three types of modified structures. We suggest that among these, an incommensurate superlattice of a × a × nc with n ≈9 superconducts, though this phase has not been isolated yet.

154 citations


Journal ArticleDOI
TL;DR: In this article, the authors studied the properties of grain boundaries in thin-film bicrystals of YBa2Cu3O7−δ and found that the critical current density Jgbc decreases as the misorientation angle between the adjoining grains increases.
Abstract: In the high temperature superconductors the weak link nature of extended defects as grain boundaries is responsible for the deterioration of the superconducting transport properties. From the study of individual grain boundaries in thin-film bicrystals of YBa2Cu3O7−δ general relations describing the superconducting transport and noise characteristics of grain boundaries in the high temperature superconductors were found. Firstly, the critical current density Jgbc decreases as the misorientation angle between the adjoining grains increases. Secondly, the Jgbc ϱN products, where ϱN is the normal resistance times unit area of the grain boundaries, scale proportional to about (1/ϱN)q with q ranging between 1 and 1.5. Thirdly, all grain boundaries show large amounts of low frequency 1/f noise. The transport and noise characteristics can be explained by a junction model which is based on an insulating layer at the grain boundary interface containing a large number of localized defect states.

152 citations


Journal ArticleDOI
TL;DR: In this article, a large variety of undoped binary and ternary transition metal oxides, including formally divalent, trivalent and tetravalent metal cations, have been examined.
Abstract: A large variety of undoped binary and ternary transition metal oxides, including formally divalent, trivalent and tetravalent metal cations, have been examined These 76 compounds are classified as either “metals”, “insulators”, or having a “metal-to-insulator” transition In an attempt to understand these variations, the Zaanen-Sawatzky-Allen framework was used in which each compound can be characterized by three parameters: the Coulomb correlation or disproportionation energy (U′), the charge-transfer energy (Δ) and the bandwidth (W) Assuming W is constant, we have calculated U′ and Δ using a simple ionic model, which includes only the gas phase ionization potentials and the bare electrostatic Coulomb interactions between the ions With this model, the occurence of metallic conductivity is remarkably well accounted for in these oxides

147 citations


Journal ArticleDOI
TL;DR: In this article, the structural and electronic properties of the recently discovered k-phase salts are examined from the viewpoint of lattice softness by calculating strain indices for short intermolecular contacts and the random potential arising from ethylene group conformational disorder prevents superconductivity in the iodide salt.
Abstract: The recently discovered k -phase salts k -(BEDT-TTF) 2 ]Cu[N(CN) 2 ]X (X=Cl,Br, I) are isostructural but differ in their superconducting properties: the Br-salt is an ambient-pressure superconductor with T c =116K, the Cl-salt becomes superconducting ( T c =128K) under a slight applied pressure of 03 kbar, and the I-salt does not exhibit superconductivity under an applied-pressure of up to 5 kbar We examine the structural and electronic properties of these three salts on the basis of their crystal structures determined at 127 K by single crystal X-ray diffraction measurements The band electronic structure of the three salts are virtually the same and do not account for the differences in the conduction properties The differences in the three salts with regard to superconductivity are examined from the viewpoint of lattice softness by calculating strain indices for short intermolecular contacts The random potential arising from ethylene group conformational disorder prevents superconductivity in the iodide salt

145 citations


Journal ArticleDOI
TL;DR: In this article, a new superconductor with infinite-layer structure and T c = 43 K was prepared applying high pressure synthesis and its T c was probably an electron-doped super-conductor because reductive atmosphere was critical for its preparation under high pressure.
Abstract: A new superconductor Sr 1- x La x CuO 2 ( x =0.1) was prepared applying high pressure synthesis. It has an infinite-layer structure and its T c equals 43 K. It is probably an electron-doped superconductor because the reductive atmosphere was critical for its preparation under high pressure.

Journal ArticleDOI
TL;DR: The microstructure of YBa2Cu3O7 films epitaxially grown on step-edge (100) SrTiO3 substrates has been characterized by means of high resolution transmission electron microscopy.
Abstract: The microstructure of YBa2Cu3O7 films epitaxially grown on step-edge (100) SrTiO3 substrates has been characterized by means of high resolution transmission electron microscopy. The results indicate a relationship between the microstructure of the film acreoss a step and the angle step makes with the substrate plane. On a steep, high-angle step, the film grows with its c-axis perpendicular to that of the film on the substrate surface so that two grain boundaries are formed. On a low-angle step, the film grows without any change in c-axis orientation across the step and without grain boundaries. Epitaxial second phases intergrowths across the steps have been found in some cases which may act as barrier layers when they cut through th YBa2Cu3O7 film.

Journal ArticleDOI
Gilles Triscone1, Jean-Yves Genoud1, T. Graf1, Alain Junod1, Jean Muller1 
TL;DR: In this article, the critical temperature of Bi 2 Sr 2 CaCU 2 O 8+δ (2212) was mapped to the (p, T ) plane for 10 −5 ≤ p ≤ 80 bar (O 2 ) and 300 ≤ T ≤ 840°C.
Abstract: We have mapped the critical temperature of Bi 2 Sr 2 CaCU 2 O 8+δ (2212) in the ( p, T ) plane for 10 −5 ≤ p ≤ 80 bar (O 2 ) and 300 ≤ T ≤ 840°C. Equilibrium conditions are frozen by quenching the samples into liquid gallium. The DC diamagnetic transitions remain sharp, and span an unusual temperature range for the 2212 phase (51.5 K to 94.4 K). This variation is attributed to a measured 2.5% change of the oxygen content ( Δδ ⋍ 0.2 ) in the structure. At the same time, we do not observe any significant variation of the lattice parameters. The Meissner effect, measured in an external magnetic field of 20 Oe, increases with T c . The slope of the normal-state susceptibility ∂ χ / ∂T becomes markedly less negative with decreasing oxygen concentration. It vanishes for the optimum doping that leads to T c ⋍ 94 K .

Journal ArticleDOI
TL;DR: In this paper, the electrochemical oxidation of La 2 CuO 4 in alkaline solution (1N KOH) has been used for preparing the superconducting phase La 2CuO 4+δ (δ⋍ 0.07).
Abstract: The electrochemical oxidation of La 2 CuO 4 in alkaline solution (1N KOH) has been used for preparing the superconducting phase La 2 CuO 4+δ (δ⋍0.07). Crystallographic data show an enhanced orthorhombic distortion and an increase of the unit cell volume. The transport properties reveal a sharp transition from a metallic behaviour to a superconducting state below 44 K which is corroborated by the noteworthy magnetic properties.

Journal ArticleDOI
TL;DR: In this article, it was shown that due to the strong electron-phonon interaction the usual Fermi-liquid description of the electron system is violated in the normal phase of HTSC metals (T > T c ).
Abstract: It is shown that due to the strong electron-phonon interaction the usual Fermi-liquid description of the electron system is violated in the normal phase of HTSC metals ( T > T c ). As a result it is impossible to use the simple formulas of the Drude type for the calculation of FIR properties of the HTSC materials. The many-body calculation of the reflectivity gives a good agreement with the FIR measurements for the single-domain crystal. The processing of the obtained results by using the “extended Drude formula” gives the effective scattering rate 1/τ ∗ ≈βħω where β is about 1 up to 2000 cm -1 (and even for higher frequencies taking into account the interband transitions).

Journal ArticleDOI
TL;DR: In this paper, the authors provide an alternative phenomenology for flux creep in high temperature superconductors by the more recent collective pinning theories, including long-time relaxation, flux-creep annealing and studies near T c.
Abstract: Magnetic studies of flux creep in high temperature superconductors show many features unexpected in the conventional Anderson-Kim theory, which assumes a thermal activation mechanism and uncorrelated motion of vortex bundles. An alternative phenomenology is provided by the more recent collective pinning theories. Experiments probing this new phenomenology are reviewed, including long-time relaxation, flux-creep annealing and studies near T c .

Journal ArticleDOI
TL;DR: In this article, a VLS-like mechanism is illustrated for the formation of a 3-Cu-layer Bi-HTSC from precursors by small, Bi/Pb rich, liquid droplets migrating over growing platelets, depositing ledges (steps) of product in their wakes.
Abstract: A VLS-like mechanism is illustrated of how 3-Cu-layer Bi-HTSC (high temperature superconductor) is slowly formed from precursors by small, Bi/Pb rich, liquid droplets migrating over growing platelets, depositing ledges (steps) of product in their wakes. The difficulty of obtaining a pure 3-layer, i.e. without syntactic intergrowths, is explained.

Journal ArticleDOI
TL;DR: In this article, the conditions for thermodynamic dynamic stability of phase-pure YBa 2 Cu 3 O 7−x and for compositions lying between the two phases were derived for the Y 2 O 3 -BaO-CuO phase diagram.
Abstract: The temperature and oxygen pressure conditions were determined by thermogravimetry for decomposition of phase-pure YBa 2 Cu 3 O 7− x and YBa 2 Cu 4 O 8 from 3 Pa to 0.1 MPA O 2 . All extant data were reviewed to determine the conditions for thermodynamic dynamic stability of phase-pure YBa 2 Cu 3 O 7− x and for compositions lying between YBa 2 Cu 3 O 7− x and CuO . Thermodynamic values for YBa 2 Cu 4 O 8 were derived, as was the 123-CuO section of the Y 2 O 3 -BaO-CuO phase diagram.

Journal ArticleDOI
TL;DR: The possibility of synthesizing phases of general composition YSr 2 Cu 3− x M x O 7± y has been examined for a wide variety of metals M.
Abstract: The possibility of synthesising phases of general composition YSr 2 Cu 3− x M x O 7± y has been examined for a wide variety of metals M. Single phase tetragonal products (typically a = 3.83 A , c = 11.5 A ) can be synthesised provided that the concentration of M is sufficiently large ( x > 0.5), and M has a preference for the Cu(1) sites (the four-coordinate sites) of the YBa 2 Cu 3 O 7 , structure. Neutron diffraction has demonstrated that a high degree of disorder exists in the (001) planes containing the M cations and, in particular, significant oxygen displacements are found. The non-existence of the unsubstituted variant YSr 2 Cu 3 O 7 is explained in terms of the high degree of compressive stress on the Cu(1) sites which could exist in such a material.

Journal ArticleDOI
TL;DR: In this article, structural changes in YBa2Cu4O8 have been studied as a function of pressure using TOF neutron powder diffraction with a sample in a helium-gas pressure cell.
Abstract: Structural changes in YBa2Cu4O8 have been studied as a function of pressure using TOF neutron powder diffraction with a sample in a helium-gas pressure cell. Its structure contains double chains of edge-sharing [CuO4] squares and two-dimensional CuO2 planes. The distance between an apical oxygen atom and an in-plane copper atom decreases with increasing pressure. The distance between barium and oxygen atoms in the chain and between in-plane copper atoms located on both sides of a Y layer also change significantly with pressure. All of these movements of atoms are closely related to charge transfer from the CuO chain to the CuO2 plane. Calculation of Madelung energies based on our structural parameters shows that the concentration of holes in the conducting CuO2 plane increases appreciably with increasing pressure, which is in good agreement with the observed rise in Tc under high pressure.

Journal ArticleDOI
TL;DR: In this article, nonsuperconducting Y 2 BaCuO 5 (211) inclusions can act as pinning centers in melt processed YBaCuO superconductors even when their size is orders of magnitude larger than the coherence length.
Abstract: While nonsuperconducting particles are known to serve as effective pinning centers in conventional superconductors, their effect in high T c superconductors is still controversial. In this paper, we give evidence that nonsuperconducting Y 2 BaCuO 5 (211) inclusions can act as pinning centers in melt processed YBaCuO superconductors even when their size is orders of magnitude larger than the coherence length.

Journal ArticleDOI
TL;DR: In this paper, it is shown that if the Fermi liquid is characterized by low energy scales, arising from narrow bandwidths or soft spin fluctuations, then canonical behavior is not expected at the relatively "high" temperatures of the normal state.
Abstract: A central issue in the field of high temperature superconductivity is the nature of the normal state. Here we review normal state transport, thermodynamic, magnetic and spectroscopic data and discuss their interpretation within a Fermi liquid based framework. Comparison with other highly correlated systems, such as the heavy fermnion metals and the quantum liquid 3 He, is also presented. It is demonstrated that the cuprate data are not consistent with canonical Fermi liquid behavior. However, if the Fermi liquid is characterized by low energy scales, arising from narrow bandwidths or soft spin fluctuations, then canonical behavior is not expected at the relatively “high” temperatures of the normal state. In this way anomalies above T c may be reconciled with a Fermi liquid (ground state) picture. Here we review the two main Fermi liquid based schools and their interpretation of the data. These are the ”almost localized” and “almost magnetic” Fermi liquid descriptions. The dichotomy between these two nearinstabilities has appeared in the context of the heavy fermoins as well as liquid 3 He and seems to be generally associated with highly correlated systems. Both approaches have addressed the data with some success and it is clear that they are, in many respects, closely related. The relationship between other Fermi-liquid-like, as well as more unconventional schemes is also addressed. Here it is claimed that the strongest support for a Fermi liquid based approach to the cuprates derives from the comparison with other strongly correlated Fermi liquid systems, which exhibit similar normal state anomalies.

Journal ArticleDOI
K. Ishida1, Yoshio Kitaoka1, T. Yoshitomi1, N. Ogata1, T. Kamino1, Kunisuke Asayama1 
TL;DR: In this article, the superconducting properties of Zn-doped YBa2Cu3O7 have been investigated by the 63Cu-NQR and -NMR technique.
Abstract: The superconducting properties of Zn-doped YBa2Cu3O7 have been investigated by the 63Cu-NQR and -NMR technique. Both the Knight shift K and the nuclear-spin-lattice relaxation time T1 were measured by using the oriented powder. The residual spin Knight shift of 63Cu at 4.2 K increases appreciably with increasing Zn-content from x=0.01 to 0.02. Correspondingly, the T1T=constant law holds at low temperatures far below Tc=79 K and 68 K for x=0.01 and 0.02 respectively, providing a clear sign of gapless superconductivity with finite density of states at the Fermi level, whereas the behavior of rapid decrease of 1/T1 just below Tc is almost unchanged even though Zn impurities are doped into the CuO2 plane. The NMR results in the Zn-doped and undoped YBa2Cu3O7 can be well interpreted by the combined effect of the gap zeros of lines at the Fermi surface originating from a d-wave pairing and the residual density of states at the Fermi level induced by non-magnetic Zn-impurities and by some imperfection of the crystal existing inevitably even in undoped YBa2Cu3O7.

Journal ArticleDOI
TL;DR: In this article, the magnetisation of a disc-shaped superconductor in a perpendicular applied field is analyzed and the differential susceptibility as the field is reduced slightly from its maximum value, is shown to give an unambiguous determination of both the critical current density and the length scale on which the current is carried.
Abstract: The magnetisation of a disc-shaped superconductor in a perpendicular applied field is analysed. The differential susceptibility as the field is reduced slightly from its maximum value, is shown to give an unambiguous determination of both the critical current density and the length scale on which the current is carried. These results can be applied to thin films and also to single crystals of large aspect ratio, and allow the behaviour of internal “weak-links”, if present, to be studied non-destructively as a function of field and temperature. Magnetisation measurements on a single crystal, a thin film, and a thick film, all of YBa 2 Cu 3 O 7 , are presented. These confirm the validity of the analysis.

Journal ArticleDOI
TL;DR: In this paper, the formation of biaxially oriented films of yttorium-stabilized ZrO2 (YSZ) on a polycrystalline, Ni-based alloy (Hastelloy c276) by Ion-Beam-Assisted Deposition (IBAD), and subsequent a-b plane aligned YBa2Cu3O7−x (YBCO) films deposited by laser ablation were obtained.
Abstract: We report the formation of biaxially oriented films of yttorium-stabilized ZrO2 (YSZ) on a polycrystalline, Ni-based alloy (Hastelloy c276) by Ion-Beam-Assisted Deposition (IBAD), and the subsequent a-b plane aligned YBa2Cu3O7−x (YBCO) films deposited by laser ablation. Jc of 6.0×104 A/cm2 (77 K, O T) and 1.4×104 A/cm2 (77 K, 0.6 T) were obtained. A new method to prevent intergranular-weak-links has been developed for application of oxide superconducting thin films, for tape-shaped cables, magnets, magnetic shields, microwave devices, etc.

Journal ArticleDOI
TL;DR: In this paper, the Hall coefficient of Tc changes from positive to negative as one goes from the underdoped to the overdoped side, and the correlation between Tc and the carrier density dependence was discussed.
Abstract: The pressure dependences of the superconducting transition temperature and the Hall coefficient have been studied for YBa2Cu3O7, YBa2Cu4O8, Bi2Sr2CaCu2O8, and Tl2Ba2CuO6+y. The pressure coefficient of Tc changes from positive to negative as one goes from the underdoped to the overdoped side. The Hall coefficient has been found to decrease by ∼ 8–10%/GPa for these cuprates. Based on these results, the correlation between the pressure dependence of Tc and the carrier density dependence of Tc is discussed. The present results are also compared with our earlier result on La2−xSrxCuO4.

Journal ArticleDOI
TL;DR: In this article, the elastic modulus and hardness of highly aligned YBa 2 Cu 3 O 7−δ obtained by melt processing was determined using a highly spatially resolved mechanical properties microprobe.
Abstract: The elastic modulus and hardness of highly aligned YBa 2 Cu 3 O 7−δ obtained by melt processing was determined using a highly spatially resolved mechanical properties microprobe. Ultra-low load indentation measurements on the (001) cleavage plane of aligned 123, indicated a Young's modulus of 143 ± 4 GPa and a hardness of 10.0 ± 1.9 GPa. For measurements on a plane perpendicular to the cleavage plane, values of 182 ± 4 GPa for the modulus and 10.8 ± 1.7 GPa for the hardness were obtained. A lower modulus in the c -direction is perhaps a result of the layer-like structure of 123, with weak coupling between the layers. Measurements on the trapped single crystal 211 particles yielded a modulus of 213 ± 5 GPa and a hardness of 14.4 ± 2 GPa. Considerations of the thermal and elastic mismatch effects between the 211 particles and the 123 matrix, the large thermal expansion anisotropy of aligned 123, and microstructural examination of polished and fracture surfaces of the aligned samples indicate that the 211 particles perhaps serve to enhance the fracture resistance behavior of 123 by energy dissipation due to interfacial delamination and crack bridging.

Journal ArticleDOI
TL;DR: In this paper, it was demonstrated that a single-phase "2223" superconductor exhibits the zero-resistance temperature at 127 K and the diamagnetic onset temperature at 130 K, when the sample is encapsulated in an evacuated quartz tube and post-annealed at 750°C for 250 h.
Abstract: It is demonstrated that a single-phase “2223” superconductor, i.e. Tl 1.7 Ba 2 Ca 2.3 Cu 3 O z , exhibits the zero-resistance temperature at 127 K and the diamagnetic onset temperature at 130 K, when the sample is encapsulated in an evacuated quartz tube and post-annealed at 750°C for 250 h. It is found that the encapsulation of the sample is crucially important for post-annealing longer than ∼10 h. Both the zero-resistance temperature and the diamagnetic onset temperature increase as the post-annealing time increases. This suggests that the increase in the superconducting critical temperature is related to a certain diffusion process such as ordering of constituent ions.

Journal ArticleDOI
TL;DR: In this paper, the homologous series of high temperature superconductors, Y2Ba4Cu6+nO14+n−ξ with n=0 (123), n=1 (247) and n=2 (124) as a function of Ca and La substitution through measurement of Tc, lattice parameters, selected Rietveld refinements and thermogravimetric analysis.
Abstract: We report the preparation and characterization of the homologous series of high temperature superconductors, Y2Ba4Cu6+nO14+n−ξ with n=0 (123), n=1 (247) and n=2 (124) as a function of Ca and La substitution through measurement of Tc, lattice parameters, selected Rietveld refinements and thermogravimetric analysis. Solubility limits of Ca and La are investigated and found to be widely different for each member. Data is presented showing that Ca substitutes on both the Y- and Ba- sites and as the Ca concentration increases the relative fraction on the Y site decreases. Systematic trends indicate that for n=0 123 is slightly overdoped, 247 is slightly underdoped and 124 is well underdoped relative to Tc(max) such that 0.06 La substitution, 0.04 Ca substitution and 0.15 Ca substitution is required to maximise Tc in 123, 247 and 124, respectively. Differences in the orthorhombicity between the members of the series as a function of substitution are interpreted in terms of differences in hole doping and in oxygen stoichoimetry for each member. For instance, the observed rapid decrease in the orthorhombicity in 124 at a high La content is interpreted as evidence that oxygen loads into the interchain site. This destabilises the ribbons leading to the formation of tetragonal 123.

Journal ArticleDOI
TL;DR: This article performed LDA calculations of structure, phonon frequencies, and electron-phonon interaction and found good agreement with available data and estimate: λ = 1.5.
Abstract: We have performed LDA calculations of structure, phonon frequencies, and the electron-phonon interaction. We find good agreement with available data and estimate: λ=1. Saddlepoints 20 meV below the Fermi level are important.