B. Roden

Nature of the specific heat anomaly in the Bi and Tl high temperature superconductors

Abstract Specific heat, field cooling Meissner effect, isothermal magnetization, AC susceptibility and resistance were measured on sintered pellets of the Bi and Tl high temperature superconductors with nominal compositions BiPb 0.3 SrCaCu 2 O 5.8 and Tl 2 Ba 2 CaCu 2 O 8 , and for comparison on some pellets of the REBa 2 Cu 3 O 6.9 system. With decreasing field, the field cooling Meissner signals of the Bi and Tl systems approach the same magnitude and transition width (Δ T ≈ 2–3 K) as in high quality pellets of the REBa 2 Cu 3 O 6.9 . The field and temperature dependence of the Meissner signals is analysed in detail. At the transition temperatures of five superconducting phases of the Bi and Tl pellets, the specific heat shows no jumps nor maxima, within the accuracy of the measurement (Δ( C/T ) 2 )). There are however sharp breaks of the slope of C/T at the T c values, which are intrinsically as large as in the REBa 2 Cu 3 O 6.9 . The transitions in the Bi and Tl systems look more like third order transitions rather than like the second order transitions of the REBa 2 Cu 3 O 6.9 .

Preparation and characterisation of single-phase Bi-Pb-Sr-Ca-Cu-O high temperature superconductors

Abstract We identify the preparation procedures to obtain polycrystalline pellets of Bi-2212 and of Bi-2223 high-temperature superconductors, which slow only the superconducting properties and the X-ray diffraction pattern of one or the other phase (“single-phase” pellets). These procedures were found by systematic variation of the preparation, with substitution of PbO for Bi2O3 as well as with addition of PbO to the stoichiometric mixture of starting powders. Bi-2223 in single-phase form has a TZR of 107.5 ± 0.5 K, whereas single-phase pellets of “Bi-2212” can have sharp transitions with TZRs between 55–95 K, depending on the oxygen partial pressure of the atmosphere in the final annealing step.

On the antiferromagnetism of CuO at 230K and its relevance for high Tc superconductivity

Abstract The magnetic susceptibility of CuO shows an anomaly at 230K, which, together with reported neutron and specific heat data, can be interpreted as an indication of antiferromagnetic order. This finding is discussed in the context of the recently discovered high T c superconductivity of CuO-derived compounds.

Preparation and characterization of RE doped La2-xSrxCuO4

Abstract We report on the systhesis of (La,RE) 2- x Sr x CuO 4 for various RE=Pr, Nd, Sm, Eu, Gd, Dy, Ho and Er as a function of the RE and Sr concentrations. The sample characterization by means of powder X-ray diffraction and EDX is described. We present our results on the solubility limit of the various RE in the T structure of La 2- x Sr x CuO 4 and discuss the systematic influence of RE and Sr doping on the structural properties. We furthermore show that doping with RE does not influence the charge carrier concentration of the samples.

The equilibrium magnetization of granular high temperature superconductors in low fields: I. The Meissner effect of YBa2(Cu1−xFex)3O7−y

Abstract We report the temperature and field dependence of the DC Meissner effect of sintered pellets of YBa 2 ( Cu 1− x Fe x ) 3 O 7− y with 0⪕ x ⪕0.08. In all samples the spontaneous flux expulsion remains incomplete ( −4πχ≈ 1 3 ) down to fields, which are much smaller than H cl , but at H ⪕ H m ≈2 Oe, they all exhibit a full Meissner effect and are therefore fully superconducting. The expulsion in 2 Oe decreases to near 30% by aging of the defect surfaces and goes back to near 100% after their reoxidation, all at a fixed and sharp T c of 93 K. The data are interpreted in terms of equilibrium properties of granular high temperature superconductors on the basis of an analysis worked out in the next paper [1]. From the temperature and field dependence of the spontaneous flux expulsion, an average cross-section of 4μm 2 is extracted for the superconducting blocks bordered by defect surfaces within the much larger grains. The superconducting loops around these surfaces are responsible for the final flux expulsion in H T c ⪕1 K in the orthorhombic phase ( x ⪕0.02). In the quasitetragonal phase, Δ T c increases from 1 to 16 K for 0.02 x ⪕0.08.

Improvement of the cation-distribution in the polycrystalline high-temperature superconductor LaSrCuO

Results obtained by high resolution EDX measurements which demonstrate that polycrystalline samples of the LaSrCuO-system prepared by means of a refined wet-chemical preparation method exhibit a much better homogeneity of the cation distribution than samples prepared by standard ceramic routes. Other characteristic properties like FC flux expulsion, phase purity and behaviour of resistivity are at least preserved. The proposed wet-chemical method is similar to several spray-drying techniques described in the literature but easier to handle and optimized for LSCO.

The specific heat anomalies at Tc of the BiSrCaCuO and TlBaCaCuO high temperature superconductors

Abstract In the specific heat of the BiSrCaCuO and TlBaCaCuO families of high temperature superconductors the usual jump of a second order mean field transition is absent. Sharp breaks of the slope of C(T) appear instead at the thermodynamic transition temperature T GL , and in high quality samples a small dip appears in C(T) at T KT , 1–1.5 K below T GL , where the dc Meissner effect sets in. Corresponding anomalies are seen in the thermal expansion, in the resistivity and in the Hall effect. It is argued that strong intrinsic fluctuations broaden the mean field anomalies, e.g by ±10K in the case of Bi2Sr2Ca2Cu3Ox.

Structure and superconductivity in La1.15-xPr0.85SrxCuO4

Abstract A phase diagram of Pr-doped La 2-x Sr x CuO 4 including structural and superconducting properties is presented. We find that Pr-doping shifts the phase boundary of the high temperature tetragonal to orthorhombic phase transition to higher Sr-concentrations without inducing the low temperature structural phase transition as other RE do. As superconductivity occurs in the same Sr-concentration range as in “pure” La 2-x Sr x CuO 4 (LSCO), the disappearance of superconductivity is not related to the high temperature structural transition, but is determined predominantly by the hole-concentration in the CuO-planes. The decrease of T c of ≈ 5–10 K in the Pr-doped samples with respect to the undoped ones can mainly be attributed to the increase of the octahedra tilting angle.

Single crystal growth and characterization of superconducting PbSr(Y,Ca)CuO

Abstract Single crystals of Pb 2 Sr 2 (Y 1−x Ca x )Cu 3 O 8+ δ (PSYCCO) have been grown from PbO KCl flux in an atmosphere of 3% O 2 in N 2 . A temperature gradient in the crucible is necessary to get crystals of PSYCCO with reproducible sizes of more than 2 × 2 × 1 mm 3 . Neutron diffraction experiments, high resolution EDX and high resolution transmission electron microscopy confirm a low mosaicity without any crystal defects, well defined lattice constants and a good cation homogeneity.