Magnetic Characterization of Rare-Earth Barium Cuprate Superconductors

We report magnetic characterization of barium rare-earth cuprate superconductors to determine critical current densities as a function of rare-earth constituent. A vibrating sample magnetometer was used to measure M-H loops for fields to 15 kOe of oriented single crystals. Critical current densities are insensitive to rare-earth constituent despite the magnetic moment on the rare-earth ion. Values of critical current densities in the a-b plane vary from 3×105 A/cm2 at 10 K to 2×104 A/cm2 at 77 K. Anisotropy in the critical current density in the a-b plane and perpendicular to this plane was a factor of 2 in a Ba2YbCu3O7 crystal at T=38 K. Aluminum impurities lower critical currents but also reduce the superconducting transition temperature

Specific Heat Near TN in (Pr, Nd, Sm, Gd, and Dy) Ba2Cu3O7Evidence for Spin-Exchange-Driven Ordering

Magnetic ordering at 2.2 K has been reported for GdBa2Cu3O7. In order to characterize the relevant magnetic interaction we have synthesized and measured the specific heat of polycrystalline RBa2Cu3O7, where R=Pr,Nd,Sm,Gd,andDy. The ordering temperatures are found to vary systematically with the de Gennes factor, suggesting the dominance of spin-spin exchange over dipolar energies for these compounds. The field dependence of the specific-heat peak for DyBa2Cu3O7 implies the order is of antiferromagnetic nature

Microwave Loss and Oxygen Annealing in Yba2cu3ox Single Crystals

Reversible thermal annealing of YBa2Cu3Ox single crystals in air has been studied by microwave loss measurements. Among the advantages of this contactless technique are the following: Crystals are kept in sealed capillaries throughout cycles of annealing and low-temperature measurement; features are resolved in loss versus temperature measurements that are not revealed by other techniques. Microwave loss is interpreted in terms of the Gittleman-Rosenblum model of fluxon motion in a viscous medium. Narrow temperature ranges are found for the formation of the 60- and 90-K phases. Optimal microwave response is obtained after but a few hours of annealing, indicating the importance of c-axis diffusion

Structure - Property Correlations in Superconducting Copper Oxides

We present initial results of a survey correlating structural features of the cuprate superconductors with the occurrence of superconductivity. The best correlation is with the Cu - Cu distance in the plane of the two-dimensional compounds. Superconductivity has been shown only over a narrow region of Cu-Cu separations for both the A2CuO4 and square pyramidal based compounds. On either side of the superconducting region semiconductivity is found for both systems. It is suggested that the key difference between the classes is that oxidation of the A2CuO4 compounds results in removal of anti-bonding electrons, whereas in the square pyramidal compounds these electrons are non-bonding, which allows for greater carrier densities befor chemical instabilities occur. The copper oxide based systems are compared to the bismuth based superconductors. Unsuccessful attempts to synthesize superconductors based on lead are described

Etching Behavior and Electrochemistry of a Bi-Sr-Ca-Cu-O Superconductor in Acidic Media

Bismuth strontium calcium cuprate superconductors with nominal composition, Bi2(Sr, Ca)3Cu2O8, have been examined by electrochemical techniques and are found to be both highly oxidizing compounds and reactive towards hydrogen ions. The chemical reactivity of 84K bismuth strontium calcium cuprate is similar to that of Ba2YCu3O7. The dissolution reaction in acid is intrinsically fast; we observe H+ mass transport control in perchlorate solution. Dissolution rates can be altered and made selective by surface filming and post reactions of Bi+3, as in Cl− solution. The stoichiometries of the acid reactions and for the etching of the Bi2(Sr, Ca)3Cu2O8 by ethylenediaminetetraacetate have been determined by rotating ring-disk methods. Several etching solutions readily adaptable to controlled patterning or removal of these substrates have been demonstrated

Thermally Activated Dissipation in Bi2.2Sr2Ca0.8Cu2O8+

A new dissipation behavior is reported in superconducting Bi2.2Sr2Ca0.8Cu2O8+ for all temperatures below Tc and all magnetic fields exceeding Hc1. The current-independent electrical resistivity is thermally activated and can be described by an Arrhenius law with a single prefactor and a magnetic-field- and orientation-dependent activation energy U0(H,). This behavior is markedly different from past observations and will be discussed in terms of flux creep and flux flow. This thermally activated behavior implies a finite resistance at all temperatures and all fields exceeding Hc1 determined by the activation energy as the only parameter

Gigahertz Ultrasound in Single Crystal Superconducting YBa2Cu3O7

Anisotropic sound propagation at 1 GHz in single-crystal YBa2Cu3O7 is described. Longitudinal phonons have been studied from 0.01 K to 300 K, with emphasis on the temperature region near the superconducting transition Tc at 88 K. A discontinuous sound velocity is observed at Tc for propagation parallel and perpendicular to the c axis. Strain derivatives of Tc are evaluated using mean field theory. Below 1 K the temperature-dependent sound velocity has positive slope, which indicates tunneling of defects

Low-Field Flux-Flow Resistivity in Bi2.2Sr2Ca0.8Cu2BO8+

Current-voltage characteristics of single crystals of the 84-K superconducting phase Bi2.2Sr2Ca0.8Cu2BO8+ show no threshold behavior in small magnetic fields at 5080 K, despite large zero-field critical currents which are only weakly field-dependent at low temperatures (\u3c20 K). We attribute this to a low-field flux-flow resistivity which shows an empirical scaling behavior near Tc and which we can use to infer some basic properties of the superconductor. This behavior is intrinsic to Bi2.2Sr2Ca0.8Cu2BO8+ and has crucial implications for its technological potential of the compound

Magnetic Studies of Strontium Lanthanum Copper Oxides

We report measurements of static magnetic susceptibilities for a series of stronium-substituted lanthanum copper oxides prepared under identical conditions. These materials have recently been shown to exhibit bulk superconductivity with high critical temperatures. Measurements of the end member La2CuO4 provie evidence for a phase transition of unknown origin at 260 K. This phase transition is suppressed by the substitution of small amounts of strontium for lanthanum. Understanding the normal-state properties of SrxLa2-xCuO4- may provide important clues to the superconducting behavior of these materials