Magnetically aligned samples of sintered YBa&Cu307 z were used to test the effects of oxygendeficiency 5 (with 5 0.2) on the superconductive magnetization M, critical current density J" irreversibility field B;",upper critical field H, t, coherence length g, condensation energy F"penetration depth A, , and related properties as a function of temperature T and applied field H~~c. In selected cases, studies were also made with H~~ab. The 0 content was monitored in situ by thermogravimetric analysis. The open porosity and granularity of the material allowed rapid and homogeneous oxygenation. We found no significant enhancement of intragrain J, with chain-site 0 defects. With few exceptions, maximum J, occurred at full oxygenation. This implies that chain-site 0 defects are not strong or effective pinning centers over most of the field-temperature regime investigated. Except for T"which was practically independent of 5 within the interval 0 & 5~0. 11 (so called "90-K T, plateau" ), most properties such as J" F"H,z, B;", (T), A, , and g were strongly and continuously influenced by the 0 deficiency. The observed abnormal magnetization with H~~c was weak at low T but became more pronounced as T and fi increased. No abnormal magnetization was detected with H~~ab. As oxygen was removed, B;"(T)and H,~(T) separated, and both lines shifted to lower T and lower B. Moreover, B;,"was strongly correlated with J, at low temperature. Determination of the thermodynamic critical field H, yielded condensation energies F,(5) that were well correlated with J,(5). The results indicate that 0 defects weaken the energy barrier of the existing pinning sites and have a negative overall effect on the capacity of YBazCu307 to carry loss-free currents.

Fine grained polycrystalline samples of MgB 2 superconductor were synthesized from the elements to contain < 5 % of impurity phases, according to x-ray powder diffractometry. The superconductive transition was sharp with a midpoint T c = 38.5 K. The magnetization in the vortex state was studied as a function of applied field H , temperature T and time t. From the equilibrium magnetization, the London penetration depth was obtained. The supercurrent density J (T , H, t) in the vortex state (derived from the irreversible magnetization) decreases approximately linearly with T , in contrast to the quasi-exponential fall-off in high-T c superconductors. The current is highly stable in time, with normalized decay rates S = − d ln(J )/d ln(t) well below those in high-T c materials. These results are compared with those of other superconductors.

The upper critical field H, & and other equilibrium properties of a magnetically aligned HgBa, Ca2Cu30, +z superconductor have been investigated. Experimental results for the reversible magnetization, both with H~~c and H)(ab, are analyzed using the theoretical formalism of Hao et al. [Phys. Rev. 8 43, 2844 (1991)]for type-II superconductors. Near T" the slopes dH, 2 jdT of the upper critical field are -1.6 T/K and -12. 1 T/K for fields parallel and perpendicular to the c axis, respectively. 0 0These correspond to coherence lengths (extrapolated to T=O) of g, b(0)=15 A and $, =1.9 A. This gives a lower bound on the superconductive anisotropy parameter y =(m, /m, b )' ) 8.

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The equilibrium magnetization was studied in magnetically aligned Hg-based cuprate superconductors with a crystal structure containing one, two, and three adjacent Cu-O layers. The mixed state magnetization was measured with the magnetic field applied perpendicular to the layers, along with low field Meissner effect and normal state susceptibility studies for volume correction factors. The internally consistent analysis is used to identify systematics as the number ''n'' of Cu-O layers increases. The study reveals a linear increase in the slope of upper critical field ϪdH c2 /dT with ''n''. Most importantly, we find a nearly constant carrier doping per Cu-O layer. This result is compared with other studies, e.g., Rietveld analysis of neutron diffractometry, and its implications are discussed.