We report measurements of the transport and the magnetic properties of high-quality, sub-millimeter-sized MgB 2 single crystals with clear hexagonal-plate shapes. The low-field magnetization and the magnetic hysteresis curves show the bulk pinning of these crystals to be very weak. The Debye temperature of ⌰ D ϳ1100 K, obtained from the zero-field resistance curve, suggests that the normal-state transport properties are dominated by electron-phonon interactions. The resistivity ratio between 40 K and 300 K was about 5, and the upper critical field anisotropy ratio was 3.0Ϯ0.2 at temperatures around 32 K.

We synthesized polycrystalline MgB2 from a stoichiometric mixture of Mg and the 11 B isotope under different conditions. All the samples showed bulk superconductivity with Tc = 38~39 K. The samples containing the least amount of unreacted Mg showed the highest Tc and the sharpest transition width (Delta_Tc). A residual resistivity ratio (RRR) of ~ 5.8, and a magnetoresistance (MR), at 40 K, of 12% were obtained for these samples. Moreover, there was no upturn of resistivity in a low temperature region at 10 Tesla. The samples containing appreciable amounts of unreacted Mg showed quite different behaviors; the values of Delta_Tc, RRR, and MR were much larger. An upturn appeared in resistivity of the samples below about 50 K at 10 T and is thought to be due to the unreacted Mg.Comment: PhysicaC (in press), 4 page with 3 figure

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M NaC 10 mM phosphate. pH 7.0) were mlxed, Acids Res. 16, 4937 (1 988)l. frozen (40 s), thawed (5 mni, and spotted with a 18. We acknowledge suppot? by grants from the Nationcapillary tube on a C, , TLC plate A blue spot deve-al Institute of General Medical Sc~ences (GM 10265) oped, In the absence of target, the color remaned and the Offce of Naval Research (NO01 4-94-1 -0703 pnk. For comparison, the lower Imlt for detectng an and N00014-97-10430) and the Department of De-ol~gonucleot~de with the use of a probe labeled with fense (MUR DAAG 55-97-1 -01 33). fluorescein In a sandwich hybridization system was reported to be 500 fmol [M. S. Urdea el a/. , R~ucleic A new high-temperature superconducting compound system, Ba,Can-,CunO,[02(n-1) n-Ba] interstitially doped with calcium or (Ca,Cu) has been identified to exhibit a transition temperature up to 126 kelvin, the highest for a superconductor without a volatile toxic element. 02(n-1)n-Ba has body-centered tetragonal symmetry and an unusual chargereservoir block. The compounds offer interesting opportunities for high-temperature superconductivity science and technology.