ADVANCES IN SINGLE-CRYSTAL BI2SR2CACU2O8+DELTA SUPERCONDUCTORS

Susceptibility, resistivity and X-ray diffraction measurements have be en performed on single crystals of Bi2Sr2CaCu2O8+delta. Very sharp (0. 5 K width) superconducting transition temperatures as low as 55 K have been reached in the overdoped part of the phase diagram. Anneals in l ow oxygen pressures have allowed us to access the entire underdoped re gion of the phase diagram. The samples have been reversibly changed fr om a high-temperature superconductor to an insulator.