Preparation and characterization of homogeneous YBCO single crystals with doping level near the SC-AFM boundary

Abstract High-purity and homogeneous YBa 2 Cu 3 O y single crystals with carrier doping level near the AFM-SC boundary have been obtained in the oxygen content range between y =6.340 and 6.370. The crystals are ortho-II phase at room temperature and undergo the orthorhombic to tetragonal transition at about 140 °C. They show sharp superconducting transitions, with T c between 4 and 20 K. T c changes by 0.8 K when the oxygen content y is changed by 0.001, and is also sensitive to annealing conditions near room temperature, due to the dependence of doping on oxygen ordering correlation lengths. Crystals with oxygen content y lower than 6.345 are non-superconducting.