Control of carrier concentration in Bi-2212

We realized highly underdoped Bi-2212 crystals by using a Bi substitution into a Sr site (x), together with the excess oxygen (delta) control. The samples with x = 0-0.3 were grown by a traveling solvent floating zone method. For each x, delta was varied using the precision annealing method [1]. Under several assumptions, the effective Bi valence was evaluated to be +2.2, which is much lower than the formal valence +3. This indicates that the Bi substitution accompanies additional uptake in excess oxygen. The x dependent delta-doping level p relation and the observed shrink in the c-axis length are consistent with this assumption. Based on the decomposition phase diagram obtained in this study, T(c) of 22 K was realized by careful annealing treatments. The samples showed a superconducting volume fraction of over 50%. (C) 2009 Published by Elsevier B.V.