The Effects of Oxygen and Strontium Vacancies on the Superconductivity of Single Crystals of Bi₂sr_2‐Xcuo_6‐Y

Abstract

Single crystals of Bi2Sr2-XCuO6-y were grown from CuO-rich melts. The Sr content in the crystals was varied from x=0.1 to x=0.5 by varying the starting composition of the melt and the oxygen content was adjusted using an appropriate heat treatment of the crystals in a thermogravimetric system. With decreasing Sr content the superconducting transition temperature,Tc, of the crystals decreased rapidly from 10 K to below 4.2 K and the resistivity in the a-b plane changed from metallic (linear in T from 30 to 300 K) to semiconducting. Reducing the oxygen content in the crystals had a similar effect on the resistivity. Only crystals with close to the maximum oxygen content (y=0) were superconducting, and removal of oxygen from previously superconducting crystals resulted in a rapid decrease of Tc , and the eventual loss of superconductivity (Tc < 4.2 K). Although crystals of Bi2Sr2-xCuO6-y could be made sequentially superconducting-semiconducting-superconducting by the addition-removal-addition of oxygen, the process was not totally reversible. After only one such cyclic heat treatment x-ray diffraction indicated that for some compositions such cycling resulted in the appearance of CuO and an another layered structure that has been identified previously and is slightly deficient in CuO. The new layered structure is semiconducting and has a layer-stacking repeat distance that is 1 A smaller than the superconducting "221" phase.