REVERSIBLE ELECTROCHEMICALLY INDUCED P(T) AND TC CHANGES IN BI2SR2CACU2O8+DELTA

The resistivity and T(c) of Bi2Sr2CaCu2O8+delta can be reversibly modified at room temperature using an electrochemical cell. Whereas reducing with up to 0.02 mole fraction of charge enhances the T(c) systematically from about 73 K to 88 K, further reduction decreases the T(c) again and eventually goes on to suppress superconductivity altogether. Above 0.10 mole fraction p(T) loses its metallic character and assumes a negative slope which, above 0.50 mole fraction, can be modeled in terms of a semiconductor with an activation energy of 30 +/- 2 meV. Oxidizing the latter material restores superconductivity, albeit the reversibility is not complete. Electrochemical and EDX data are compatible with the reversible solid state diffusion of copper; we propose that copper is intercalating.