LOW TEMPERATURE RESISTANCE BEHAVIOR AND SUPERCONDUCTIVITY OF La–Sr–Cu–O SYSTEM: ROLE OF ALTERED Cu–VALENCE AND O–CONTENT

In this note we are communicating the results obtained on various samples of lanthanum-based cuprate in which Sr and Zn have been partially substituted for La and Cu, respectively over a range where the superconductivity is gradually destroyed. The decrease in T c is accompanied by a changeover from metallic to semiconducting-like resistance behavior occurring at lower temperatures. The observed behavior is related to the effective copper valence and the change in the oxygen content of the samples and the results are discussed in terms of a tentative phase diagram which would be necessary to examine the mechanism of superconductivity.