Superconducting properties of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mi mathvariant="normal">−</mml:mi><mml:mi mathvariant="normal">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ba</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuO</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Abstract

We show that the superconducting transition temperature ${T}_{c}$ of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ as a function of barium content (0\ensuremath{\le}x\ensuremath{\le}0.25, determined by mutual inductance measurements) has two maxima, both ${T}_{c}$\ensuremath{\approxeq}25 K, near compositions x=0.09 and x-0.15. Between these two maxima is a local minimum (${T}_{c}$ about 5 K) for x=0.12. dc magnetization data are also reported for six compositions. Anomalies in electrical resistance appear near T=50--60 K at compositions 0.10\ensuremath{\le}x\ensuremath{\le}0.125. Many samples clearly show a second resistive superconducting transition near 30 K in addition to the bulk transition most clearly observed magnetically. The variation of ${T}_{c}$ with composition is discussed in relation to the occurrence of resistance anomalies. The tetragonal lattice parameters at room temperature are consistent with previous work, and show no obvious anomalies. The sample preparation procedures are discussed in detail.