Magnetism, Superconductivity, and Chemical Substitutions in YBa2Cu3O7-δ

Abstract

Twelve compounds with the formula RBa2Cu3O7-δ where δ≍0.1 and R=Y or a lanthanide element except Ce, Pm, or Tb, crystallize in the same layered orthorhombic perovskite-like structure (see the article by I.K. Schuller and J.D. Jorgensen in this issue). All such compounds are superconducting with superconducting transition temperatures Tc≍92-94 K, except for R=Pr. The compound PrBa2Cu3O7-δ appears to be a special case, since it does not exhibit metallic behavior and is not superconducting. Historically, the prototype YBa2Cu3O7-δ compound is singularly important since it was the first superconducting material with a Tc greater than 77 K, the boiling point of liquid nitrogen. In the meantime, yet higher Tc's have been obtained in YBa2Cu3O7-δ at very high pressure (Tc˜107 K at 150 kbar)5 and in new layered compounds in the Bi-Sr-Ca-Cu-O (maximum Tc≍110 K)6 and Tl-Ba-Ca-Cu-O (maximum Tc≍125 K)7 systems (see the article by A.W. Sleight, M.A. Subramanian, and C.C. Torardi in this issue).The origin and nature of the high Tc superconductivity of the RBa2Cu3O7-δ compounds, and the other layered copper oxide compounds as well, are presently not understood and constitute a formidable challenge to experimentalists and theorists alike. One of the most intriguing possibilities is that a magnetic mechanism, rather than the electronphonon interaction, is responsible for the formation of the superconducting electron pairs in the high Tc copper oxides. The primary evidence for a magnetic pairing mechanism is the proximity of antiferromagnetism and superconductivity as the concentration of holes in the conducting CuO2 planes is varied,8 as discussed below for the RBa2Cu3O7-δ compounds.